Hello all, So, I have another question related to my previous post. Brief data: New construction, 3200 ft2 rambler, slab on grade, E. Washington state, with a standard heat pump for HVAC. Tile floor in the great room with high ceiling so I've installed radiant floor in that space and, while I was at it, in the master bath, too. The goal for the radiant system is to keep the tile floors warm during the winter and, I suppose, help with the heating, too. The heat loss for the great room and master bath combined is 16k Btu/hour. I placed a thermistor in the slab to control the radiant system and there's a conventional wall thermostat for the heat pump. So, here's my question: What percentage of the heat loss should I assign to the radiant system when sizing my boiler? Like 25% or? Or can someone just cut straight to the chase and tell me what size boiler they'd install. I was thinking about the Thermoelectric 20k Btu model B-6tmb. OR, do you think that my needs are so small that I'd do fine with a heat exchanger on my domestic WH? Remember, this is only supplemental heat for these two rooms. The remainder of the house will be served by the heat pump. For what it's worth, our electric costs are 2.3 cents a Kwh...don't mean to rub it in. Thanks for all your insight here. ~martin

why bother with a heat pump with kwh so cheap? size the radiant for the full load and have at it.

Homeowner wants heat pump HVAC for air conditioning. Real hot here in summer. And slab is poured....

How much insulation under the slab and are you North Eastern, Central Basin or Palouse?

2" of blueboard. It's in Wenatchee. thirties in the winter and nineties in the summer. thanks, ~martin

You can run the numbers to get a required btu/hr for a given floor temp. My guess is that a heat exchanger on the DHW is a little less expensive than a dedicated boiler for the floor.

So, here's my question: What percentage of the heat loss should I assign to the radiant system when sizing my boiler?

So, here’s my answer: If the radiant system will be used to replace 100% of heat loss from these rooms, you already know the answer…16 Btu/hour…assuming this is an accurate heat loss number which must be the case since no one would be dumb enough to design/construct a radiant system without first accurately knowing the heat loss of each room. If the radiant system will be supplemental and only used to replace X% of the heat loss from these rooms, then the radiant system will provide X% times 16 Btu/hours and the heat pump will provide 100% minus X% times 16 Btu/hour. So how best to do this depends on the X value you use. You might want to select X so as to just maintain a desired floor surface temp which most design hydronic radiant software will readily determine including the DIY software on our website.

"since no one would be dumb enough to design/construct a radiant system without first accurately knowing the heat loss of each room."

We make a nice living doing the first heat load on many such systems; redesign, re-pipe, new controls, perfect comfort and a healthy bill to boot.

Nice plug for the DIY software though.

Happy Mother's Day

We make a nice living doing initial designs properly, but we don’t bother with remodels or redesigns at all…or even initial designs right now as we are always deep into actual construction projects this time of the year. However, we did send a couple referrals your way recently. Happy Mother’s Day too!

Thanks for the input. I guess that was a pretty obvious answer. SO, I'm going with 20% or 3200 btu/hour to provide enough heat to keep the floors warm in the winter. Remember, this whole system is just for supplementary heat to keep living room and bathroom tile floor warm and primary heat source is heap pump Since even a small electric boiler would be over-kill, I'm going to go with a heat exchanger. Good call? I'm basing that on 800 bucks for a 10k btu ThermoElectric boiler vs 250 bucks going heat exchanger route (160 for a smaller flatplate exchanger and 90 for a Grunfos pump) Would you folks agree with this? I'm thinking flatplate exchanger model FG-5X12-4 with a btu/hour number of 18k - 40k or should I go even smaller? I was thinking it can't hurt to oversize this component, both to keep my required flow rate down as well as have higher capacity just in case I end up needing it. I ask because I've never used an exchanger before, but always have just direct plumbed between Pex and dedicated boiler. And thanks for the help. I do the "expert" thing on some other topics on forums and I know it can be tough to have to deal with newbie questions and the hand-holding involved. So thanks, ~martin MA, I'm a twin cities born and raised guy who lives out in Washington state. We have a cabin up at Detroit lakes and a friend told me the ice isn't even off yet!?! What the heck?....

you need to include the cost of a stainless steel pump in the comparison, not a cast iron model. Secondarily you'd need to consider how you want to control it... thermolec has a rudimentary form of outdoor reset built in.

You're right. oops, I had switched over in my mind to the closed side and threw a number at it. Let's make that 250 for the SS pump. So, it's 800 boiler vs 425 heat exchanger route. Still a good call and the way to go, right?