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Radiant versus forced air for low load homes

Hey all. I fight clients on this one all the time. We build passive solar ICF homes with <1.0 ACH50. Many of our homes feature finished concrete floors. EVERYBODY wants radiant heat for the "warm" floors. I tell them the floors will not be warm, and they are wasting cost since we have to ventilate, filter, and dehumidify the air and thus need two systems. (We are right on the edge of CZ 5 and 6).
I know years ago BuildingGreen had an article called "When radiant heating does and doesn't make sense." But it appears to be behind a pay wall now. Is there any such articles or empirical data that you know of I can direct them to? Something a little more trustworthy than some young uneducated tradesman punk trying to tell them it is not worth it?


  • Sean WiensSean Wiens Posts: 331
    Do not have a lead on your article. But why are you saying floor would not be warm, of course it would be warm. The real issue with radiant in a high performance home with lots of solar heat gain, is that with a high mass slab radiant system, you will regularly overshoot your target temps because the panel will keep puking out heat for hours and hours after a call for heat has been cancelled by the thermostat. That is why I am putting in a low mass ceiling and wall radiant system (which by the way will also heat up any floor that the ceiling/wall panel will 'see' at least a degree or two). Also, if you accept best practice that the HRV should be piped separately from a forced air heating system anyway, then the cost for the hydronic is not that bad especially when you take into account all of the extra hidden costs for forced air (like deeper floor trusses to fit the ducts, or the loss of ceiling heights due to bulkheads). And the reduced costs to run a hydronic system will quickly pay back any extra installed costs.
  • David ButlerDavid Butler Posts: 3,889
    edited March 2017
    @Sean, in low load homes, the floor distribution temperature in some rooms may only need to be in the mid-70's at design conditions. Even 80 feels cool to the touch. So it's important to explain to homeowners that they can't have a super efficient home AND warm floors. The radiant industry has harped on 'warm toes' for so long that's it's going to be difficult to back away from this expectation.

    There are other good reasons not to mix distribution with a home's thermal mass, such as the overshoot problem you mentioned. I've designed systems for homes in the upper midwest with < 5 btu/ft2 load in most rooms. In situations like that it becomes really important not to encumber the heating distribution system with a lot of mass. Bottom line: avoid radiant slabs in low load homes. Low mass radiant works well, but unfortunately is much less familiar to homeowners and contractors in the US.

    I'm sure you knew I would take issue with your characterizations of forced air! There's always been a lot of bad-mounting of forced air among wet-heads, a lot of it deserved (ok, I'll admit it, I'm an dyed-in-the-wool air-head!). But that's all about quality design and installation. The fact is, the more efficient the enclosure, the harder it is to make the argument that radiant is more comfortable. And the harder to justify the additional first cost.

    Also, your comment re: hidden costs of forced air doesn't really apply to low load homes like Jake's. You don't need deep floor trusses or bulkheads when the trunk is maybe 6" high and the largest branch is maybe 5 inches! Also, in many areas of the US, residential contractors simply aren't trained to design & install hydronic distribution systems. In many markets, I'm doing good to get them to install a hydronic air handler (ideal in areas with high electric rates, cheap gas and loads way too small for the smallest furnace). And the commercial guys who know hydronics are typically unfamiliar with the idiosyncrasies of low-load design, and most are unwilling to work on under- or low-5-figure residential projects.

