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Radiant for comfort, not heating

I put radiant tubes under my slab when I built my house, there is 5 or 6 loops around 200'.

I want to install something, preferably electric, to warm my (wifes) feet, but not toss a lot of heat and disrupt how my current propane furnace operates, something simple. Just an expensive foot warmer, is that possible? Just under 2000 sf, I wouldnt have to, and may not want to use all the zones, but 1000 sf at a minimum. Thanks.

Asked by T Carlson
Posted Mar 8, 2018 8:42 PM ET


18 Answers

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T. Carlson,
Lots of questions here. At a minimum, we need to know:

1. Your climate zone or geographical location.

2. How much horizontal rigid insulation, if any, you installed under your slab.

3. How much vertical rigid insulation, if any, you installed at the perimeter of your slab.

In general, if your slab ever gets warm enough for your wife to feel heat through her feet, your slab is blasting enough heat into your space to overwhelm a well-insulated house. That's why energy experts always say, "A radiant floor is a great way to heat a poorly insulated building."

If you house is relatively airtight and well insulated, a radiantly heated slab will almost never get warm enough for your wife to notice.

For more information on these issues, see All About Radiant Floors.

One possible solution to your marital problems is to install electric resistance heating in your bathroom floor, followed by new ceramic tile. When your wife complains of cold feet, you can tell her to stand in the bathroom for a while. Or you can buy her slippers for Christmas.

Answered by Martin Holladay
Posted Mar 9, 2018 6:08 AM ET
Edited Mar 9, 2018 6:10 AM ET.


Zone 6/7 border, 2" EPS underslab and 2" perimeter, 2" eps interior wall insulation, 2x4 wall with r13 unfaced fiberglass. Rims are spray foamed 2-3". HPWH air conditioning basement year round, approximately 5-6 degrees cooler than other levels, although there is no cold air return for the basement, and only 5 supplies which are basically shut off at the moment.

I will start reading, your wife solutions make sense to me, however Im guessing either you're not married or married to one of those rare wives that pick up shovels. Mine comes from a magical land where hardships such as slight temperature differentials apparently don't exist.

I will start reading and come up with a plan for review, thanks.

Answered by T Carlson
Posted Mar 9, 2018 9:22 AM ET


T. Carlson,
I'm very lucky in the wife department. I'm at the NESEA conference in Boston, and yesterday Karyn jumped on our diesel tractor and cleared 6 inches of show from our long driveway with the Kubota-mounted snowblower. She has never complained of cold floors.

Answered by Martin Holladay
Posted Mar 9, 2018 9:33 AM ET


A plate isolated loop running off the water heater, managed by a floor thermostat and an on/off switch could work, probably for less money than a small electric boiler solution.

The operational cost differences between propane and electricity vary widely by local market. A propane water heater solution (or even the propane furnace) may or may not be cheaper to operate than an electric boiler.

Low voltage electric radiant under select sections of the floor (as Martin suggests), would be cheaper to set u and operate than heating the entire slab or large sections thereof.

Answered by Dana Dorsett
Posted Mar 9, 2018 10:24 AM ET


Dont I need a water heater with side taps to use a plate isolator ( Im assuming you are referring to a plate heat exchanger? ) I dont have that. I would have to buy another water heater which would be solely for radiant, options are electric or propane.

I like the simplicity of the water heater diagrams Im looking at.

Answered by T Carlson
Posted Mar 11, 2018 6:56 PM ET


This is always an interesting discussion, and probably one of those where the solution is always very dependent on the sensitivity of the occupants. If the floor slab is within a couple of degrees of the air temperature now, do you think you could increase the slab temp enough to make your wife comfortable without overheating the house? I wonder what most people mean by warm to the touch in terms of a slab? Is it body temp? Slightly warmer than it was without heat? the answer to that probably determines whether you can make this work or not.

Answered by Malcolm Taylor
Posted Mar 11, 2018 8:28 PM ET


Optimal foot comfort (for most) is around 75F and this will produce around 5 btu-hr/ft2. When a home needs less than 5000 btu/hr, it won't be able to warm 1000 ft2 of floor all the way to 75F without overheating the air (edit: but see below for a HPWH exception).


Your water heater should work fine with a plate heat exchanger.

Answered by Jon R
Posted Mar 11, 2018 9:50 PM ET
Edited Mar 12, 2018 12:57 AM ET.


The basement is unfinished so everything is open. The HPWH is what is cooling the basement. Its running right now as I type with 46f air coming out of it. Main and second are at 70, and basement walls AND slab is pretty much at 62 all over according to my Flir. So I can add heat in the winter, or I could switch the HPWH to electric only, finish the duct install and forget about the tubes, but Im leaning towards screwing around with it for fun.

I dont need the slab like an electric mat heater, we arent going to lay on it in the buff, I just need to somehow balance supplemental heat to get the basement up 8 degrees without screwing up the floor above. I could insulate the ceiling since Id get a slight sound deadening bonus out of it but I think really her complaint is that its 62, not necessarily her cold feet although that has come up more than once.

So static wall and slab is 62, what if the slab was at 70 with the assistance of a water heater, will wall surfaces and air temp be at 70 or should they be less since the slab is the emitter?

And can I just buy a regular water heater, no side taps and make it a closed system?

Answered by T Carlson
Posted Mar 11, 2018 11:49 PM ET


Are you saying add a heat exchanger to my existing?

