New radiant heat install

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Nathan Perry

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I’m building a new 2100sq ft home. All one level. I’ve received mixed input on what to use as far as a boiler, hot water heater and electric boiler for the system. The pex will be going in the floor joist. One proposal was also to use 7/8 pex and I’ve read some guys would never use that. What would you guys recommend?
 

Plumber69

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I’m building a new 2100sq ft home. All one level. I’ve received mixed input on what to use as far as a boiler, hot water heater and electric boiler for the system. The pex will be going in the floor joist. One proposal was also to use 7/8 pex and I’ve read some guys would never use that. What would you guys recommend?
Thats not to big, im sure 1/2 pex (5/8) with multiple runs would be fine
 

Dana

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What type of radiant are you planning on using? (Stapled up on the underside of the subfloor? Heat spreader plates on the underside of the subfloor? Bare tubing suspended tube between the floor joists? Suspended tubing with finned convectors?) It matters.

How many bathrooms? Any big tubs to fill?

For NEW 2100 houses in RI a right sized cold climate ducted air source heat pump is probably going to be a better option, with low-voltage mesh radiant heat operated off a floor thermostat only in the rooms/locations where you really care about the barefoot comfort. Most tight code minimum R homes that size can be heated/cooled with a 2-ton or 2.5 ton modulating Fujitsu or Mitsubishi all the way down to -5F or cooler, and would use about 1/3 the amount of heating electricity as an electric boiler. If properly sized you'll almost never hear the system running. These things are not at all comparable to the typical noisy hot flash followed by the extended drafty chill of a 3-5x oversized scorched air furnace.
 

Nathan Perry

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What type of radiant are you planning on using? (Stapled up on the underside of the subfloor? Heat spreader plates on the underside of the subfloor? Bare tubing suspended tube between the floor joists? Suspended tubing with finned convectors?) It matters.

How many bathrooms? Any big tubs to fill?

For NEW 2100 houses in RI a right sized cold climate ducted air source heat pump is probably going to be a better option, with low-voltage mesh radiant heat operated off a floor thermostat only in the rooms/locations where you really care about the barefoot comfort. Most tight code minimum R homes that size can be heated/cooled with a 2-ton or 2.5 ton modulating Fujitsu or Mitsubishi all the way down to -5F or cooler, and would use about 1/3 the amount of heating electricity as an electric boiler. If properly sized you'll almost never hear the system running. These things are not at all comparable to the typical noisy hot flash followed by the extended drafty chill of a 3-5x oversized scorched air furnace.

I was going to do the pex under sub floor with the spreader plates. 2 bathrooms. One big tub one normal.
 

Dana

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I was going to do the pex under sub floor with the spreader plates. 2 bathrooms. One big tub one normal.

Half inch PEX would be more than enough to deliver 20 BTU/hr per square foot (more than you should ever need with a 2x6/R20, U0.30 window type house with R49 in the attic.)

Has anybody done the room by room heat load calculations yet? That's important to get it right, and to know just how much heat the floor/other needs to deliver. A typical 2100' ranch house over a 2100' insulated to code min basement with 8 or fewer corners to the footprint would have a total heat load of about 25,000 BTU/hr @ +10F (a typical 99% outside design temp for RI locations) or a bit less. An 900 square foot over 1200' 2 story with a 1200 square foot code-min basement would run closer to 20,000 BTU/hr @ 10F. The most you would ever need to cover the Polar Vortex disturbance cold snap coolth is 1.4x the 99% load so assuming my WAG heat loads are right (could be higher or lower, depending on the particulars) call it 28-35,000 BTU/hr, which is ~10 kw of electric boiler, not more.

The cooling loads for houses that size will normally come in around 1.5- 2 tons in our area (I'm ~35 miles north of Providence). A ducted 2 ton Fujitsu AOU/ARU 24RLGX puts out a bit more than 23,000 BTU/hr @ +10F, the 2.5 ton AOU/ARU30RLGX puts out about 29,000 BTU @ 10F. They come bigger in that series (in half ton increments), but unless you're building the crummiest 2100'new house in RI you won't need anything bigger than the 3 tonner, but even that's a stretch. Even the 3 tonner can drop back to as low as ~10,000 BTU/hr @ 82F in cooling mode, or ~9000 BTU/hr @ 47F in heating mode, and even if that's sub-optimally oversized it won't be a terrible fit for cooling or heating, but the 2 ton would be better if the numbers work, since it has a wider modulation range with a lower minimum modulation, which means it will idle along running nearly continuously for maximum quiet comfort. Some of the 2 and 3 ton Carrier Infinity w/GreenSpeed have less modulation range, others more depending on the air handler selection, but those too are VERY comfortable and quiet if sized correctly. Any of these would deliver about 3x the amount of heat per kwh compared to an electric boiler as a seasonal average, and even during cold snaps at temps cooler than 10F would be delivering 2x the efficiency.