    Lastly, hydronics is only competitive if the home has access to cheap natural gas. Even then, the difference in first cost between a gas furnace or hydronics air handler versus radiant with forced air cooling usually can't be recovered thru savings on distribution energy. A blower in a well designed forced air heating system for a low load home is only going to consume 10's of dollars a year.
  • Good discussion Sean and David! I do like keeping the HRV separate from the forced air system, but not every home design is conducive to this; and I don't get to pick my clients' homes--though I can often have some influence.
    Your point about low mass ceilings is a good reminder, and well taken; but allow me to explain what I mean about warm floors; which was the point of the question: Keep in mind we are using exposed concrete. In my own forced air home, I took some temperature readings this winter. The wood floor right next to ceramic tile was the exact same temp, but it felt warmer. The concrete floor was 1/2 degree cooler than the rug on top of it,which was right at setpoint. As expected, the concrete felt cooler (but not cold). Now if the home were heated with radiant floors, the floor temp at design temperature would need to be 2* warmer than setpoint. But we spend most of the season above design temp. And in a passive solar home, we also spend a good deal of time without ever calling for heat. Thus, when calling for heat maybe a couple times a day when it is 25* out (where we spend much of the winter), the floor would not be noticeably warm. It would still be significantly cooler than your body temp, and since it is concrete, it has a higher conductivity than wood, LVT, etc., so it would still feel just as cold. 1*-2* isn't enough to swing the feeling on your bare foot from cool to warm.
    I would be fine with installing dual systems for folks who had an extra $40K laying around, but such a client is rare. So I feel like the bad guy because they can't get radiant in their $400K home, but I can deliver a home that has nearly all of the comfort, noise, and air quality benefits of radiant. And at $0.097 cents per square foot heat bills last year, energy savings are not a compelling argument for radiant.
  • David ButlerDavid Butler Posts: 3,889
    edited March 2017

    ...in a passive solar home, we also spend a good deal of time without ever calling for heat... calling for heat maybe a couple times a day when it is 25* out (where we spend much of the winter)...$0.097 cents per square foot heat bills last year...

    Now that's what I'm talking about. Eat your heart out, you folks with leaky old homes that have barely a sweater for insulation! B)

  • I give a strong second and third to Jake and David's comments.

    Jake - I think you have most of the empirical data you need already...the main one being the upfront cost difference for adding on a hydronic system.

  • Hi Jake, This should become an interesting topic to follow. I read your blog on thermal mass (part 1) and wish you would let us know when part 2 is published. You basically have your answer embedded in part 1.

    I disagree with David on whether it is best to manipulate the thermal mass or not. Not too long ago I assisted on a very low load home with passive components and a discussion about the pros/cons ensued. Here's a short list of the main points of discussion.

    1) The homes you build should minimally have a usable life of 100 years. In that lifespan, what do you believe energy will look like 10, 20 30 years from now. I believe we will be using a LOT more intermittent energy sources - solar -wind - tidal - wave - etc..

    2) Because we are going to use more intermittent sources, the ability to manipulate energy (energy storage) will become more important. In the homes you are building, there is enough mass and the loads are low enough to allow preloading the home and coasting through any "peak" scenarios. With radiant the home is ready to be loaded with any number of technologies.

    3) Overshoot -- David is correct on the distribution temperatures of radiant in a low load home - my guess is your homes would require something like 77F +/- distribution temps and floor temps of 73F +/- . These would be "design load" temps. To minimize any overshoot of temps you would use "outdoor reset" to manipulate the distribution temps according to the outdoor temp, Rarely do weather fronts come through that dramatically drop/raise temps over a short period of time. Even if temps overshoot a couple of times a year, the temps in a low load home will only overshoot by a couple of degrees (look at distribution and design floor surface temps). I don't think you will get too many complaints about being slightly warmer in Michigan on a few days per year.

    4) Warm Toes - You will not have "warm toes", but the floor will be 4-5 degrees warmer than an air system and we feel that difference.

    5) Comfort -- Jake - you say it in "part 1" of your blog on mass. There is nothing more comfortable than having everything around you the same temp. It's very noticeable with people who have granite countertops.

    6) The low temp distribution will be absolutely perfect for any future energy inputs to the house.

    7) Keep in mind you can't store warm air unless you use batteries

    8) Use ECM circulator technology and your electrical cost of distribution becomes very small

    I won't argue whether
  • I won't argue about which system costs less or more to install. I believe consumers respond to simulating discussions about the pros and cons of their thoughts on all aspects of a new home. I can't begin to count the number of upgrades I've done in the last 30 years after a good discussion and I have been very surprised many times at the cost of the upgrade chosen. Why fight with a consumer, have a good discussion and let them choose.

    The ultra low distribution temps of radiant is very complimentary to the most efficient thermal technologies and that will always be true.