Answered by T Carlson
Posted Mar 11, 2018 11:51 PM ET


I was responding only to the need for side taps.

Bigger picture, if you want to shift heat from the air to the floor and only slightly warm the room, then HP mode make sense. If you want the room much warmer, then you want to add heat via resistance heat or propane.

Answered by Jon R
Posted Mar 12, 2018 12:31 AM ET
Edited Mar 12, 2018 1:06 AM ET.


I think the key response you received so far was from Dana: "managed by a floor thermostat".

My home is hydronically heated. This winter, which was unusually cold, put a lot of load on my ground source heat pump system, and the well temperatures got pretty low. I decided that I would "shed" our ground floor (walkout basement) area, which is generally unoccupied unless my wife is working from the farm and using the computer workstation there. Set the air thermostat back to 66, and the slab/floor thermostat back to 64.

I just ran down downstairs, barefoot, and grabbed the IR thermometer. Yea, it's "cool" down there, but comparatively, at least for my feet, it's entirely comfortable compared to the air temp. And yes the slab measures 64, except for a few areas proximate to external door openings. (Of course the flip side of a hydronically heated slab, is that I have to start a couple days in advance of my wife arriving here to bring the slab/room temp back up.)

BTW, a bit of a non sequitur, but still related, I just finished a year long set of laboratory and local metering radon tests in the house. Interestingly, it appears, that radon levels elevated somewhat when I would heat the slab. Wasn't something I anticipated, so didn't record data with enough granularity to verify that, but I think that makes sense due to stack effect. Also noticed that it seemed to be a bit harder to get my 1st floor masonry (wood) heater to initially fire, ground floor slab highly heated versus not, again I think a stack effect issue. Some additional thoughts to mull over under the "unanticipated consequences" category.

Answered by Andrew Bater
Posted Mar 12, 2018 5:33 AM ET
Edited Mar 12, 2018 5:48 AM ET.


Side taps on the water heater can make setting up a radiant loop easier- a convenience, not a necessity.

Tapping the cool return from the heat exchanger into the cold-feed input to the water heater, and hot supply from the output also works.

Using the temperature & pressure valve port (with a tee to still accommodate the t & p valve) and the drain port (again teed, to be able to run a drain valve), is another option, if that's any easier.

For non-condensing water heaters keeping the turbulence low by limiting the flow on the potable side can protect the water heaters center-flue heat exchanger from potentially destructive chronic condensing. (With condensing water heaters or electric water heaters that's not an issue, of course.)

Answered by Dana Dorsett
Posted Mar 13, 2018 9:10 AM ET


T Carlson .

Is this house one or two stories with a basement ? You stated you put radiant tubing in the slab , at what location is that tubing ? How are you heating the house as a whole ? Where are your wife's feet cold , all floors , one or another floor ?

Answered by Richard McGrath
Posted Mar 13, 2018 2:13 PM ET


This is strictly convenience. 2 story house with full basement, zoned modulating propane fired forced air.

I designed the house with the basement floor plan and installed tubing to that plan. The basement is unfinished but we hang out down there since the kids like to play down there. There are some heat runs but no cold air return, heat runs are basically closed off cause I'm stingy. It's setup to accommodate traditional ceiling heat runs and 2 cold air returns if I want to do that.

I turned the hpwh to electric only a couple days ago to see what would happen under the same conditions.

Answered by T Carlson
Posted Mar 13, 2018 4:01 PM ET


In a well insulated home it should be quite easy to heat the space using the tubing and an inexpensive electric water heater . Keep that floor warmer along with the air all while not hurting the HPWH performance . You have a radiant ready slab , why not use it ?

Answered by Richard McGrath
Posted Mar 13, 2018 5:22 PM ET


Richard, that's what I was hoping to hear. I just didnt know if you could dial radiant back for what would be a pretty low load, enough so it didnt start heating the rest of the house and what equipment would accomplish that. I really like the water heater solution for its simplicity and price.

My water heater choices would be a cheap electric, my current hpwh with heat exchanger, or a propane power vent, any comments on those choices?

Answered by T Carlson
Posted Mar 13, 2018 8:44 PM ET


Allow the HPWH to heat DHW only , dont' wanna tax those units . Inexpensive water heater dedicated to space heating is probably best bet , electric could work due to anticipated low loads , an inexpensive modulating / condensing water heater would be best . See HTP Crossover floor . Illustartion sshow unit performing fan coil duties but my question is this , do we really think any appliance knows what it is sending heat to ?

Return would connect to cold inlet and supply would obviously connect to hot outlet . Use same equipment as you would for a heating system . I suggest an outdoor reset mixing valve for supply water adjustment and season long comfort , a quality air eliminator like a Caleffi or Taco 4900 , a small ECM circ that will consume about 13 watts on average to move the fluid and a sufficiently sized expansion tank taking into account the system volume which will be higher than a system with a 2 gallon cap, boiler . I can offer assistance if you'd like . not sure how much can be done on a forum and pro bono .

Answered by Richard McGrath
Posted Mar 13, 2018 10:02 PM ET


Richard, much thanks, your suggested response was exactly what I was looking for. I'm not looking for someone to tell me exactly what to do or do design or calcs or anything like that, especially for free, just an example setup, appreciate it. I'll keep you posted.

Answered by T Carlson
Posted Mar 13, 2018 10:24 PM ET

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