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Community and Q&A

Hydronic radiant floor heat loads when combined with a forced-air system

airfix | Posted in Mechanicals on

Pretty good home, Climate Zone 6a ~ 3900sq.ft.  This is not technically a green building question but more of a systems integration question.

We had always planned Hydronic Radiant floor heating in our home based on our general liking of the type of heat and warm toes in the winter.  That was supposed to be my only system, no cooling.  However my general contractor has been pushing me to add a forced air heating system to prevent pipes freezing should the boiler go out.  He tells me he typically installs forced air with a radiant floor system for that reason, and to accommodate quick changes in temperature of the house such as unexpected cold spells.

I had been resisting until this summer.  It has been so stinking hot in our area and we are just miserable with the heat even although I’m managing the house temp with open windows and fans at night and closing the windows during the day it’s not enough.  So I’ve decided we are definitely going to put in a forced air system, primarily for cooling but with the quick heating benefits of a furnace system too.

My contractor and subs seem to be treating each system as a separate entity.  The radiant floor guy doesn’t talk to the HVAC guy and visa versa. My gut tells me I should be designing conditioning of my home as a single system with two subsystems.  How do I go about doing that or finding someone who can help me?

My thought process is this:

1) I’ll only do forced air on the upstairs level, but have radiant floor on both levels.  We have large enough stair well and open area to connect both levels that there will be “some” air exchange between levels.

2) I’ll size the radiant floor system to accommodate the full heat load.

3) I’ll plan the forced air heating system to accommodate only a partial heat load.  Let’s say I plan the  radiant heat load to the 99% Dry Bulb of 2.6F I’ll plan the forced air heat load using a 20F or warmer cold condition.

This way I’m not over sizing the furnace for what really is a supplemental role.

Other thought’s I’ve had about whole system design is how would I set up the controls (if it can be done) so that the radiant floor heats the house to say 60F and then the forced air takes over the load to get me up to the 68F?  The forced air would also deal with any quick temperature changes required too.

My thinking is that the thermal mass of radiant floor doesn’t like to change its temperature and when it does, it is very slow.  Having the furnace provide the heat from 60F to 68F means that at night we can drop the temp to 60F to keep it cooler for sleeping.  I understand I need to ensure I don’t short cycle the furnace.  Can this type of system be designed?  Does anybody of any info on how to do it?

p.s. In general I like the new site

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  1. this_page_left_blank | | #1

    One contradiction is immediately obvious. You say you want the in floor to bring it up to 60, and the forced air bring it the rest of the way to 68, and you also said you want the in floor as your primary heat. While it may seem like "topping up" from 60 to 68 is supplemental, in practise the in floor heating will almost never run. Well, it would, because you will have undersized the forced air. So what will happen is the forced air will run balls to the wall trying to maintain 68, won't be able to, and your in floor heat won't come on until you get down to 60. So you'll spend a lot of time below 68 when you don't want to.

    You have to set your primary heating to the higher set point, with the supplemental coming on if it can't keep up. Your slab will cool down at the same rate night whether it was it or the forced air heating during the day.

    I think if there's actually a chance of the pipes freezing due to boiler failure, your house isn't designed properly. Maybe I'm wrong there, but regardless I'd be getting some kind of a heat pump sized based on the cooling you need alone. Whatever heating that also get you will be plenty to keep your house modestly warm for any short term boiler failure.

  2. STEPHEN SHEEHY | | #2

    Even if money is no object (3900 square feet?) two separate heating systems? Why? Most people think in floor heat makes no sense for a tight, well insulated house, since for much of the year, it won't be on enough to stay warm.

  3. airfix | | #3


    That was my thought exactly. If they do this dual system all the time, how does it work? If the forced air and the radiant floor heating are set to different set points I drew the same conclusion as you. If they are both set to the same set point then from a cold condition (cold slab) the forced air would likely heat the home quicker (assuming we are not lower than my cold design point) and so the radiant floor heating would barely heat the floor. I need to understand the logic, which currently escapes me, hence my post.

    I'm not sure what you mean by "getting some kind of a heat pump sized based on the cooling", can you clarify?