    Jake, do you use natural gas or are you all electric ?
  • Sean WiensSean Wiens Posts: 331
    @ David - thanks for comments. Re floor truss depth - have to disagree, even a 6" trunk requires a dramatically deeper truss. Remember other things need to get in there too, like plumbing drains. I have a 12" deep truss, and it was a HUGE struggle to get in the 3" & 4" drains AND my 6" HRV piping. I had to do cross overs in a couple of areas where the ceiling is dropped, or would have had to do bulkheads. It would have been impossible to also get in any sized air ducts in that cavity as well. And in my case, I would have had to give up ceiling height for deeper floor trusses, due to zoning roof height maximums.

    @Jake/David - point taken about sensing floor temps in a radiant slab.

    @Edward "The homes you build should minimally have a usable life of 100 years. " I highly disagree for many urban areas. With fast population increase that is expected in many regions, a single family dwellings life span will probably be well short of even 50 years. In my region (Greater Vancouver), they are expecting to double in population in the next 15 -20 years (another million people). Great swaths of single family neighbourhoods are already transitioning to much denser multi-family and this is just going to escalate going forward. I put a life span of 25 years on the house I am building. Re comments on overshoot - based on feedback provided by individuals like John Siegenthaler, your characterization is not accurate. The overshoots happen throughout the day when sun comes out from behind clouds, and the temps can overshoot by 10*-20* quite easily in a HP home with lots of south glazing. Outdoor resets do not work to combat this issue because of the slow response time of the high mass panel.

    In general, sounds like the main reason many choose air over hydronic is the lack of contractors to install hydronic. This is also evident by the stated install costs by Jake. A traditional hydronic in slab (for clients without a tight envelope) should be in the $10K - $15K range for a high efficient gas boiler and panel installation. Yes it does get more expensive when doing ceiling wall panels, and even more expensive when doing air source to water heat pumps. But then you truly do have a system that can transition to low energy inputs like solar thermal and PV.
  • Sean, In a high performance home the radiant floor is not going to be the cause of a 10-20 degree overshoot since the floor is maybe 5 degrees warmer than the t-stat set point, therefore, when the room reaches 73-74 degrees the floor stops emitting heat and begins to absorb heat. This is at design temps - when the outside temp is higher than design temps the floor will be less than 73-74.

    In Jakes comment he has taken a floor temp reading that is +/- 1 degree below t-stat set point ( probably 70F). If you have a 10-20 overshoot with the radiant floor because of solar input - you are going to have a 10-20 degree overshoot with the non-radiant floor and you should consider drawing the curtains a bit or reconsider how much solar input you really want for that room. A room in a HP home may only take 5000 btus at design load and that's not very much southerly glass. Since the average temps are much higher than the design load temps the actual heat load could be 2000 btus/hr and the overshoot problem is worse.

    If you were talking about a house that has high heat load and you were pushing 140f water into your radiant than yes I think I could agree with you about the radiant slab causing the overshoot.
  • Sean WiensSean Wiens Posts: 331
    Ed - a high mass slab does not stop producing significant heat for at least 12 hours after the call for heat has ended. This has been well documented and presented by individuals like John Siegenthaler. In a poorly performing home, there is less not more risk of overshoot. But in a HP home, unless you are dramatically limiting solar gain (which would reduce efficiency of dwelling and be the opposite goal of what you are trying to achieve), you will have overshoots with a high mass system. In John's seminar, they showed that a HP home could easily shoot up to 80 degrees or higher when a high mass panel is present and the sun comes out on a cloudy winter day.
  • Sean, Think about this closely - if the slab is 74 degrees and everything (including the air) that surround that slab is 74 degrees there is nowhere for heat exchange to occur. At 74f the slab is no longer the driver of heat gain - the solar is. Heat gain to the room is limited to the actual temperature of the floor. And keep in mind that these would be "design load" temps or going from a very cold night to a very bright sunny day.

    In a less efficient house with a high mass slab the input temps to the slab are much higher (for higher load) and could have input temps of 100f or more. Couple a 100F slab with significant solar gain and the slab does have the capacity to be the driver of 80F room temps.

