Radiant in concrete - Higher temp/more cycles versus Lower temp/constant circulation

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I have a 2500 sqft concrete basement floor that I insulated and pex'd before the pour. I have foam and that double foil bubble radiant insulation under it all. Even brought it up the edges of the poured walls 4 inches above the footings (the concrete crew thought I was insane but it's one of those things you can only do once). The pex is on 8 fairly equal length loops, spaced about 8 " apart, coming into a manifold with 1" pipe connections. I tried to balance my loop lengths while kinda sorta creating square zones but my goal was a balanced system not zones. The basement is wide open. No rooms or partitions and it's staying that way as long as I'm living here. Even though it's a big area I feel like I did an exceptional job with the insulation and want to try and use my water heaters to warm this puppy up...

I installed 2 state power vent 40 gallon heaters instead of one bigger tank thinking I'd be able to keep the family with hot water even if I had a failure of a unit. They are piped in parallel currently and it's got me thinking...

I know I'm going with the closed flate plate exchanger setup. I could keep the parallel piping arrangement and feed the exchanger with some nice hot water and let the system cycle using pumps as things heat up and cool down but I had a thought.

Say I re-pipe the heaters in series. Set the first one anywhere between 80-100 degrees and only feed the exchanger from that unit and just let the pumps run constantly. I'd adjust the temp as needed to maintain a comfy floor. Or even cycle the pumps based on the concrete temp. Then feed the second unit from the first and crank it to 150 or even higher to kill that legionella bacteria and temper it with a mixing valve before feeding it to the fixtures.

Here's my thinking... The second scenario would eliminate the cycling of the system at the expense of being able to change temps quickly. I don't care too much about that. I want a comfy floor that doesn't chill your feet. I don't need to be able to run down there and try and recover a cold floor quickly. The floor temp would basically be controlled by what I set the H2O heater to. Even if I did want minor control and used a thermostat on the concrete surface to energize pumps it would still cut the cycles of the system way down. I also feel like the entire floor temp would be more consistent.

Tell me why I'm wrong. I can't see any reasons this wouldn't work.
 
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One more thing... I'm thinking trying to cycle hotter water to that much pex in that many loops using one pump isn't going to cut it. I could see return temps at the manifold way cooler than the feed temps if the system is on and off in cycles. BUT a nice slow constant flow would keep things even. It may take a week to get the floor up to temp at the beginning of the heating season but I really don't care about that. :D
 

WorthFlorida

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As you know the slab is a lot of mass to warm up before you feel it in your toes and where the walls touch the floor acts like a heat sink. All the insulation under the concrete you'll still lost some heat to the ground. I'm sure there are tables out there you can find to calculate the BTU's you'll need for the space. Then size up for the proper furnace. Residential water heaters are designed for domestic hot water use only where it will cycle on and off, not for continuous use especially gas fired ones. It's like using your gas stove to heat the house. With your two heaters you probably have 80K BTUs available. Usually that is the amount to heat entire home. The recommend maximum water temp for floor radiant heat is 130 degrees. One for safety and two for the finish floor material. Don't forget, the concrete will want to expand and contract with temperature. At a low temperature there be no problems but if you get the water too hot it could put a lot of stress on the slab.

It really doesn't matter that much if you cycle or run for longer times, you need to delivery a certain amount of BTU's to the area to maintain a certain temp. Where the cycling matters is can the heating appliance work on the duty cycles your asking for it to do and the efficiency of the appliance, i.e. heat lost up the flute.
 

Dana

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Operating a non-condensing hot water heater at 80-100F on a regular basis will usually result in condensation on the center-flue heat exchanger and a much shorter lifespan. It's better to keep the water heater at 130-140F and do the mix down to lower temp on the other side.

If you run 80F water in the floor at a 100% duty cycle you'll be sweating down there, most, or even all the time, unless the basement walls have no insulation and/or you have a lot of window spacing. With 8" o.c. spacing and 1" tubing you have extreme overkill for most basements.

No matter what you won't be able to change the temperature of a 4" slab very quickly.

If the basement walls are insulated to IRC 2012 or newer code minimums (R15 continuous insulation in northern IL, R10 in southern IL) and you have at least R8 under the slab your heat load is likely to come in under 10,000 BTU/hr, maybe even under 5000 BTU/hr. If the walls are not insulated and you have 2' of above grade exposure or more and you only have a half-inch of foam under the shiny bubbly stuff (= glorified vapor barrier) it could be as high as 20,000 BTU/hr. The range of possibilities scales by more than a factor of 4, and it makes a real difference as to what your peak water temperature requirements are, and what makes sense for control scheme.

Stop hacking, start designing, beginning with at least a rough heat load calculation of the basement:

What are the basement walls insulated with, at what R-value, and how much of the wall is above grade?

How many square feet of window, and what are the published U-factors for those windows?

Any basement doors to the outside? If yes, published U-factor for the door, and it's size in square feet?

What is your 99% outside design temperature (or ZIP code, so's we can estimate it)?

The bubble foil insulation is worthless under slabs (maybe R0.5-R1, but not R2), but the foam isn't. How much foam?
 
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Operating a non-condensing hot water heater at 80-100F on a regular basis will usually result in condensation on the center-flue heat exchanger and a much shorter lifespan. It's better to keep the water heater at 130-140F and do the mix down to lower temp on the other side.

If you run 80F water in the floor at a 100% duty cycle you'll be sweating down there, most, or even all the time, unless the basement walls have no insulation and/or you have a lot of window spacing. With 8" o.c. spacing and 1" tubing you have extreme overkill for most basements.

No matter what you won't be able to change the temperature of a 4" slab very quickly.

