Hello everyone .
 We are  in the process of constructing our  retirement home - Radiant floor heat on both  floors - 820 sq. ft. wood framed main floor , with equal size basement using icf blocks -  .  I would like to use gypsumcrete  on the main floor . My issue is Im beginning to doubt  my HVAC guys interest in this project . NO matter how many times I express my lack of interest in any other types of heating ,  he keeps pushing baseboard heat as the way to go  . His latest claim is that Radiant heat will dry out and destroy the sub floor and he is asking if Id be willing to sign a waiver  letting him off the hook " when " it happens .Has anyone ever heard this happening , or are my instincts on this guy correct ?

I think your instincts are indeed correct...

I would definitely NOT recommend an under-floor HR emitter (e.g., staple up with plates) as expense will be high and performance will be low. Instead put PEX in your basement slab (along with under-slab insulation) and do an above-floor (e.g., Gypcrete or Warmboard, etc). Perhaps get yourself a copy of John Siegenthaler’s “Modern Hydronic Heating” and get fully educated on this subject. We have free DIY design calculators on our website should you be so inclined too. The first step is a proper room-by-room heat loss analysis (ACCA Manual J8 or ASHRAE).

Do you need AC too? If so, mini splits might be something to consider in lieu of HR heating or in addition.

Posted By sailawayrb on 27 Mar 2019 02:30 AM


Do you need AC too? If so, mini splits might be something to consider in lieu of HR heating or in addition.

That's right- 1-ton mini-split with a cold-climate compressor technology (ducted or ductless) can heat/cool most code-min 820' houses in the US, and cover the cooling load too.

There are no fossil-fired boilers appropriately sized for the likely heat load, but adding that load to a condensing water heater can work. In very low load houses unless one limits the amount of heated floor areas to where it matters the flor temp won't necessarily be warm enough to notice except during the coldest temperature extremes.

Where is this house located? The outside design temperatures are ...??

Thanks for the reply's you two ... Im in zone 3b ( Iowa/ Minnesota border . )Right smack dap in the middle of a corn field . I went to Sailaways page to to use the calculator , but it seems to wanna out smart me. Jan. is our coldest month with an average temp of 6 degrees ... Can you dumb this down for me please "There are no fossil-fired boilers appropriately sized for the likely heat load, but adding that load to a condensing water heater can work."

Posted By Muddy on 28 Mar 2019 01:05 AM
Thanks for the reply's you two ... Im in zone 3b ( Iowa/ Minnesota border . )Right smack dap in the middle of a corn field . I went to Sailaways page to to use the calculator , but it seems to wanna out smart me. Jan. is our coldest month with an average temp of 6 degrees ... Can you dumb this down for me please "There are no fossil-fired boilers appropriately sized for the likely heat load, but adding that load to a condensing water heater can work."

Basically what he's saying is that even the smallest oil and gas-fired boilers are far larger than your needs call for.

Condensing water heaters (essentially a water heater with two exchange loops, the primary and a secondary that reuses hot exhaust gas for a bit of extra efficiency).

Please read the calculator instructions very carefully before using the calculators. The instructions are very detailed and many folks have successfully used them. Start with this calculator:

Borst Heat Loss Analysis Calculator

Then you can use this calculator to sort out the cost of heating with various fuels and heat source options:

Borst Integrated Heating System Performance Calculator

And then you can design a HR heated floor with this one:

Borst Hydronic Radiant Floor Heating Design Calculator

These calculators are really engineering software tools and they do require some thought before using them. And I would say that if you hire someone to do this for you, they should be an engineer or at least have that aptitude.

Posted By Muddy on 28 Mar 2019 01:05 AM
Thanks for the reply's you two ... Im in zone 3b ( Iowa/ Minnesota border . )Right smack dap in the middle of a corn field . I went to Sailaways page to to use the calculator , but it seems to wanna out smart me. Jan. is our coldest month with an average temp of 6 degrees ... Can you dumb this down for me please "There are no fossil-fired boilers appropriately sized for the likely heat load, but adding that load to a condensing water heater can work."

You are in DOE climate zone 6A, which is the climate zoning used in building codes.

ASHRAE recommends heating systems 1.4x the heat rate needed to cover the heat loss of the structure at the 99th percentile temperature bin for the location. (The 99th percentile bin means 1% of hours over a 25 year period will be colder than that.) The 99% outside design temperature for Mason City IA is -10F, so that's probably within a degree of your design temperature.

Even code-minimum 820' house with a typical window/floor area ratio would have a heat load of less than 12,000 BTU/hr @ -10F, and house with better than code (but still cost-rational) insulation & windows is likely to come in under 10,000 BTU/hr. Per the ASHRAE recommendation the "ideal" furnace or boiler would be 1.4x that, or 14,000 BTU/hr. A Dettson Chinook might be a good fit, or it might not- run a real heat load calculation, don't size it based on a WAG from some anonymous internet poster like me, but that's probably where it will come out. Most US equipment runs more than 2x that sized.