    In the early days of radiant distribution and slab temps were much higher than we see today and thus the tag "warm toes"
  • John SemmelhackJohn Semmelhack Posts: 150
    edited March 2017
    Regarding cost - it's important to remember Jake's situation...he's in CZ 5/6 in Michigan with heating, cooling and dehumidification loads. The cooling and dehu loads are relatively small, but one can assume that most custom home clients want to be comfortable in the summer as well as in the winter. If Jake puts in a combustion-hydronic heating system, then he also has to put in a separate system for cooling and dehu. Or...if he puts in a more expensive air-to-water heat pump hydronic system for sensible heating/cooling, then he still has to provide a means for controlling humidity. Either way, costs go up compared to air based heating/cooling. For those in heating-only regions, the economics of hydronics fare better...and you also tend to have more competition and lower prices as a result (or vice versa, depending on if you're a chicken or egg kind of person).

    For custom houses in my area (central Virginia), installed HVAC prices on low-load houses we consult on are typically in the range of $5-$6 per square foot of floor area. Our designs typically include ducted mini-split heat pumps and an ERV. I'd happily put any of our air-based designs up against the very best residential hydronic systems on comfort, energy use, upfront cost and IAQ for houses with similar peak loads.

    Regarding distribution system efficiency (circulating water vs. air) - there's a false notion that because water's heat capacity is so much greater than air that it must be much more energy efficient to pump water around than air. This may be true in large commercial buildings where duct size limitations significantly increase the fan power needed to move air...but with good residential designs we can achieve about 0.1W/cfm air handler fan efficacy at peak loads...in the range of 40W-50W for a ~2,000ft2 house....btw, we get "free" MERV 13 filtration with that fan as well!

    Regarding using hydronic systems and mass in low-load residential construction for load shifting - I'm highly skeptical of the cost/benefit. Is there really going to be a bunch of utility cost savings if I shift my 1,000W peak electricity load several hours a handful of times per year....or shift my average 300-400W load several hours daily? I'm not buying it.
  • Side note - I don't think there's been a discussion this good on the LinkedIn forum in over 6 months...and this is only with a small set of people at the moment! Thanks, David!...and Jake, Sean, and Ed!
  • Sean WiensSean Wiens Posts: 331

    Side note - I don't think there's been a discussion this good on the LinkedIn forum in over 6 months...and this is only with a small set of people at the moment! Thanks, David!...and Jake, Sean, and Ed!

  • John, Pex is now a commodity and anyone can buy pex w/O2 barrier at less than $0.40/ft. . Tubing for a 2000 sq/ft no basement for well under a $1000 at 1linear ft per sq/ft of house. Every distributor or manufacturer will do a layout for you and it is simple to lay down for high mass. $1000.00 or less of material and two men one day. If you use an air>water machine you can use it for sensible cooling load without risk of condensation by using controlled water temps.

    Electricity How are we as a society going to handle peak loads ? Good question There seem to be two emerging ways to tackle it with one being electrical storage and the other being inducing behavioral changes through "peak" TOU rates. Those being coupled with fast ramp natural gas. Where will we be in 20 years ?

    Sean, I have a hard time imagining $ 400K homes being demolished for multiple unit housing. Maybe after all of the 100K-200K are demolished and converted.
  • Sean WiensSean Wiens Posts: 331
    @ John - point taken re need for de-humidification.
    @ Ed - I have not seen designs with panels at 74 degree supply. More like 90* to 100* on low temp systems.
  • Sean WiensSean Wiens Posts: 331
    Ed - here it is due to land value and yes they are demolishing $400K houses. In my region, pretty much any house starts at over $1M and that would typically be land value with no value to the house. They currently are clearing out large swaths of Granville Street in Vancouver. This is an affluent neighbourhood with multi-million dollar homes. Yes many were older, but the newer ones came down as well in order to assemble the land for the large multi family projects. Re my comment of doubling the population. This would mean every house in Vancouver would have to be torn down to make into a duplex. Obviously that is not going to happen, but what they are doing is tearing down entire neighbourhoods to put in mid and high rise developments. Point was 100 year life span for single family is not realistic in most urban regions now for better or worse.
  • 74F actual slab temp with +/- 77F distribution temp (input) for an ICF home like Jake is describing.