For reference, I live in a sub-code 2x4 framed antique 2400' 1.5 story bungalow (400' upstairs, 2000' on the first floor) over 1600' of insulated basement (with a very inefficient 14 corners to the foot-print) a bit north of you in Worcester. Most of the windows are circa 1923 single pane wood sash double-hungs with 1980s vintage clear glass storm windows. My design load (at +5F) is about 35,000 BTU/hr, and would still be a less than 40K if fully heating the basement. Your house has to be substantially better than that, if any attention at all is paid to the energy efficiency of the building envelope.

How big is the big tub? (A 60 gallon soaker tub, or is it a 120 gallon spa?)
 

Nathan Perry

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Half inch PEX would be more than enough to deliver 20 BTU/hr per square foot (more than you should ever need with a 2x6/R20, U0.30 window type house with R49 in the attic.)

Has anybody done the room by room heat load calculations yet? That's important to get it right, and to know just how much heat the floor/other needs to deliver. A typical 2100' ranch house over a 2100' insulated to code min basement with 8 or fewer corners to the footprint would have a total heat load of about 25,000 BTU/hr @ +10F (a typical 99% outside design temp for RI locations) or a bit less. An 900 square foot over 1200' 2 story with a 1200 square foot code-min basement would run closer to 20,000 BTU/hr @ 10F. The most you would ever need to cover the Polar Vortex disturbance cold snap coolth is 1.4x the 99% load so assuming my WAG heat loads are right (could be higher or lower, depending on the particulars) call it 28-35,000 BTU/hr, which is ~10 kw of electric boiler, not more.

The cooling loads for houses that size will normally come in around 1.5- 2 tons in our area (I'm ~35 miles north of Providence). A ducted 2 ton Fujitsu AOU/ARU 24RLGX puts out a bit more than 23,000 BTU/hr @ +10F, the 2.5 ton AOU/ARU30RLGX puts out about 29,000 BTU @ 10F. They come bigger in that series (in half ton increments), but unless you're building the crummiest 2100'new house in RI you won't need anything bigger than the 3 tonner, but even that's a stretch. Even the 3 tonner can drop back to as low as ~10,000 BTU/hr @ 82F in cooling mode, or ~9000 BTU/hr @ 47F in heating mode, and even if that's sub-optimally oversized it won't be a terrible fit for cooling or heating, but the 2 ton would be better if the numbers work, since it has a wider modulation range with a lower minimum modulation, which means it will idle along running nearly continuously for maximum quiet comfort. Some of the 2 and 3 ton Carrier Infinity w/GreenSpeed have less modulation range, others more depending on the air handler selection, but those too are VERY comfortable and quiet if sized correctly. Any of these would deliver about 3x the amount of heat per kwh compared to an electric boiler as a seasonal average, and even during cold snaps at temps cooler than 10F would be delivering 2x the efficiency.

For reference, I live in a sub-code 2x4 framed antique 2400' 1.5 story bungalow (400' upstairs, 2000' on the first floor) over 1600' of insulated basement (with a very inefficient 14 corners to the foot-print) a bit north of you in Worcester. Most of the windows are circa 1923 single pane wood sash double-hungs with 1980s vintage clear glass storm windows. My design load (at +5F) is about 35,000 BTU/hr, and would still be a less than 40K if fully heating the basement. Your house has to be substantially better than that, if any attention at all is paid to the energy efficiency of the building envelope.

How big is the big tub? (A 60 gallon soaker tub, or is it a 120 gallon spa?)

the heat load calculations have not been done. I’m just trying to reach out and get the best system I can afford to do without just jumping to the first thing I was told. I appreciate all your input big time, this is what I needed. The tub is a 60 gal smoker tub. I’ll try to upload a floor joist plan pic to give you an idea.
 