    I'm not sure why you are questioning the 3900 sq.ft home? Are you implying it is too big? Or are you implying it's too small for two heating systems? Many contractors in my area install forced air as well as radiant floor heating. The reason I'm looking into it, like I said in my original post, is because my contractor is recommending it and two, I need to do AC which likely (not always) requires ducting so it seems plausible to me that adding a furnace might have some merit.

    What other options do I have for AC apart from ductless mini-splits which by the time I put in 3 or 4 I'll have paid for my forced air AC.


  4. GBA Editor
    Martin Holladay | | #4

    I agree with Trevor and Stephen.

    There are several reasons why GBA usually doesn't recommend radiant floor heating, ranging from (a) these systems don't provide air conditioning, (b) if the house is well insulated, you almost never get the "warm toes" feeling, (c) these systems are often slow to respond, and (d) these systems are expensive to install. For more information, see "All About Radiant Floors."

    It sounds like you are committed to installing a radiant floor heating system, so it's probably too late to back away from that plan.

    You've provided several rationales for installing a forced air system in addition to a radiant floor system. Only one of these rationales makes any sense -- "to provide air conditioning." The other reasons -- "to prevent pipes freezing should the boiler go out" and "The forced air would also deal with any quick temperature changes" -- don't make any sense.

    If your house has a good thermal envelope -- with attention to airtightness and properly installed insulation -- your pipes won't freeze even if the house has no heat for days.

    Installing two heating systems is expensive and unnecessary. If your house has a boiler, you don't need a furnace.

    Clearly, you want air conditioning. Your choices are (1) ductless or ducted minisplits, or (2) a central air conditioning system that distributes cold air with ductwork. Forget the furnace.

    If you choose to install ductless minisplits, you will soon realize that these amazing devices also provide heating (making the radiant floor system unnecessary). If you had realized that fact in time, you could have saved $20,000 or more and left out the boiler and radiant floor heating system. The $20,000 you saved could have been invested in air-sealing measures, high-performance windows, thicker insulation, or a PV array.

  5. this_page_left_blank | | #5

    I made the same mistake (put way too much faith in the builder before doing a lot of reading here), but thankfully the dollar value was nowhere near that amount for me. The cost was bundled with the install of the slab insulation and pre-conditioning loop, but my best guess is at most $6000 (about the same as air source heat pump, coincidentally). If I had it to do over again, I definitely would have gone with a air source heat pump, if only for the A/C. I suspect that one is in my future some time.

  6. Jon_R | | #6

    If you want warm feet, put the radiant heat only in select areas - perhaps a bathroom, under an office desk, etc. High mass radiators are not very compatible with setback.

  7. Expert Member
    Akos | | #7


    The way to handle the dual heating systems is to set the floor heat to maintain a certain slab temperature and the air temperature slightly above what you want the room at (that is if the slab is hot, the radiant would shut off). I find that 24C slab feels normal, about 26C feels warm. You probably only want to do this in the areas where you normally walk. You can then set the central air to maintain the air temperature as you normally would.

    What should happen is in the shoulder season most of your heat will come from the radiant. The furnace should only kick in on the cold days. This should work with a "standard build", super insulated house might have issues with overheating. I have something similar in a 110 year old balloon framed house that is mostly insulated/air sealed, the extra heat in that case comes from a couple of mini splits (which are not needed unless the thermometer really dips).

    Not sure I get the logic of backup heat. Well commissioned SIMPLE hydroponic systems are reliable, I would rather trust that over most heating sources (maybe not a wood stove, can never beat that for reliability).



  8. airfix | | #8


    Thanks for the clarity you bring to the discussion. It's not too late to change our mechanical systems we are still in the planning stage and needed some redesign due to county code issues and are about to start work on new construction drawings. I myself am not 100% committed to the radiant floor method. It has been driven by my wife's style choices in selecting a finish concrete floor throughout (even on the 1st floor), which led to the radiant choice.

    I did some chatting with a local heating contractor about these dual systems that go in all over our area. He says they are very common. It comes from people wanting the radiant floor warm floor feeling but needing AC. With most homes only being insulated to code values mini-splits are not very common here. So ducted AC is the system of choice. Then he talked about the cost comparison of buying a fan system to distribute the AC and comparing that to the cost of a furnace and using the integral furnace fan to distribute the AC. He said to buy a furnace is only a few hundred dollars more than having a dedicated AC blower system. As the furnace is a secondary system it doesn't get used much so it doesn't need to be efficient, you are buying it for its fan system. He posed the question why not spend an extra $200-$500 for the redundancy it provides?