    Jake, what's the heat load of a typical R Value Home at design load ? Sq/ft and load
  • 90F to 100F for staple up and transfer plates but much lower for embedded.
  • David ButlerDavid Butler Posts: 3,889
    Wow, I go to bed for a few hours and my mailbox is full of comment notifications!

    @Ed, you spent a lot of words in your initial comment defending the value of thermal mass. I love thermal mass. Even without utility peak price signals, well designed thermal mass provides comfort as well as economic benefit (educes the load the system sees. Agreed. Agreed.

    What I was trying to covey was not to mix your distribution with the home's thermal mass.

    Re: overshoot discussion...
    To the extent that direct solar gain (through glazing) is responsible for overshoot, I hold window design largely responsible. In a home that's purpose-built as passive solar (or any home with high-gain glass), you have to be very careful with glazing design to avoid overshoot. While overshoot may feel nice in cold weather, the problem is the contrast that creates with rooms that have little or no solar gain. Window coverings can be effective in managing solar gain but that requires constant attention (or an expensive parametric automation system).

    Clearly, a 75F radiant slab can't push the room to the high 70's, but an unheated floor that starts out at, say, 68F at daybreak can help prevent overshoot due to solar gain.
  • I've always struggled a bit with true passive solar designs. I actually have never been in true high performance (low load) home and seen what a good passive design looks like. I have been in many earlier passive homes that have higher loads and always felt they were a bit chilly -especially in the northern end of the homes. I've also thought it would be a good idea to have the passive collection outside the exterior walls and mechanically moved into the home (like a fan). We have a lot of log homes with glass gable ends pointed south and the heat loss in the winter is very large, especially considering we can have so many dark days where that glass is just a heat loss.
  • David ButlerDavid Butler Posts: 3,889
    It's sad that passive solar design has fallen out of favor, probably because so few advocates and skilled designers are left. Passive solar homes of the 80's and early 90's, largely roll-your-own experiments, were a natural outgrowth of the energy crisis and self-sufficiency movement. Comfort is relative, but homes of this era do not compare well with today's code built home.

    By the mid-90's, serious efforts were underway to support PS designers with guidance and modeling tools. NREL and the Passive Solar Industries Council (PSIC) published Passive Solar Design Strategies: Guidelines for Home Building, with regional editions for different latitudes and climates. The 'Builders Guide' passive solar modeling software sprung from that project, and by the late 90's it morphed into Energy-10, a DoE2-based simulation program that was optimized to account for solar gain, thermal mass and day-lighting. E-10 was targeted at light commercial but also supported residential and was embraced by the fledgling passive solar home industry.

    I relied heavily on the "Guidelines" manual and Energy-10 software to design my previous home in Charlotte in 1999. At the time, the best high-gain IG units (surface 3 pyrolytic coating, aka 'northern low-e) had mediocre u-values in mid-to-upper 30's. Because the house was very well insulated and relatively tight, it all worked very well, with a mere 2-ton heat pump for 3100+ ft2, pretty unusual at the time.

    About 10 years ago, the major IG manufacturers introduced spectrally selective high-gain glazing with modern sputter coat technology. First came Cardinal 179, and more recently, Cardinal 180, with a respectable 0.26/0.64 (dual-pane, cog u-value/SHGC), and 0.21/0.62 when combined with their new i89 coating on surface 4 (interior). Tri-pane 180/i89 clocks in at 0.13/0.53 (cog), perfect for super low load homes. Thin-framed composite windows optimize the whole-unit values. These new glazing options are a game changer for passive solar design. Too bad are so few are around who care.

    That said, probably half of my projects over the past 8 years -- from beyond-code to Passive House -- have been passive solar design. In particular, I've done a lot of work with Debra Rucker Coleman, one of the pioneers of passive solar design. Although we didn't know each other back in the late 90's, we both beta tested Energy-10. Sorry to see that project die.

