Accessory radiant slab heat source
I recently completed a new family room/ mud room addition on my house and I was hoping to get some advice on the best heat source option for my radiant slab.
Climate zone 5A
Existing home conditioned area: 1500SF – one story
New addition area 320 sf – same story as existing house
The addition includes a slab on grade floor with 2″ rigid foam under slab and slab edge insulation along with pex radiant tubing. The radiant was not planned to be the main source of heat, only supplemental for comfort reasons and as it sits the space is conditioned just fine from the heat source in the adjacent room. The new addition rooms are open directly to the main kitchen/dining area of the house which is heated by a 12k btu fujitsu mini split. At this point floors are cold to the touch and I’d like to fire up the radiant system. I’d also like to upgrade my existing electric resistance tank style water heater to something more efficient. This brings me to my question…which heat source would be the best option for DHW needs and the small load of the radiant system? I have been working with a plumber throughout the process but I’d like to reach out to the GBA community to see if there are any other suggestions. My plumber is recommending a Tankless propane water heater with indirect tank, heat exchange plate, and necessary pumps for the radiant side. Any suggestions are much appreciated and please let me know if more information is needed.
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Replies
Jon,
Tankless plus an indirect tank is no a cheap setup, not sure why that is the recommendation.
For small zone like a kitchen, I go with an open system running off the existing tank water heater (this is by the simpler/cheaper setup). You have to take care not to have stagnant water in the radiant loop, this can be done by pulling the cold water feed through the loop (preferred) or running a daily timer on the pump.
It is possible to make this work with a tankless unit, but the small size of the zone would add complications. If you must have the tankless, the easiest is to go with a unit that has re-circulation function (ie Ranni RUR98iP ) and use the re-circulation loop for the floor heat.
Thanks for the response. I'm committed to a closed system at this point because the piping is O2 barrier pex. Plumber recommended the closed system and countless articles on the web spoke against an open system because of stagnation issues. I now realize open would have worked due to the small size of the system..
I'm not committed to the tankless set up, and honestly it seems a bit over complicated but the local utility is offering a $600 rebate and my plumber is a friend so parts and labor will be reasonable. With that said I'm still open to other options..I was thinking just a direct vent propane tank water heater with a heat exchange plate and accessories would be a simple install but the plumber has concerns about efficiency and costs of operation.
Would the Rinnai unit you mentioned with the re-circ feature only benefit me with an open system?
I was even looking into a HPWH but all the info I found spoke against them keeping up with the radiant demand.
Jon,
I'm with Dana on this one. If you electricity is cheap, propane doesn't make much sense. Just get a new electric tank and call it a day.
The mini boiler Dana is suggesting might be cheaper than the parts you would need to connect a closed system to a tank. Although if you go with a sidearm, that might not be the case.
You can also check the pex pipe that is in your radiant slab, some of them have the correct ratings for potable use (check out https://canarsee.com/learning-center/pex-pipe-markings for how to decipher them). The important one is the NSF/CSA and the chlorine rating if you are on city water. If the piping is fine, you can use it in an open system.
As for your plumbers concern with the plate/efficinecy. I wouldn't worry about it, this zone is too small to matter much. The small load means you can probably even use a sidearm heat exchanger on a propane/electric tank to feed this zone saving you one pump.
As for the Rinnai. I have not used that specific unit for radiant heat, I've use one of their older ones and it had no issues with a small zone. Typically anything that can do recirc has a low enough fire rate for a small zone. I have tried to use a different manufacturer on a small zone and the burner would cycle constantly. Getting that combination to work was a pain (buffer tank extra valves and controls, PIA). You can always connect a close system to an open system, just add in the heat exchanger/pumps/expansion tank.
Also keep in mind, depending on your water hardness, tankless heaters are a maintenance item.
A heat pump water heater for the hot water, and a tiny electric boiler for the slab, controlling the slab temp with a floor thermostat, letting the mini-split maintain the room temp makes a lot more $en$e than a ridiculously oversized tankless propane burner + indirect. In many markets even the operating cost of the propane burner is more than resistance electricity.