    With respect to the control systems and set points his company has one set point for the zone. He sets it up so the radiant floor as the primary system. If the zone set point is 68F then behind the scenes (from the home owner) the radiant floor will be set to 68F and the forced air will be set to 66F. The forced air will only come on for a short time if there is an immediate need such as an outside door being opened long enough. The reality is the forced air will be off 99.5% of the time, it is really just a surrogate for the blower to blow AC in the summer and a backup should it be needed.

    The mini-split idea keeps coming up but I think I'd need 4 units based on the planned zone usage. Now I have to think about where those units will be mounted and how they will affect the outside aesthetics. I assume each one has their own air source heat pump that mounts directly outside of where the inside distribution head is? If I have one in the bedroom, will it heat the bathroom enough on those cold mornings without direct distribution to the bathroom? How well do they work at on the 2.6F day and will I need supplemental heating for those super cold spells?

    My house will be a 2 floor building. The lower floor will be a mostly below grade basement that includes a separate rental apartment with a polished concrete floor. I don't think I'll need AC in the basement so AC is only for upstairs. So maybe what I do is have forced air heating and cooling upstairs only, then radiant floor heating only down stairs for the concrete floor warming. That way I'm not having 2 systems compete in the same zone. The decision to go with AC really threw a spanner in the works.

    It seems the downside of going "pretty good home" compared to fully net zero is that you're spending money on insulation but you still have a significant outlay for mechanical systems where a net zero house can really cut back on the mechanical costs.

    PS a good website improvement would be to have a message preview option so I can proof read more easily before posting.

  9. walta100 | | #9

    If you decide to do want the heated floors a way to get the system to play together may be to set the floor heat thermostat higher than the forced air but add an outdoor sensor that will disable the floor heat until the outdoor temp falls below 15°and disables it once it gets over 35°. In that way only one system will be operating at a time.

    The toasty warm floors you remember were only possible because the attic had R10 insulation, the walls had 0 insulation, the windows were single pain glass with aluminum had frames and very drafty. The floors had to be 105° to get the room up to 70°. With today insulation if you get the floor up 77 you will sweating you tail off and your feet will still be cold. If you want your warm feet feeling you will need to insulate to 1955 levels and install that big honking boiler and brace yourself when the fuel comes. We were children and remember the systems fondly because we did not pay the bills at the time.


  10. Expert Member
    Dana Dorsett | | #10

    Using a 2-stage thermostat letting the hot air system be the second "stage" will work, but this is all extreme overkill for a pretty-good house.

    Separated wall-coil type mini-splits will usually be ridiculously (and sub-optimally) oversized for the heating load of a bedroom (even a big bedroom) in a pretty good house. Instead of 4 ridiculously oversized mini-splits, one or two mini-ducted mini-splits sized correctly for the zone loads is more likely to deliver the comfort & efficiency you're looking for. (How many zones was the hot air furnace going to be?)

    What are your design temps, and your (zone by zone, if you have them) Manual-J load numbers?

  11. Yupster | | #11

    I'm working on an Hvac design for exactly the system you are describing and I've done a number of them before. (Albeit not the control you were suggesting in your original post) The comments about the radiant floor not being what you expect are completely valid. 4"+ of subslab insulation will keep your floor warm enough to not be uncomfortable if that's your concern. The radiant floor will just not run warm to your feet that often. knew there would be an unless...your Manual J calculation shows rooms that have high heat loss with a small floor area. Or you can decrease the area of a large room to be heated only where you expect to be walking/standing. This will cause the btu per square foot to need to increase, which you do by raising the floor temperature. Of course, your hvac designer should be able to tell you all this. Make sure they are doing a design. Often with radiant it ends up being "Throw some tubing in and run some water through." That won't get you a satisfactory anything. Your contractor should be able to show you calculations showing the required flow and supply water temperature through each loop, as well as a loop layout. If he can't, he is just making it up as he goes.

    If you want the backup heat of the forced air, install the smallest furnace that can still provide the right amount of airflow for your A/C, approximately 400 cfm per ton. Just remember that it comes with the extra cost of additional gas piping, possibly bigger piping. That adds up quick.