    Now, without good modeling tools, we're left with a vanishing breed of passive solar advocate/designers like Debra who instead draw upon decades of of experience and judgement.

    From where I sit, passive solar and thermal mass is a no-brainer when it comes to high performance home design. The lowest of low-hanging fruit. What other cliches can I invoke?

  • Sean WiensSean Wiens Posts: 331
    David - I have the Cardinal 180/180/i89 triple pane IGU in Cascadia fibreglass frames installed on my build. It will be a year before the house is done, but I am expecting very good performance with them. I am only at the framing stage but can attest that on a day with freezing temps, the windows in the sun are warm to the touch on the inside surface.
  • David ButlerDavid Butler Posts: 3,889
    edited March 2017
    @Sean, your comment "warm to the touch" caught my attention. One of the concerns about the new i89 coating is that it lowers the surface temperature, potentially increasing condensation risk in tight homes with high RH (see table on page 20 in Cardinal's tech guide). As you can see, the MRT of i89 is higher than other IG products even though surface temps are considerably cooler. Magic. But the surface temps in this table are not based on direct sun exposure, since it's important to consider worst case surface temps in any condensation analysis.

    As an aside, a couple of years ago Robert Bean and I collaborated on a residential project in Alberta with lots of glass. We were considering using i89. I recall speaking with Jim Larson, Dir. of Technology at Cardinal, about the assumptions behind his MRT values. He had developed an MRT calculator based on ASHRAE 55, but it turns out there's no standard way to do this, so the results depend on the assumptions he had made. At the time, Robert and I discussed getting involved in the 55 standing committee to propose a uniform method for IG manufacturers to develop MRT comparisons like this. Interesting stuff, but we were both very busy so nothing ever came of this.
  • 2000 ft 1/2 pex with O2 barrier $ 800.00 easy and fast to install in high mass floor --- much more time for staple up/plates
    200 ft supply lines $ 200.00
    Fittings $ ??????
    Prefabricated mechanical panels +/- $ 4,500.00 4 Zones -1 DHW - 1 Aux - 2 radiant w/reset mix valve 40 minutes to install
    Miscellaneous $ 2,000.00 pipe, fittings, supports, parts etc.. T-stats, wire, manifolds
    DHW indirect $ 1,500.00
    Subtotal $ 9,500.00

    Gas boiler $ 3,500.00

    Air to water heat pump with buffer $ 8,500.00

    Labor 2 men 5 days +/- $ 8,000.00

    Heat pump -- $ 24,000 Boiler -- $ 19,000 $ 40,000 is a lot of money , I'll cut you a break and help you for $ 36K
  • Danny GoughDanny Gough Posts: 185
    Re: overshoot discussion...
    Greetings Gents, High mass overshoot can be a problem even without the passive attributes. Passive just complicates the issue. One stratgey is to embed sensors in the sunlit floors and use weather responsive controls to predict the demand. Of course it doesnt take much area to push the control scheme to customized programming. But it does work exceptionally well. It reminds me of predictive cruise control on a car.
  • A well designed radiant system is not the cause of overshoot in a performance home. Well designed means using a program like loopcad and using a relatively accurate "Design Load" to determine supply temperatures and supply flow that fulfills the load requirement.
    10 plus years ago (when I was contracting) I did a system for a slab on grade HP home - 12" walls and R-80 something in the cap. There was a good amount of eastern and southern glass, but the house was never referred to as a passive house. The owners intended to do most of the heating via a small woodstove when they were home -they are gone 12 hrs a day at their jobs. The design requested was to use radiant to keep the home from going true cold while they were away and keeping the house in the mid 60's until the next fire upon return. We used R-40 rigid under the slab since it was pinned to ledge on one side (heat sink) and R-20 on the slab edges. The slab was constructed with 10" of aggregate and 4-5" of concrete and a slab sensor was used to control the slab temp to 68F. This radiant system is set to operate 24/7/365. The supply temps were very low (can't remember) and it has worked perfectly for 10 plus years. I know these people personally and they have never discussed an overheat with me in 10 years. In the warmer months they come home to a 68F house and in the coldest weather they come home to a house around 64F. That slab can never drive the home over 68F.
    Overheat is dependent on the amount and temp of the energy supplied to the slab. In HP homes the supply temps (high mass) are extremely low at design load and the use of reset drives that supply temp even lower when ambient is above design temp. It would require an extreme event - like design temp to design plus 40 in a few hours to cause the house to reach even 75F.
    If you're experiencing these overheats, that you're describing, bring in either a very good radiant company or bring in the manufacturer of your radiant system components and they will help you go through an accurate redesign. Good news is that it's just a matter of adjustment.
    Radiant systems in high mass for higher load homes are a different discussion because you're then talking about supply temps high enough to cause an excessive overheat.
  • David ButlerDavid Butler Posts: 3,889
    edited March 2017

