Radiant Heat Under Bathtub

Hi I'm new to this forum and looking for some practical advice. I remodeled the bathroom and tossed out the old Slantfin. To match it's heat output (the bathroom is comfortable on cold days -just right) we need to produce 40 BTU/hr/sq.ft. over the entire bathroom floor including the tub. I believe I compared apples to apples using constant water temperature tables to compare the Slantfin to the radiant. The tub is an old cast iron 60" model (avocado green of course but they don't make them like this anymore we'll get it painted). The sub-floor under the tub is 3/4 boards. So I'm considering running 2 runs in each bay of extruded Al track and 1/2" Pex. The boiler is a new Triangle high efficiency but I don't think that matters much given the calibration I mentioned above comparing 160 F Slantfin to 160 F radiant using tables. The rest of the house is Slantfin. We live in New England and it would be nice to warm our feet in the shower so we're really in love with the idea of radiant heat in that room.

Long story short: does anybody have experience running the extruded Al radiant under cast iron tubs? Will that big airspace around the tub above the subfloor kill the heat transfer? On the other hand, will the tub get too hot?

Thanks!

A true 40BTU/ft heat load would be the heat load of an uninsulated room with single pane glass in a MA location. A floor emitting 40BTU/ft would be an uncomfortably hot ~90F (usually tile) surface that feels a bit like hot asphalt on a July afternoon to bare feet. I'm going to assume that you're just trying to match the room-to-room balance, and don't have an actual heat load of 40BTU/ft.

The air space between the floor and most of the c.i. tub will have an approximate R-value of R2-R4. If there's nothing but flat plank, ship-lap or t & g plank flooring figure on an R-value of the planking of about R1.2. If 3/4" plywood, R0.9. Getting 40BTU/ft through ~R3-R5 is going to take water hotter than 160F. But at least the tub won't get too hot, eh?

A better approach is to either install a convecting panel radiator in addition to any floor radiant to approximately match the output curves of the fin-tube, or to install radiant ceiling in addition to any floor radiant.

Hopefully you didn't succumb to anxiety and install anything bigger than the Solo-60, unless you did a careful heat load calculation that prove you're in one of the ~2% of MA homes (that's a WAG, not a hard statistic) that might actually need something bigger? Oversizing it the most common error with mod-cons for both rookies & seasoned HVAC installers alike, but to get the most comfort & efficiency out of it requires down sizing to the smallest unit that will actually serve the load, so that it can spend most of it's time modulating rather than cycling. The -60 is already more than enough boiler for the vast majority of homes in MA. The minimum fire output of the -110 is more than half the true 99% design condition heat load of the average home in MA, and well above the average condition heat load.

Hi Dana,

I like your profile photo. Cannon?

Thanks for the interest in my heating system, and the thought you put into the R-values and the suggestion.

I did indeed get the -110. It's used for HWH as well as primary heating. My contractor sized it. I think at 800 sf living space any plumber would have been ashamed to install anything larger. Nice system - you don't hear it and with the ambient temperature sensor it modulates its discharge temperature so that temperature swings are unnoticeable.

I think you made good points about the radiant heat in the bath. Your assumption is right on, I am just matching heat balances. And with the Triangle system, it's likely that things won't get too hot. On the other hand, a really cold day with high demand the tub could get hot. But it won't because the planks and the air gap will cushion it. So we'll give it a try first as planned. You know, it's easier in this case to just try it. We'll learn a lot. And I'm planning to use Sharkbite push-ons to create the 4 loops so it'll be really easy change things around. It will be good to get some heat on the feet, and with the gap it shouldn't be too hot. So if the room turns out too cold because the balance if off, we can add a loop to heat the wall next to the tub. This requires yanking drywall on an inside room so I'll just plan on it in just in case the plan A doesn't work.

I have an infrared thermometer and will post the results. It'll be a few months, though. Am on vacation enjoying perfect weather 80's no rain light breezes all week. No place nicer in the world in summer. My wife and I are planning a Presidential Traverse in two weeks (White Mountains Pierce to Madison) and it's slowing down the plumbing. Didn't think about skiing until I saw your photo.