    There are plenty of controls that will run the radiant with forced air as backup. The Tekmar 563 thermostat is one.

    One final bit about minisplits, if you want to look into minisplits show an accurate Manual J and a floor plan to an hvac designer who understands minisplits. They can tell you how many you would need, approximately where, what the expected temperature differential would be. Ducted minisplits are always an option for segmented rooms. And the outdoor units can go pretty well anywhere, they don't have to be directly outside the indoor units.

  12. walta100 | | #12

    I made my post before I understood the main level the homes finished floor would be slab on grade concrete if that is the case then I might consider heating the slab but floor heating thermostats sensors would be inbedded in the slab so the floor is not trying to heat the room just the slab.

    I would insist that the floor heat and the AC systems be installed by the same contractor so they point fingers at each other.

    Do not allow any equipment or duct work to be installed in the attic as it horribly inefficient a stupid idea.


  13. airfix | | #13


    I've not got my manual J numbers yet. Based on a thread here last month and the recommendations therein I've chosen to have an independent HVAC consultant run my manual J numbers. He is 10 days out in completing that process. I'll find out if he has experience with ducted mini-splits.

    The hot air furnace was going to be 3 zones on the main level with an additional 2 zones in the basement that were planned radiant zones.

    Yupster, does the $200 to $500 sound about right for cost difference between a blower dedicated to AC compared to one integral to a furnace. I understand that gas line needs to be run but it's likely to be a short distance of gas line branching off from elsewhere in the mechanical room.

    The mini-splits are tempting then I can put electrical resistance heat pads beneath the tile for the warm bathroom floor effect.

    It's hard for a non-industry professional like me to get a handle on the relative costs of different systems. There are so many variables in the house design, the system design and the installation that I don't have all the answers yet therefore I can't get accurate pricing to make good decisions. I haven't found one "go to guy" that I can use to help me design my mechanical systems and choose the best options for my situation. It's quite frustrating coming from the aerospace industry.


  14. DunSpannerin | | #14

    Slab sense thermostats at each zone are essential to get any hydronic floor system running properly. Never use wall sense thermostats they just never get the message.

    If you want the Rolls Royce of heating, and you have the money to pay for it, just do it. It's way better than the "cold feet, hot head" feeling that you'll get with mini-splits. Along with no dust, place furniture where you want etc.

    Think about it. If the contractor has done his job properly, you should wind up with a very very large radiant surface, capable of providing the total heating demand to the house, and passing most of it's heat via radiation to all objects in the room.

    If the floor thermostat is set to 20°C(68°F), the floor is at 20°C(68°F) and the radiant receivers in the room are at 20°C(68°F), guess what, there's nothing happening here at this point in time - equilibrium(rarely attained, I might add).

    One is able to cool using hydronic flooring also, but floor temps and dew points must be carefully managed, and it depends on the cooling capacity required. You'd need an air to water or water to water (geothermal) heat pump for this, with the heat pump able to be switched between heating and cooling.

    You mention adding forced air heating to
    1) prevent pipes freezing, and to
    2) accommodate quick changes in temperature such unexpected cold spells

    Covering off point 1). If the house is properly designed you should never suffer freezing pipes. Anyway, although I don't recommend it, there's always antifreeze.

    Point 2) hydronic systems don't do sudden heating demand. These systems are brought up to temp at the start of the season, and there they operate until turned off at seasons end.

    If you're intending on putting in a forced air system, then HRV\ERV ventilation should be your first consideration.

    Open stairwells and voids are bad news for any heating system, so ensure that losses to outside walls are seriously reduced via the very best in insulation.

    The next hydronic floor I do will also have a floor thermostat (or two) embedded at a point where the highest solar gain to the floor is achieved (ours is obviously a nett heating climate), then circulating water from this zone to selected other zones, without operating the heat pump.

  15. Expert Member
    Dana Dorsett | | #15

    Designing the mechanical systems and deciding the zoning BEFORE the Manual-J is done is a serious waste of effort!

    It's fine to have a few ideas about possible approaches, but this is seriously out of control, with multiple zones and backup systems, dual stages, slab thermostats & hydronic ground source heat pumps bla bla bla

    Get the Manual-J done first, preferably by a competent third party who is NOT in the business of installing & maintaining HVAC equipment. The optimal solutions are probably going to be much simpler than most of where this discussion has been going.