    ...use radiant to keep...house in the mid 60's until the next fire...

    Setting slab well below the desired space temp would definitely prevent overshoot, but the premise of Jake's question was folks wanting warm to-the-touch floors. That's simply not possible in tight, beyond-code homes. My earlier comment about overshoot assumes slab is only heat source. It's easy to avoid overshoot with a hybrid system.

    I have a few homes with radiant floors + heat pump. In those cases, I argued slab heat was unnecessary (I lost). In your project with R40 on the bottom, any space heat system would yield similar slab temps without the expense of radiant floors, even with daily setback. I consider radiant slabs akin to heated windows, (albeit losses are much smaller, especially with R40 sub-slab!)

    Overshoot is obviously a bigger issue when there's a significant cooling load (most of the country). With slab potentially operating 24/7/365, sounds like the house you described didn't need A/C.
  • What I forgot to mention was the home is off grid solar with battery and the house is basically "dead" when they leave for work. That was the impetus for using the extra aggregate - storage. We talked in depth about adding solar thermal but they were very content using a small amount of propane for DHW and the slab heat. The house has proper sun angle shading, the slab is 68F and able to absorb sensible heat, so they do what many people do here, they open the house at night and cool the house down and then close it up during the hotter days. The slab absorbs during the day and releases at night (depending on ambient being below slab tem) out the windows.

    Most people operate their AC at 72F-76F and that is well below slab temp, so the slab has no energy input to the home when the home rises above 68F. We can leave that slab on 365 and it has no impact or input when the home temp rises. That was the beauty of using a high mass slab and low supply temp to satisfy the design criteria. This is the same whether the slab is 68F or 74F.

    The mass of the building is everything inside the thermal insulation - concrete, furniture , book, dishes wood sheetrock virtually everything. The entire content of the mass is ready to absorb heat from air that is warmer than the mass. A radiant slab is a portion of that mass and is no warmer than what it takes to keep the home at setpoint. If you have a temp setpoint of 70F the entire mass (except floor) is at or slightly below 70F . Any overshoot of 10-20 degrees is an issue of solar input and not of the slab being 4 degrees warmer than the rest of the mass.

    Similar to the project I described the ability to actively manipulate the mass of a home opens up many possibilities. Low load homes with high mass are perfect for effectively utilizing cheap off-peak electricity for air>water and ground/water based heat pumps. The ultra low temps required significantly raise the COPs and then couple that to off-peak rates.

  • I love the discussion guys. Thanks to everyone for the input, I learn a lot.
    To me, the discussion is so often theoretical it becomes unhelpful. I appreciate hearing from those with experience living in low-load homes for years.
    To the question of our total heat load at design temps, we are seeing 18-30 kBtu, depending on the size of the home. It's pretty hard to get lower than that; and also pretty impractical. Power was out here from Thursday through Saturday. Didn't need to run the heat pump since it was sunny. How nice!
    I just got off the phone with a client who did radiant, and was willing to pay for 2 systems (the other to dehumidify/cool). He has standard concrete floors in the basement, polished concrete floors on the main level, and wood floors in the second level. He reports that the floor temp is typically "neutral"; not warm, not cold. When there is a cold snap, he can tell that they are warmer, but they still are not warm. The polished concrete feels a bit cooler than bare concrete, probably due to a conductivity of the surface difference.