If the basement walls are insulated to IRC 2012 or newer code minimums (R15 continuous insulation in northern IL, R10 in southern IL) and you have at least R8 under the slab your heat load is likely to come in under 10,000 BTU/hr, maybe even under 5000 BTU/hr. If the walls are not insulated and you have 2' of above grade exposure or more and you only have a half-inch of foam under the shiny bubbly stuff (= glorified vapor barrier) it could be as high as 20,000 BTU/hr. The range of possibilities scales by more than a factor of 4, and it makes a real difference as to what your peak water temperature requirements are, and what makes sense for control scheme.

Stop hacking, start designing, beginning with at least a rough heat load calculation of the basement:

What are the basement walls insulated with, at what R-value, and how much of the wall is above grade?

How many square feet of window, and what are the published U-factors for those windows?

Any basement doors to the outside? If yes, published U-factor for the door, and it's size in square feet?

What is your 99% outside design temperature (or ZIP code, so's we can estimate it)?

The bubble foil insulation is worthless under slabs (maybe R0.5-R1, but not R2), but the foam isn't. How much foam?

Good reply. Stop hacking and start designing. As you can tell from my first post I like to dig in and think I'm designing on my own when this has been done already by people in the know...

Poured basement walls are insulated on the outside with 2" xps foam. Lower end of that would be R-9. 2' of walls are above grade and uninsulated. 4" of spray foam between exposed floor joists where rim board meets joists. Under slab is 1" foam for R-4 plus the double bubble foil which at the time I read was the most important part. The slab doesn't touch the ground, footings, or walls anywhere. I basically made an insulation and bubble foil bowl or liner for the entire pour. There is no heat sink action into the walls as the other poster described. The foil is supposedly an excellent radiant reflector which is what I'd want here. Windows are negligible. I have 4 that are less than 2 sqft area each. No doors leading outside. the 99% outside design temp is 2 for my area. Also, the pex is 3/8" spaced 8" o.c. coming into a manifold with a main 1" connection. I wasn't clear there. At the time I read smaller pex spaced closer is better than bigger pex spacer further so I went with the 3/8".

I didn't think I'd have a condensation issue with the heater at 80F but it's a good point.
 
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So I've been reading a lot more online and found those honeywell mixing valves designed for closed systems with greater temp range selections. Sounds like it would be something I could install and then tinker with temps fairly easily if I wanted to. I found this diagram which would be exactly what I'd do except my heat source would be the flat plate exchanger fed by my water heaters. My question here is why are there two pumps installed basically doing the exact same thing? Does that increase flow because that's something I definitely think I'd need with the number and size of my loops. I can't really figure how it would help that much though.

Screenshot 2017-01-12 at 23.22.17.png
 

Dana

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Am I understanding that the 2" XPS stops at grade, and that ~24" of above grade concrete on the perimeter is uninsulated? That only cuts only about 5K off the heat load- the major heat loss is through the highly conductive above-grade concrete.

An 8" concrete wall has an R-value of about 0.5. If you include the interior & exterior air films you're a bit over R1, that's-it. In southern IL (US climate zone 4) you can get to code-min performance at low mold risk with 1" of cheap box store Type-I EPS (about R3.9-ish) trapped to the wall with a 2x4/R11 batt insulated studwall (kraft facers or unfaced, but not foil-faced) and no interior side vapor barriers/retarders other than standard latex paint on gypsum. In northern IL (US climate zone 5) it would take 1" of foil faced polyiso (R6- R6.5 rated, but derates to ~R5 for this application) with a 2x4/R13 studwall. If there is capillary break between the footing and foundation wall it's fine to put the bottom plate of the studwall directly on the slab, but if there isn't you'll want to put an inch of EPS or XPS (but not polyiso) between the bottom plate & slab as a capillary & thermal break. The wall-foam and studwall insulation has to be full lenght, otherwise the thermal bridging of the concrete with the 24" exposure would cut the performance significantly. Six vertical feet of concrete still only adds up to ~R3.

At outdoor temperatures that matter the XPS will (at least initially) exceed it's rated R-value, but eventually falls to about R4.2/inch (at 75F average temp through the foam) or R4.5/inch (at 40F average temp through the foam- say 65F indoors, 15F outdoors), as it loses it's climate damaging HFC blowing agents over a few decades. Call it R9, plus the R1 for the concrete + interior air film, which would be close enough to code for southern IL if it went all the way up to the foundation sill.

Foil is an excellent reflector, but it's also a great conductor. To have any beneficial thermal effect it needs a significant air space on at least one side, but preferably both sides of the foil. So, did you include an inch or more of air between the slab and the bubble pack, and an inch of air between the bubble pack & foam? (Didn't think so...) But an inch of polystyrene (EPS or XPS) is way better than bubble-pack alone. The 1/8" of air in the bubbles is thermally bridged by the plastic, and still not enough to be worth even R1 before it got flattened to something thinner by the weight of the slab.

Any number of adjustable mixing valves out there will work, and they are a common feature in radiant systems. If you're doing the zoning with zone valves (preferred), use an ECM drive smart pump set up to operate with constant-pressure feedback, otherwise with a dumb AC pump your pumping costs may equal or exceed your natural gas costs.

Natural gas exhaust condenses copiously at temperatures below 120F, even when there is a fair amount of excess combustion air as there is in most center flue water heaters (that all have a steady-state combustion efficiency of about 80%). The minor condensation of a center-flue heat exchanger during the start of a firing cycle is of no consequence if the burn continues until the heat exchanger's temperature rises to well above 120F for the duration of the burn, since the condensate re-evaporates and is purged well before the end of the burn. But at storage temperatures of 80-100F it will condense continuously whenever it's burning, and the acidity of the condensate will eat up the heat exchanger. How fast that happens depends on the number of hours per year the burner operates.
 
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