When the burner is oversized for the load it's as-used efficiency won't necessarily hit near its rated AFUE. But with a buffering thermal mass (like a tank full of water) it can, since the minimum burn times can be long enough (it's more complicated than that, but I'm keeping it simple.) Water heater based heating systems can be a better bang/buck than a small (but still oversized) modulating condensing boiler + buffer tank. The peak BTU/hr of a 2gpm shower is likely to be more than 5x your peak space heating load, and just adding the heating load isn't going to impact your hot water performance very much if you have burner big enough. A common burner size for condensing tan water heaters is 76,000 BTU/hr, which is plenty. With a heating system sized to be pulling heat out of the tank at 15,000 BTU/hr there is still more burner output than the typical atmospheric drafted standalone 50 gallon propane or gas water heater.

Your likely heat load is also well within the range of several cold climate ductless mini-split air source heat pumps. The 3/4 ton Fujitsu 9RLS3H can deliver 11,000 BTU/hr @ -15F and it's biggest brother in the series, the 1.25 ton 15RLS3H can deliver 16,000 BTU/hr @ -15F. Similarly, a Mitsubishi -FH12NA can deliver 14,600 BTU/hr @ -13F, and the bigger versions a bit more. So if your load really is on the order of 9-11,000 BTU/hr @ -10F there is probably a modulating ductless heat pump solution better suited to your actual loads than a condensing propane boiler or furnace, and unlike a propane water heater, it can also air condition at high efficiency.

You can probably heat an 820' ICF basement with an insulated slab with a half-ton Mitsubishi FH06NA (~6400 BTU/hr @ -13F) or at most an FH09NA (~7600 BTU/hr @ -13F).

http://meus1.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NA_ProductDataSheet.pdf

https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

Since those two models can modulate down to 1600 BTU/hr @ +47F they will modulate efficiently over a much bigger fraction of the heating season than the Fujitsus (which all modulate down to 3100 BTU/hr @ +47F).

Mitsubishis will turn off then the outdoor air sensor is out of range, which could be as "warm" as -18F (though most are still running a -25F according to field reports), but they automatically re-start when it warms back up to about -13F or so. In your area you'd probably be better off using Fujitsu equipment upstairs- they keep going no matter how cold it gets, even though the lowest temp at which the manufacturer specifies the output is -15F.

Getting the house off of fossil-fuel dependency now will be cheaper that doing it later. Iowa's grid is fairly low carbon and cheap compared to most of the US, and the trends are that it will be cheaper in the future, as the levelized cost of renewables continue to dive, rendering fossil fueled generators uneconomic. About 3 out of 4 existing coal plants in the US could be shuttered and replaced by local renewables this year at a net cost savings, and by 2030 that will likely be true for combined cycle natural gas too. Leveraging the lower than US average electricity cost with a cold climate heat pump is still cost-rational in your area compared to cheap resistance heat + central air.

Note: the more efficient you build the house the less heat you are going to need to keep it warm, thus the less work the heated floors are going to need to do to keep the house warm, thus the floors wont actually need to be that warm to maintain house temperature.

To combat this in my newest house I only heated the areas where people could walk. Thus, nothing under cabinets, nothing in regular closets, and so on. The heavy traffic areas I spaced the tubes at 8 inches instead of the normal 12 to make them do more of the heating so we could feel the heated floor effect.

So, they will be more along the lines of not cold not warm. Just a FYI on what to expect.

Posted By newbostonconst on 30 Mar 2019 10:27 AM
Note: the more efficient you build the house the less heat you are going to need to keep it warm, thus the less work the heated floors are going to need to do to keep the house warm, thus the floors wont actually need to be that warm to maintain house temperature.

To combat this in my newest house I only heated the areas where people could walk. Thus, nothing under cabinets, nothing in regular closets, and so on. The heavy traffic areas I spaced the tubes at 8 inches instead of the normal 12 to make them do more of the heating so we could feel the heated floor effect.

So, they will be more along the lines of not cold not warm. Just a FYI on what to expect.

Dumb question on this.

Doesn't doing "just the active areas" make it harder to maintain the temp overall?  As you've got "cold spots" or at least "less warm spots".


Also, if this is an in-slab floor, wouldn't that sort of differential heating stress the slab and (potentially) lead to possible cracking and slab failure?
Or is the slab mean temperature enough to avoid that?

You should never put PEX under bathroom/kitchen cabinets, under closets, under kitchen pantry rooms and under toilets (doing so could melt the wax seals). You reduce the spacing in areas that need a higher heat gain Btu/h/sf (e.g., areas with lots of windows or larger percentage of exterior walls). This is why it is so important to do a proper room-by-room heat loss analysis before designing it. And a “room” isn’t just an area surrounded by walls with a door. E.g., it could be kitchen nook or dining room area with lots of windows in an open concept floor plan.