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Dana

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An 80 gallon heat pump water heater can fill a 60 gallon tub, and is probably the best choice in an all-electric house in New England. The heat pump will partially dehumidify the air in the house during the summer, taking some of the load off the AC or dehumidifer, turning the "latent heat of vaporization" into heat inside the insulated tank rather than heating up the room air, the way a room dehumidfier does. They use about 1/3 the amount of electricity of a plain old electric tank. During the heating season 2/3 of the heat is being drawn from the room, but if it's located in a 2100' open basement it won't result in an appreciable room temperature drop.

There is a significant upcharge for going with heat pump water heater, but at typical Nat'l Grid & other large utility residential pricing in our region it pays for itself a few times times over within the anticipated lifecycle of a water heater. (Some local municipal electric companies in MA have cheap enough electricity that it's not a no-brainer, but it'll still pay off, just not 2-4x.)
 

Jadnashua

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I'll take radiant floor heating over perimeter radiators or forced air, but then that can lead to needing ducts for a/c that are only used part of the year. Radiant floor lets you pretty much ignore where the duct outlets are, and arrange your furniture any way you want. Trying to put heat into a room from the ceiling is a tough job to get things even. And, using ducts in the floor make for a lousy cooling job. A radiant floor (or ceiling or walls) just makes you feel better, and sometimes will allow a room temperature to be lower while still being comfortable.

But, it's not inexpensive to install. It can be to operate.

If I were building a new house, I'd consider using Schluter's Bekotec for the heating. It will give you a good heat sink to even out the temperature and would work with wood, tile, surfaces. The pex, if used, is snapped around the pillars, then deck mud is used to fill in around it for your finished surface.
Bekotec.jpg
 
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Nathan Perry

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An 80 gallon heat pump water heater can fill a 60 gallon tub, and is probably the best choice in an all-electric house in New England. The heat pump will partially dehumidify the air in the house during the summer, taking some of the load off the AC or dehumidifer, turning the "latent heat of vaporization" into heat inside the insulated tank rather than heating up the room air, the way a room dehumidfier does. They use about 1/3 the amount of electricity of a plain old electric tank. During the heating season 2/3 of the heat is being drawn from the room, but if it's located in a 2100' open basement it won't result in an appreciable room temperature drop.

There is a significant upcharge for going with heat pump water heater, but at typical Nat'l Grid & other large utility residential pricing in our region it pays for itself a few times times over within the anticipated lifecycle of a water heater. (Some local municipal electric companies in MA have cheap enough electricity that it's not a no-brainer, but it'll still pay off, just not 2-4x.)

i actually have a small municipality. Cheaper than the grid. Excuse my ignorance but how does a heat pump system heat the water for the radiant and do the cooling? This sounds too good to be true.
 

Dana

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i actually have a small municipality. Cheaper than the grid. Excuse my ignorance but how does a heat pump system heat the water for the radiant and do the cooling? This sounds too good to be true.

I wasn't proposing a hydronic heat pump for cooling & heating, but a modulating air-delivery system (ductless or ducted, or a combination thereof.) I was suggesting controlling the room temperature with the heat pump, and using low-voltage above the subfloor electric radiant (not hydronic) to control the floor temps in areas in the parts of the house where it matters.

While there are a few vendors of modulating reversible chiller type heat pumps that are up to the task, the summertime outdoor air dew points are too high in this region to make hydronic cooled floors easy to pull off. The lower the heating water temp requirements, the more cold weather capacity there is, and the higher average efficiency. Using a chiller for the sub-floor hydronic radiant may require higher temps during the coldest days that would need to be boosted with a tiny electric boiler or a hydro-air coil as a second stage when it gets TOO cold, but you may be able to get there without it. Using an above-the-subfloor system such as WarmBoard or Roth panels might get the water temp requirements low enough that it can work well enough even during cold weather to not need it. This takes a bit of design work- probably not worth the engineering cost if you have 5 cent electricity, but can be if it's north of 10 cents /kwh (all-in, delivered).

LG's Multi V-S system could do it all (including the domestic hot water) at high efficiency. A house in Jamestown RI using one of those systems was featured on This Old House last year. An advantage of the Multi V-S is that it can heat and cool different zones (including the domestic hot water zone) simultaneously, efficiently moving heat from one part of the house to the other. The capacity of that system is overkill for heating/cooling loads of their superinsulated house is overkill, more necessary to keep up with the hot water load.
 
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