  16. GBA Editor
    Martin Holladay | | #16

    I agree. This seems like a classic example of overinvesting in hydronic equipment and forced-air heating and cooling equipment.

    It wouldn't surprise me if Steve Mackay is looking at heating and cooling systems that will cost $30,000 or $40,000 to install. Imagine if this house had, instead, $15,000 or $18,000 worth of minisplits -- imagine the envelope improvements or the PV system that you could buy with the savings.

  17. Yupster | | #17

    Yes Steve, $200 to $500 sounds about right. Martin is correct, a system I designed that was similar to yours but smaller was installed a few months ago. I believe the quote exceeded $35,000 for the zoned hvac ductwork and install, furnace and a/c unit, hrv ductwork and install, boiler, radiant controls, and DHW install. Radiant piping install was separate. It is a great system though, customer loves it and the contractor did an exceptional job installing.

  18. user-7131510 | | #18

    I am neither a contractor nor HVAC expert, but I started following this thread because of Steve's initial concern: "prevent pipes freezing should the boiler go out". Our home was built in 1880, is roughly 1400 sq. ft, two floors / colonial style, aluminum (yes...) siding, 15 miles west of Boston, MA. The basement is fieldstone. Horsehair plaster still sits behind our walls. We have lived here now for 10 years, and until this past winter and summer, all has been fine. Typically, we only had to use our two window AC's (one on each floor), for a max of two weeks. We have the magnificent shade trees to thank for that. This year, with the high heat and humidity, they have been on almost everyday and most evenings. Most winters have been good to us too. But this past winter we were hit with the major cold spells that would linger for weeks. Wind chill temps reached down to the -20s. We have forced hot air (oil) and a fabulous soapstone stove for heat. As soon as that nasty frigid spell hit us we cranked on the woodstove; kept us toasty for days. A few days in and what happens? Our pipes freeze! High 70's in the house, we're home, and our upstairs bathroom pipes freeze! We couldn't believe it / couldn't figure out what happened? We decided to get a small ceramic heater (with fan) and positioned it facing the upstairs bathroom plumbing. About 36-40 hours later, the water was working again. We eventually concluded this happened because the main furnace was off while the woodstove was on and the basement must've gotten too cold for the pipes. So we needed to start using the furnace again, which ultimately, along with the woodstove, made it too warm for my liking. It might help to add that the upstairs bathroom having the problem is also on the north side of the house. I am not a fan of forced hot air heat and have been trying to figure out how to prepare for this winter... some options considered included a heated floor, mini splits, electric heaters, etc. Personally, I prefer the quiet of electric heat and might just use portable units for this winter. I also don't care for the drafts associated with forced air heating. But that still leaves the basement and pipes problem unresolved? Then I thought - what about a woodstove in the basement? or pellet stove? or electric heater? or a mini split that also leads to the basement? or how about setting up a wire system to keep those bathroom pipes heated? Too many things to think about....but I'm doing the research, which is why I am here ;-)

  19. Expert Member
    Dana Dorsett | | #19

    Martin: I doubt it would take even $15000 worth of mini-split to heat & cool 3900' of "...Pretty good home..." in zone 6A if the equipment is specified by a qualified third party and put up to competitive bid. A ~3000' SUB code-min house in zone 5A I got involved with made it on a bit less than $15K of ductless (in a high contractor priced town even). That equipment would have been oversized for a 3900' PGH in zone 6A. It was even a retrofit installation, which is usually more expensive than if designed-in from the get go. A ~2000' deep energy retrofit from a handful of years ago made it on $13K of ductless, equipment that proved to be 2x oversized for the actual loads. (The project owner balked, nervous that a single 3/4 ton mini-split per floor wouldn't be able to keep up.)

    In pretty good house the notion that you can have...

    " ...the furnace provide the heat from 60F to 68F" so that " night we can drop the temp to 60F to keep it cooler for sleeping" ...

    ...seems kind of quaint. In a PGH turning ALL the heating in a room off overnight (not just the furnace, leaving the radiant floor on) would only result in a significant temperature drop on the coldest handful of nights per year, unless the windows were opened. A drop of fully 8F overnight would require opening a window even when it's in negative double digits outside. In a PGH leave the bedrooms at the preferred sleeping temperature, which in a high-R house is still pretty comfortable even when up & about from a mean radiant temperature perspective.