Thanks again,

Dave

<< oops, repeated the post by mistake>>

As much as it pains be to be the bearer of bad news, your contractor was a hack!

I can't imagine an 800' space anywhere in MA having a heat load as high as the minimum-modulated ~28KBTU/hr output of the -110 even at -5F (which is below the 99% design temp for any MA location), so it'll be doing a lot of cycling, especially during the shoulder seasons if you're running it under outdoor reset control (and it sounds like you are.) For reference, my ~2400' + 1500' of semi-conditioned basement antique bungalow in Worcester with sub-code insulation has a heat load less than 35,000BTU/hr @ the local 99% design temp of +5F. My biggest heat loss factor is from the ~13KBTU/hr loss from my antique single pane double-hungs w/clear-glass storms, followed by wall-losses. If you live in a glass house you might have as much heat load as I do, but I don't think that's very likely.

I expect that your true heat load is under 20,000BTU/hr, probably even under 15K, as long as you have glass in the windows and doors that latch. ;-) Got a ZIP code, and some mid-winter heating bills with meter-reading dates, from which we could establish an upper bound using fuel use against heating degree-days?

If it's all run as a single zone the short cycling may not be terrible, and there are ways to deal with it if it is. Ideally you'd want it set up so that even at condensing temperatures you get minimum burn times >5 minutes, but even ~3 minutes isn't going to kill this boiler or it's efficiency. But 1-minute burns & 20 burns/hour would have signifcant negative impact on boiler life and eat into efficiency, so when the heating season starts measure it, and check back if it's doing anything unseemly (which it very well might.) You may have to set a higher low-limit to the reset curve, run a higher differential around the reset point (is that programmable on this unit?) or add thermal mass to system if it's short cycling. Even the -60 might have been bigger than ideal for running radiant floors, but the -110 is way overkill.

I have no idea where the profile photo was shot, and it's unlikely any of those folks are me- the site admin just pasted it up there, but I kept it since it's not far off- I'm definitely an avid skier. If one of them were actually me it would be the telemarker on the right- I usually only ski fixed-heel mode when racing in the beer leagues @ Wachusett.

Is it snowing yet? I haven't gotten skied since July 8th when I skinned up from Paradise to Camp Muir on Mt. Rainier (WA)- my quads are twitching sitting in this chair just thinking about it! It turned out to be a real bluebird day- could see as far as Mt. Jefferson in OR (more than 100 miles away) and was able to ski most of the ~4500' of vertical down to the parking lot, with only a couple of shoulder-carries.

It will be tough to measure my actual heat load because every year we've lived here we've mixed the heating oil / gas use with the wood stove. And we both work so the house drops to 50's during daytime and we "crank it" back up to 60's in the evenings and mornings so the swing is large. In any case, the new boiler has an adjustable reset curve and it trends ignitions so we can get an idea how things are doing and at some point I can declare in the house that we're not burning any wood until we get some data points on the heating system and we can get a good week's worth of measurement. This may work best, if at all, on some vacation (perhaps a ski trip to Utah!). I'm not afraid to tweak it. My zip is 01984, coastal eastern MA.

I grew up in Wyoming skiing the USSA racing circuit then a little bit of high school racing before my school cancelled it. The point of that is that I got around a lot of Wyo, Mont, and Colo, grew to appreciate the amazing mountains of the west, and I miss it. About every two years I meet my Dad in Salt Lake for a week of pow-pow. The tele skiers impress me but not enough to try it. One day I rented a snowboard and discovered that if I hit my head any more I'll be left a boarder permanently. So I stick with the alpine equipment and I'm happy. Mountaineering we stick with the snowshoes and crampons.

This summer we summit-ted Cloud Peak in the Bighorns of Wyoming (13k, 4th highest prominence in US, but easy hike). This was a big deal because my wife gets altitude sickness and we were all over the place working the acclimatization thing. I had no idea how much work that is. No snow except a couple stray snowfields. Just a couple more weeks it'll start.

Yeah, with mixed fuel use it's harder and ultra-deep setbacks it's tough to do a fuel-use heat load calc. A simple I=B=R approach works though.