Ideally, you design it so one supply temperature and one thermostat could keep every area at the desired indoor temperature. Doing this heat gain/loss balancing significantly reduces the heat source ON time and the cycling rate which significantly increases the heat source operational life. Of course, you actually zone it with more than one thermostat because people like some areas warmer or colder than the average building temperature (e.g., warmer bathrooms and cooler bedrooms).

There really isn't any significant temperature differential gradient in the slab that can cause cracking as the heat readily moves through the thickness of the slab and evens out the temperature no matter if there is PEX in it or not. However, you can get into a situation where you can’t supply enough heat gain to an area to offset the heat loss in that area because because you didn't properly design it (e.g., determine proper PEX spacing, flow rate and supply temperature). If you need the floor surface temperature to be warmer than the maximum recommended 85F to have a comfortable area, you know that you have a bad HR floor heating design and/or an energy inefficient building (e.g., not well-insulated and well-sealed).

In an energy efficient building, the floors are never going to feel warm, but they will never feel cold either. The floor surface temperature will be about 70-72F which I personally find to be the maximum comfort level as feeling any cool or warm spots across the floor doesn't feel natural or good to me. If I feel the need for a very warm spot, spending some time on the heated bench of our once-a-day wood-fired masonry heater is as good as it gets. This low floor surface temperature also means your supply temperature requirements are less than 90F and you are not wasting energy by having to heat the water to a higher temperature. You can even use a solar collector and heat storage tank as your primary heat source. So it is all good if having a comfortable, draft free, energy efficient, low maintenance and quiet heating system is your goal.

WOW !!!! Thanks for the reply's ... If I went with the Mitsubishis type heater would I put one in every room .Example the Blue prints don't show it , but the basement will have two bed rooms , a bathroom and a kitchenette/ living room .Would each room require its own heat source ?

test test test

Well, each room will need to receive heat from some source, either directly from a source located in the room or from air circulation from another heat source located elsewhere. A room-by-room heat loss analysis (e.g. ACCA Manual J8 or ASHRAE) would help sort this out. Isolated rooms with many windows typically require a heat source located in the room. Rooms that are less isolated and have little or no windows may receive adequate heating from air circulation. One solution might be to do HR floor heating the basement and mini split in remainder of house with perhaps HR floor heating in some additional rooms (e.g., bathrooms). You should also consider ventilation using an ERV/HRV.

Posted By Muddy on 04 Apr 2019 12:41 AM
WOW !!!! Thanks for the reply's ... If I went with the Mitsubishis type heater would I put one in every room .Example the Blue prints don't show it , but the basement will have two bed rooms , a bathroom and a kitchenette/ living room .Would each room require its own heat source ?

Basement heating & cooling loads in a code-min or better house are pretty low, but there is no substitute for actually running the load numbers to adequately sort that out.

The "one head in every room" solutions for ductless are really GREAT for the ductless installer's ability to make their boat payments, but really SUCKS for you, on comfort/efficiency/up-front cost basis. It usually leads to grotesque oversizing factors- capacity that you neither need nor want.

Even when set to "off", there is still refrigerant flowing through the heads/cassettes on Mitsubishi multi-split systems, enough to overheat/overcool rooms with tiny loads.What's more, the modulation range the compressors are finite, and a bunch of oversized heads will short-cycle the main compressor like crazy during mild to middlin' weather, which takes bite out of efficiency.

It's better to RUN THE LOAD NUMBERS, and only install heads in rooms that have design loads of about 2/3 pr more the design capacity of the head (which is specified differently for use with multi-splits vs. single zone mini-splits), and use ducted cassettes and split output between multiple low-load rooms.

Never let the HVAC contractor run the numbers, especially on better than code houses. Have a certified engineer or some other qualified third party like a RESNET rater do it- someone who makes their living on reputation on the accuracy of their numbers rather than on selling/installing/maintaining equipment. While there are competent HVAC contractors with the tools and skills to do the load analysis, it the extreme exception rather than the rule- MAY 1 out of 20. Even among those who will run a Manual-J for you most do a pretty crappy job of it. I can't count the number of times I've review an HVAC contractor's Manual-J that had large, often critical errors toward oversizing in it. Running the numbers isn't their primary business, and it seems too much to expect due diligence on that front from them, the way you would with an engineer. Even if it costs a grand up front (it's usually less in my area) it's going to save that much on equipment oversizing by the rules-of-thumb contractors, or contractors with huge thumbs on the scale in their Manual-Js.

“...Have a certified engineer or some other qualified third party like a RESNET rater do it- someone who makes their living on reputation on the accuracy of their numbers rather than on selling/installing/maintaining equipment...”

+10

Posted By sailawayrb on 04 Apr 2019 07:55 PM
“...Have a certified engineer or some other qualified third party like a RESNET rater do it- someone who makes their living on reputation on the accuracy of their numbers rather than on selling/installing/maintaining equipment...”

+10

^^^^^^^^^^^^^^^
Infinite THIS!