    This kind of stuff can become apparent if looking for it when reviewing a competently done Manual-J, if one bothers to BELIEVE THE NUMBERS, both the Manual-J numbers and the equipment capacity numbers at the relevant outdoor temperatures.

  20. GBA Editor
    Martin Holladay | | #20

    User-7131510 (the person who wrote comment #18),
    First of all, can you tell us your name? (I'm Martin.)

    The problem you experienced last winter (frozen pipes in your basement) has nothing to do with your heating system, and everything to do with air leakage through your home's thermal envelope.

    If you are keeping your indoors at 70 degrees F, and your pipes freeze, that means you have air leakage.

    The solution is to track down the air leaks and seal them. This type of work -- called blower-door-direct air sealing -- is performed by contractors called home performance contractors or weatherization contractors. If you can find someone in your area who fits that description, you won't regret spending the dollars you invest in air sealing work.

    1. user-7131510 | | #21

      #18 = Lydia
      The 70+ degrees was registered using the woodstove only, with the furnace off. This is the first and ONLY time we had the pipes freeze in 10 years. I doubt it's merely coincidental that it was at the same time we had the furnace off for days during the coldest days of winter, while using only a woodstove for heat. And the fact that everything went back to normal (pipes unfroze with direct heat application, heat / furnace came back on - reaching temps in house we've experienced over the years), led me to believe the pipes temporarily freezing was a result of having the furnace off, resulting with not enough heat reaching the basement pipes, while we were comfortably residing in our woodstove heated levels above. I am sharing this story mostly because Steve said "my general contractor has been pushing me to add a forced air heating system to prevent pipes freezing should the boiler go out." - does seem to carry merit, and our experience seems to support that hypothesis, as difficult as it is to believe or accept.

  21. GBA Editor
    Martin Holladay | | #22

    You're right, of course, that when your basement furnace is operating, the furnace keeps your basement warmer than when the furnace isn't operating. But this is a crude and energy-intensive way of keeping your pipes from freezing.

    The nice thing about my suggested approach (sealing the air leaks) is that my way keeps your pipes from freezing with less expenditure of fuel.

  22. Expert Member
    Dana Dorsett | | #23

    +1 on the air sealing recommendation.

    I have a friend in Worcester living in a brick clad 2x4 house with no wall insulation and a fieldstone that had a history of pipes freezing during Polar Vortex type coolth, ALL of which was attributable to air leaks in unfortunate locations. An air leak in the foundation seal at the water feed to the kitchen sink was the worst. Without gutting it and doing the whole wall, a bit of can-foam in the gap that could be reached, and hand-packed cellulose between the leak area and the plumbing has it to where it'll sail through negative double-digit temps without a problem, even though the basement temps still drop into the dangerously low 40F range during those cold snaps. She could do a whole lot better energy-use and comfort-wise with more work on the air leaks though.

    The better the air seal at the attic floor/upper floor ceiling (the MOST critical air leaks), and the basement (second most critical), the lower the stack effect pressure will be, which lowers the amount of chilly infiltration air that gets drawn into the basement and elsewhere. A 70F conditioned space is a significant stack-effect driver when it's 0F outside, but blocking the bubble of super-warmed air from leaving out of the top of the house keeps the house from being depressurized and drawing air in via leaks at lower levels.

    1. Jon_R | | #27

      Air sealing won't reduce stack effect pressure. But it will help with stack effect *flow*.

  23. user-7131510 | | #24

    Thanks Martin. Dana - your Worcester example is one I would probably mimic, especially since we only had a problem those 2 days. We just repointed our basement, the attic is appropriately insulated. The warmth from a woodstove cannot be beat - especially compared with forced air. Someone had suggested redoing the insulation where the water pump rises to the 2nd floor. Another person suggested wrapping the pipe (which is copper) with some heating wire that could be switched on automatically with a temp drop. No high cost or major reconstruction with that. But I know that isn't a solution. Frankly, I wouldn't mind trying timer controlled electric heaters in each room. I don't know how that would compare with our current oil heating bills, but at least I would feel more comfortable with it. I hate forced air heating.

    1. user-7131510 | | #25

      PS - our floor above the basement is not insulated. Would insulating that floor with mineral wool help keep the basement warmer?