The 99% outside design temp for nearby Gloucester is + 5F so we can use that as the outside temp, 70F for the inside temp for a delta-T of 65F.

Assuming it's a ~23'x 35' retangular cape/ranch and fiber-insulated 2x4 construction that's a perimeter of 116', times 10' of height (includes joist depth) comes in at 1160' of gross wall area.

At a 15% window/floor ratio that would be 120' of window, figuring two exterior doors at 20' each brings the total clear-wall area down to ~1020', with a U-factor of about 0.1. So the wall losses would be:

U0.1 x 1000' x 65F= 6500 BTU/hr.

With 2" solid doors and single-panes with storms you get 160' of U 0.5. So the window & door losses add up to:

U0.5 x 160' x 65F= 5200 BTU/hr

Assuming R19 in the attic,that's about U0.06, for 800' comes in at:

U0.06 x 800' x 65F= 3120 BTU/hr.

Assuming you have NO insulation in the basement, 1.5' of exposed foundation & band joist at an average U-factor of 1, with 116' of perimeter and a 55F basement (when it's +5F out), you have ~175 of U1, and a 50F delta-T, which is a loss of:

U1 x 175' x 50F= 8750 BTU/hr.

Add it all up and it's around 23,570 BTU/hr.

If it leaks air like a sieve at the attic & foundation add another 4-5K, call it 29,000BTU/hr, which is the MINIMUM fire output of the -110. If your foundation is insulated & air sealed your design heat load is almost certainly under 20K. Eyeballing the Weatherspark.com cursors when zoomed out to cover the whole heating season, binned-hourly mean temp January temp in Wenham is about 27-28F, so even in the leaky-house scenario the average mid-winter heat load is about 19,000 BTU/hr, 15K if it's pretty tight. The min-mod output of the -60 is about 15K, which means you'd be much better off with the -60, since it could actually track your load, whereas with the -110 it's always cycling, even on design-day.

This is a really common error, for both DIYers and pros alike, but even a rule-of-thumb guy should have gotten it right for an 800' house- even a grossly-oversizing 40BTU/foot x 800' only comes in at 32KBTU/hr, well within the non-condensing output of the -60. Most of those hacks use 35 BTU/ft, which would hit the above I=B=R estimate for a VERY leaky house. If your windows & insulation levels & tightness are than that, your heat load will of course be much lower. Code min new construction in MA is usually under 15BTU/ft, with rare exceptions.

So I guess the good news on this only slightly better than a WAG heat load calc is that assuming a 24,000BTU/hr design load and say 650' of exposed radiant floor you'd only need ~35BTU/ft out of the floor. At a more likely (or at least possible, if you insulate the foundation & band joist) 15,000 BTU/hr puts you under a very reasonable 20-25 BTU/ft requirement from the radiant, with more comfortable floor ~80F peak temps.

I have relatives in both Sheridan & Greybull- the Bighorns are very pretty indeed! I originally hail from the Seattle area- got way too used to seeing glaciated volcano horizons for the backdrop on clear days, and I too miss it, living in the comparatively flat swamps of MA.

Hey radiant heat under the bathtub is working great! I made sure I had two extruded plates in each bay at any given spot (there's room for 4). To keep the pressure loss equivalent to the old 3/4" slantfin lines, I made manifolds from 3/4" copper and sharkbite pex crimp fittings. I ran 5 parallel 1/2" pex circuits snaked around the bathroom floor. So there were at times up to 3 pex lines in each bay at a location. Only 2 were ever plated. Under the tub I followed the same pattern. The floor turned out to be about 76 degrees and the tub about 72 degrees while the floor in the rest of the house is about 65. Those numbers are about what I would have expected from common sense and mounting the plates under the 3/4 board subfloor successfully kept the tub from getting too hot. The bathroom is comfortable just like it was with the slantfin. The bride is thrilled with the warm tub! I like the comfy feeling when standing by the sink.

My only disappointment was not covering all the floor underneath with plates. There are decided cold spots in the corners (if I bother to step there). And next to the toilet where I was worried about the wax ring the floor is about 68 degrees. It's a bit chilly compared to the rest of the floor. I had expected the heat to dissipate more uniformly.