We built our SIP home last year and our primary heat source is a masonry heater that works excellent and heats the house very well. The only issue that I had a feeling may be a weakness in the design of our house is our master ensuite is on a corner of the house and the furthest from the heater. The issue is, with the porcelain tiles, the bathroom floor is cold and the air is 'cool', not comfortable like the rest of the house. Originally I had planned radiant floor heat as our backup heat source however due to time contraints and budget I went with a propane furnace instead. I would like install some kind of supplemental heat for that room only and have looked at: http://www.warmzone.com/retrofit-radiant-heat.asp My concern with the above is truly how much heat will one experience? The manufacturer did say it will heat the floor but not necessarily the room. I am just looking for something that would make it a little more comfortable. Does anyone have any suggestions? Tks.

Electric resistance floor heating will definitely take the chill off the bare-foot-on-tile experience.

But you can probably do it greener & cleaner & higher capacity with a hydronic loop off your hot water heater. Assuming you have a propane water heater, that would have a very MINIMAL impact on your hot water capacity. Given the ultra-low heat load, a staple-up or even suspended-tube radiant system would get you there, but if you want to limit pumping time and increase responsiveness, the thin sheet-metal heat spreader plates would do.


If you don't have good access to the sub-floor from below, it might be easier to implement a heated towel rack type radiator as a hydronic loop off the hot-water heater rather than a radiant floor.
In either case, plumb it the same way as a recirculation loop (often used to reduce or eliminate delays) drawing hot water to the bathroom, but controlled with a room thermostat rather than a pipe thermostat. Done that way you probably won't run into code or health issues just running it open-loop (no heat exchangers), and may be able to tap the existing plumbing for making the loops. As long as that water can't stagnate, and is fully purged with normal hot water use, the extra few quarts of water for a small section of radiant floor or towel rack don't present much of a health risk the way "open systems" for larger radiant apps might. Assuming you're putting in something less than 40' of half-inch PEX, that's not a lot of water. In series with the shower it'll add less than a minute to the wait for water in summer, and if you wanted to speed that up you could just turn the thermostat for the radiant up, count to 10, then off again, using the circulator EXACTLY like a recirc-pump. Since it's all potable water, any pump you use should have a bronze impeller, as the oxygen in the water of an open-system would wreck iron pumps pretty quickly.

Tks. I have considered hydronic radiant floor but haven't delved into the details of just how to do it. I have left the joists open underneath the so my options are open. I also looked briefly at heater towel rack (hydronic) as that would be an easy install. Of course the one I liked was $3,600 - ouch! I think I'll look at your first suggestion.

Posted By Kyle241 on 07 Dec 2010 05:30 PM
Tks. I have considered hydronic radiant floor but haven't delved into the details of just how to do it. I have left the joists open underneath the so my options are open. I also looked briefly at heater towel rack (hydronic) as that would be an easy install. Of course the one I liked was $3,600 - ouch! I think I'll look at your first suggestion.

Ouch indeed! (Dang, I wouldn't pay that much for a BOILER, for my place, let alone a freakin' RADIATOR!)  Platinum or gold plating?

With the joists open from below a staple up with stamped aluminum sheet  heat spreader plates, a bronze pump and some simple recirculation controls should get you there.  You'll likely spend more than $500, but less than a grand as a DIY project.

Dana,  would this work with an electric water heater?  And how hot does the water need to be?  I thought for a radiator that the temps needed to be much higher than what you nornally get from a water heater.

Posted By bmancanfly on 07 Dec 2010 06:16 PM
Dana,  would this work with an electric water heater?  And how hot does the water need to be?  I thought for a radiator that the temps needed to be much higher than what you nornally get from a water heater.

You don't WANT more than 120F water in a heated towel-rack type radiator.

Unlike fin-tube baseboards, radiators work pretty well at low temp. Taller radiators work better than shorter radiators since they get better convection currents going.

How hot the water temp needs to be is a function of radiation area (and type), and the actual heat load.  Since this is only intended to raise the temp of a room in an otherwise conditioned-heated home, the heat load that the patch of radiant floor or radiator needs to carry on it's own is quite small.

Whether the heat load is going to be enough to cut noticeably into the capacity of an electric hot water also depends on the actual heat load it's supporting.  A typical 50 gallon propane tank heater can deliver 35-50KBTU/hour to the water, but a typical 50 gallon electric tank delivers 7.5-9kilowatts (=26-31KBTU/hr).  Even 5kbtu/hr won't be much missed out of a propane fired unit, but might be with an electric tank if you like to take long showers.  Mind you, 5KBTU/hr would be a LOT for a small bathroom if you keep the windows closed.  If it's a 250+ square foot potty-palace with dual showers and a soaking tub you may be looking for a different solution (or maybe you'd already have ridiculous hot water heating capacity to handle the 6 sidespray showers. )

Thinking further, if you needed a lot more than 300-400BTU/hr out of it you're probably not going to get there with just a towel rack radiator @ 120F water, but you could easily with a small convecting panel-radiator,eg: http://s3.pexsupply.com/manuals/1249544647674/17236_PROD_FILE.pdf

Output with 120F water will be something like 1/3 the 180F numbers in the charts/specs.

This would be also true for most cast-iron radiators and cast-iron baseboard, even if they don't have a low-temp spec for it. (This would NOT be true for fin-tube baseboard, which really starts to crap out below 130F.) A 4' section of Burnham BaseRay RC cast iron baseboard delivers ~ 750-800BTU/hr at domestic hot water temps. (As it happens that's what I'm heating my tiny barely-insulated upstairs bathroom with, @ ~125F average water temp, which is why the li'l light bulb went off in my head.) It tends to run ~$40-45/foot, if you're price-comparing different solutions.

When purchasing heaters, it is also important to get the right heater for each area of the house. Some heaters are more suited to the kitchen or bathroom, while others best suit living rooms or bedrooms. Even if you aren’t sure exactly what you’re looking for, spending some time looking around here may help you decide on important topics such as whether you need to get gas wall heaters or electric wall heaters.

Thank you for this information. I guess people are having a lot of trouble with cold bathroom.. especially the weather's chilly, not to mention on early mornings. This could be the answer! Say goodbye to ice-cold sensation during bath times. Make yourself as warm as possible. Now that's a good bath.

I am taking 3 approaches avoid a cold bathroom.
We are close to wrapping up an addition with Master Bath.
I am not the General Contractor on the job,
so there is a limit to what I can afford to do, but...

1. I had the contractor arrange for the plumber to staple up
1/2" Pex and plates from radiantec.com - note, TWO circuits
in each joist cavity. So that's 8" spacing,
and if I use separate pumps or if I feed the two
1/2" circuits from a 3/4" feed, then I am getting
more heat flow than a single 1/2" circuit.

2. I had the plumber make both curcuits
go around the inside cavity of the tub.

My logic is: If the tub is insulated, heat will be
trapped and make very little difference.
I doubt it is insulated much.
If the tub is NOT insulated, then it will keep
the occupant and water in it hot.
If there is no-one in the tub, the room
will be warm. If it is too hot,
we will fine-tune it with mixing valves.

AND
3. I had the plumber make a loop
in the wall where the towel bar will go.
In all likelihood, this will wait until
the General Captor releases my house.

(One thing I get out of this deal
is a few hundred square feet workshop :-)

Note, this will be connected to a future
GSHP, either DIY or professionally installed,
so the operating temperature is close to 100
degrees, not 120 or 130 that a water heater
would be.