An Electro Industries EMB-S-1 would do it for less than a grand, a boiler targeted well for radiant floors, it's DIY-able for most people with reasonable plumbing & electrical skills.
https://www.ecomfort.com/manuals/Mini_Electric_Boiler_Specifications.pdf
Thanks Dana. I was hoping to use a single heat source but it does seem to make more sense to split up the radiant and DHW. BTW I'm in Massachusetts which has fairly high electricity rates.
I get it (I'm on Nat'l Grid in Worcester), but with a couple of Taco-007s, one on each side of the heat exchanger the pumping power alone would be something like 25% of the heat you're trying to deliver to the slab. The pumps are their own little radiators, and probably NOT located in the room with the slab. How efficient is that?
For the same $200 or so it would cost for a pair of -007s you could get a single AquaMotion AM55-FVL for the electric boiler solution, which at it's lowest speed would be 5 watts and probably no more than 20 watts (depending on the pumping head of your tubing- you may have to bump it up from the absolute lowest to get the right delta-E) instead of 150W for a pair of cheaper pumps.
Ignoring the pumping power, the MA average three-buck propane burned at 95% efficiency delivers 87,000BTU/gallon into the system, which is the equivalent of 87000/3412= 25.5 kwh worth of heat via an electric boiler. So the fuel cost portion of the operating cost of the propane system would be the equivalent of $3.00/25.5= ~12 cent electricity.
By the time you add in the pumping power costs (unless buying ridiculously expensive pumps) would be the equivalent of 15 cent electricity. That may be ~25% lower operational cost than running the electric boiler, but there's still no "payoff" on the additional upfront cost and maintenance of a propane burner. The levelized cost of rooftop solar is well under 15 cents in MA after the various subsidies, and just one "extra" panel would produce more power than the slab would use with an electric boiler in a year, at a marginal up-front cost less than what it takes to do the propane version.
The water temperatures you would need for keeping the slab at 72F are also well below domestic hot water temperatures, so it would need to be mixed down with a thermostatic mixing valve. Every little detail just keeps adding up-front cost that's never recovered on the operating cost end. A keep-it-simple-stupid approach with the tiniest electric boiler is really the right solution here. The only time the isolated water approach would start to make sense is if it were heating a much bigger part of the house than a 320' addition.
The EMB-S-1 is good for 3800 BTU/hr and at a BTU/square foot ratio of 12 BTU/hr per square foot of conditioned space most likely COULD heat the entire addition down to about 0F outdoors or so without assist from the mini-split if needed. If the slab is kept 2F warmer than the average room temp it'll be emitting about 1200-1300 BTU/hr, about 1/3 the full-on output of the EMB-S-1, which can live on a single 120V/15A circuit- nothing special, cheaper to wire up than a typical electric water heater. (Your toaster probably draws more power than that boiler!)
If the domestic hot water is using propane for the source, a condensing tank is generally a better/smaller/cheaper thing to install than a tankless + indirect. But in your case I suspect a heat pump water heater sized for the biggest tub you need to fill is the "right" approach.
Great info Dana. I'm leaning toward the electric boiler for the radiant. The DHW is now something I can decide on at a different time but I'll explore both a propane direct vent tank unit and the HPWH. The location on the water heater will complicate the heat pump option though. The mechanical room that the water heater is located in is about 550 SF and has no heat source to steal heat from. Average temp in the winter is probably about 45-50*. The "Attaching ducts to a heat pump water heater" article looked very interesting but unfortunately I'm not a prime member and couldn't read the whole thing. I'll do some more digging.
Heat pump water heaters can indeed be ducted, but you need to be careful where the cold output is being directed in winter. But a 550' room is still big enough, and 50F is still warm enough for most tank-top compressor heat pump water heaters. It's not as efficient at 40F as at 60F, but it's still WAY more efficient than a standard electric, and it'll dehumidify that space in summer.
At the higher-cost/highest-efficiency end of the scale for about $4K a Sanden split system heat pump water heater puts the compressor outdoors (looks like a mini-split) and can still deliver storage temps north of 150F at reasonable efficiency even when it's -20F outside.
https://www.sandenwaterheater.com/