      1. this_page_left_blank | | #26

        Insulating the floor above the basement in your scenario would make the basement colder.

  24. Expert Member
    Dana Dorsett | | #28

    What Trevor said- putting insulation between the warm first floor and cold, probably leaky basement only makes the basement colder.

    Insulating & air sealing the band joists with 2" of closed cell foam is something that MassSave ( ) still subsidizes, but in most cases they won't subsidize insulating the basement walls.

    Using can-foam to seal around any electric or plumbing penetrations of the foundation helps. If you have a clothes dryer down there, find a better-class backflow preventer cap (MassSave will probably subsidize that too.) Any flue chases that run from the basement through to the attic are potenially MAJOR stack effect drivers, and can be air sealed (both up top in the attic and at the bottom), using fireproof methods.

    If/when you take the MassSave initial audit emphasize your concern about air tightness, and DO point out the frozen pipes problem. If the auditor deems your place leaky enough they will subsidize a more thorough round of blower-door directed air sealing.

  25. airfix | | #29

    Dana and Martin,

    I appreciate your input and comments. They are exactly why I come here. I'm navigating building a house, something I've had no prior experience at, as a result I'm relying on my general, local subs and experience from friends in the area. Unfortunately even in a supposed progressive area such as this three are very few generals and subs that have accurate information when building an energy efficient home.

    I'm taking it one step at a time. One problem at a time.

    There is a local parade of homes this weekend, maybe I'll make some good contacts during that tour.


  26. Jrod | | #30

    How did you make out with this? My first thought was I would use wall mounted thermostats for the main space and I would do slab thermostats for the concrete. I’m planning a build right now and I’m on the fence right now if I will install ducting in the basement along side the in floor heat. I’m definitely doing forced air upstairs using a high velocity air system.

    1. DC_Contrarian_ | | #31

      I don't understand at all the appeal of floor-mounted thermostats. You want to regulate the temperature of the room, not the floor.

      1. Jrod | | #34

        If you have 2 heating system one heating your floor and one heating your space. The appeal would be heating the concrete slab to a nice temp that doesn’t cause your feet to sweat. Which then your room temp can be maintained by the forced air heating.

        1. DC_Contrarian_ | | #36

          How do you keep that from overheating the space when the heating load is low? You can't heat the floor separately from heating the space.

          It seems what you want is to set the water temperature to the highest temperature your feet can stand. Then use a two-stage thermostat, stage one is the floor and stage two is the forced air. When the floor alone can handle the load of the room the forced air is off and the floor modulates on and off to hold temperature. When the floor alone can't keep up it is on 100%, and the air modulates to hold temperature.

          That's literally what a two-stage thermostat is meant for -- having a preferred and a secondary source of heat.

          1. Jrod | | #37

            You install the temp probe in the concrete and set the temperature to a comfortable temp say 19-20 degrees Celsius and then your thermostat on your wall is what you adjust to maintain the space. Say during the day you get home and you have a chill rather than increasing your floor temp to heat the room you use your forced air. To bring the temp up and leave the slab at a comfortable temperature. I found the home I grew up in as a kid the temp would often overshoot if you wanted a bigger change.

          2. DC_Contrarian_ | | #38

            At 20C -- 68F -- the floor isn't contributing any heat to the room. You could get the same effect just by insulating the floor, much more simply.

  27. user-723121 | | #32

    When I attended the Better Buildings, Better Business conference in Green Bay in 2003 or 2004, high velocity heating and cooling was the next big thing. The idea was the smaller supply lines offered more options for placement. Have not heard much about high velocity since. To me high velocity means noise in operation. Plus, as I remember, the supply lines I saw in the demonstration home we looked at were of a flex type, lots of resistance.

    1. Jrod | | #33

      I visited a few show homes in my local town that are running them. The variable speed fan is awesome. With a small temp change the fan blows at a reduced speed to heat your space but if the temp continues to fall it speeds up more. It also varies its speed based on duct pressure. It’s a nice system that’s very easy to install. They also have the option for multiple zones which makes balancing temps in multi level homes way easier.

      1. user-723121 | | #35

        Sounds like the system is much improved, ECM fans were not in the discussion in 2003/2004 as I recall. With the low volume of air you describe, the efficiency of the building envelope will need to be high.

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