Underperforming radiant floor heating, how can I improve it?

pipecurious

Hi Everyone,

I've been doing a lot of reading on here and was looking for some advice.

We bought a house (built 2005) a couple years ago that has all radiant floor heating (in unfinished basement, first floor, and upstairs bedrooms). It's a two-story, ~3000 sqft (not counting basement), living room has cathedral ceilings. We live in a cold climate (less than an hour from Montreal) and we've noticed that on some of the cold and windy days (especially single digit Fahrenheit temperatures and 20+ mph winds) we get in the winter, the house doesn't stay warm. We keep the thermostats all set at 73 degrees F, but on these cold and windy days our master bath was only able to get to 66 degrees F and the living room was only able to get to the upper 60's as well. One of the bedrooms upstairs is our nursery and that couldn't get above 67 degrees F. Overall the house felt noticeably colder than normal.

Overall the house is well air-sealed and we don't have any major air leaks. The house is well insulated, considering the year it was built, we have 2x6 walls with interior fiberglass batt insulation and exterior foam insulation (1 inch). Rim joists are sealed and insulated with fiberglass batts. Overall the house has a lot of glass (many large triple pane casement windows). Upstairs rooms have fairly large double pane, double hung windows, and on the very windy days I did notice some air leakage around the double hung windows, so I sealed those with some flexible rope caulk, but that only helped to increase the room temps there by a degree).

For heat source we have a 154 MBH oil boiler, but we seldom this (it's just backup) as we have an outdoor wood boiler that is tied to the oil boiler via a heat exchanger. The plumbing is primary secondary, and each room is on its own zone with a circulator pump and mixing valve (6 zones total). This whole setup is original to the house.

I noticed the radiant floor temperature gauges located right after the circulator pumps for the first floor and upstairs bedrooms were reading 150 degrees F). So after doing some reading I wanted to see what radiant floor setup we had. I checked the first floor joists from the basement and can see that there's radiant barrier bubble insulation stapled up, a few inches below the radiant lines, and the radiant floor lines behind that are fastened to the subfloor with PEX clips, so they hang below the subfloor by about an inch. This seems like some version of either plateless staple up or suspended radiant floor from what I've been reading.

My question is, what can I do to improve the setup so that we don't see temperature drops in the rooms? I'm assuming if heat loss calculations were done when the system was designed, the design temp is surely lower than single digits, as historically we've had much colder weather in the winter (the last few years have been mild). Might I see an improvement if I put insulation in the joist bays (e.g. rockwool)? What about adding plates? How about both? At some point we'd like to replace the oil boiler for a heat pump and I figure plates would be necessary to get the flow temp lower to allow for that. Is there anything I'm missing that would help?

Any suggestions are very much appreciated. I'd like to make changes sometime in the spring/summer once the heating season is over so that next winter the house will stay warmer.

Jamie Hall

You're not missing much there. Better insulation below the piping (but don't cover it) will help — the bubble wrap isn't doing much for you. Plates would help — but only if you could move all the piping so everything was in contact with the floor rather than spaced.

That said, can you measure the actual floor temperature when the system is running full out? Not the air temperature just above the floor — but the actual material temperature? That will be the best guide to whether the system can actually provide the heat you need, as there is a definite upper limit on how warm the floors can be! 80 F is about as warm as is nice to live with…

bjohnhy

Post some photos of you boiler, circulators, mixing valves, manifold, etc.

What are the return temps from floor loop?

DCContrarian

When it's running full out, how are the floor temperatures? Are they comfortable to walk on? Because the only way to get more out of the floors is to make them hotter, we can talk about ways to do that, but if they're already at the edge of comfort that may not be the way to go.

There's basically two ways to approach this. One is to get more heat out of the floors, making the surface hotter. If you're already seeing 150F for the water temperature the easiest step of raising the water temperature has already been taken, so it's going to involve reconfiguring the piping below the floor. The other approach is to leave the floors as they are and add more heat emitters. Those could be radiators, fan units, towel warmers, all kinds of things. If you're already running pretty hot water it would be pretty straightforward to add them.

If you have a way of measuring the existing floor temperatures we can estimate the existing heat output, which will gives an idea of how much capacity needs to be added. We can also estimate from fuel usage, and from return water temperature and flow.

hot_rod

You have a joist bay installation method of tube installation. The tube heats the air in the joist bay, which convects to the floor. Those generally need to run pretty hot 150°F plus.

An outdoor reset control is helpful, it would ramp that 150 up when temperature drops outside.

An infrared camera is a good way to assure it was tubed properly. Use it for looking at the structure for leaks also. The phone attachment IR cameras actually work well enough for this. This is a common tool on hydronic service trucks around here.

You'll find many other things to look at with an IR camera😎

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