Adding Radiant Floor Heat to Existing Radiator System

clifton_clowers

I live in an old house in New England built in the late 19th century. Over the years it was converted from a coal/steam system to an oil burner with water and then to a gas boiler. We have about 4000sqft, 3 levels, and our newish boiler (about 4 years old) is rated at 220000 BTU. And for what it's worth, I rarely turn the heat above 64.

We're about to do a kitchen renovation and we're thinking about replacing the kitchen radiator with a floor heat system. The kitchen is about 200 square feet (5% of the house) and not open to any other rooms - we keep the door closed to keep smells in and cats out. Based on both the percent of the house, and the percent of the radiator fins in the kitchen, I think we need about 12000BTU. I know the are better ways to calculate that, but those numbers are consistent.

Here's the question - what is the preferred/most cost effective way to lower/regulate the temperature of the water going to the kitchen floor? My plumber has suggested a Viega hydronic mixing block. This is extremely expensive and when combined with the labor to install it, prohibitive. I assume there must be other solutions.

Alternatively, how unreasonable would it be to use a our hot water heater, a heat exchanger and a couple of pumps for this small space?

It would be neat if the room were it's own zone, but frankly it is not that important.

Thanks

hot_rod

Those numbers don't work so well, 12,000 ÷200 square feet would be 60 BTU/ sq ft. Mid to high 20 BTU/ sq ft. would be more realistic for radiant floor systems.

The size of the current radiator may give you an idea of what the space requires to stay comfortable.

Load calc here to try your own calculation.

http://www.slantfin.com/slantfin-heat-loss-calculator/


Kitchens are often handicapped by not having enough exposed floor to cover the heatload with radiant floor alone. Cabinets and appliances need to be subtracted out of the floor space for the floor calculation.

Ironman

You'll spend about as much for pumps (one of them has to be stainless or bronze), a heat exchanger, expansion tank, relief and fill valves as you would for the Viega mixing block. And you heat source would be 70% efficient.

As hot rod pointed out, the floor will probably not be sufficient by itself as a heat source. You could use it as a warming system and control the water temp with a thermostatic mixing valve set to 120* max. Make sure that heat transfer plates are installed or you'll get very little output.

If you're set on eliminating the radiator, a toe kick heater could be installed. The largest one is rated about 10-12k btus.

KC_Jones

clifton_clowers said:

Based on both the percent of the house, and the percent of the radiator fins in the kitchen, I think we need about 12000BTU. I know the are better ways to calculate that, but those numbers are consistent.

This sounds like you measured your radiation and basing your heatloss off of that. You need to calculate the heat loss of the space, not measure radiation. I would suggest, it's not so much "better ways", there is a correct way, and then there is everything else.

Slant fin has a nice ap for doing heat loss of a space, I'd suggest downloading that and starting there. Also if you are working with a professional and this has not been suggested, that would be a reason to pause IMHO.

clifton_clowers

Thanks for all the comments.

There is a pro involved - someone with whom I have worked many times in the past and who has always been very good to us - but in this case he is being oddly cryptic about calculated numbers or pricing. But the estimate is in the $ range (with me doing the labor to lay the actual floor) and we feel like we're being gouged.

thermostatic mixing valve is the term I was looking for. Thanks. I'll talk to the plumber about it - I am a pretty competent DIYer, but I would never connect anything to the boiler on my own.

I used an online heat loss calculator at https://www.usboiler.net/heat-loss-calculator.html (slant/fin app kept crashing) and it gives me the same 12000 number However, if we pull down the outside wall (we'd rather not in order to save the old plaster) and add some insulation, it goes down to 7000. I suspect thick plaster being a bit better than drywall in this regard, and many of the walls being inside walls puts the number somewhere in the middle.

We're looking at Warmboard or Thermofin U as the metal plate. The plumber has suggested Viega as he has used it. But to my eye the PEX to aluminum coupling is comparatively poorly done, the aluminum looks too thin and is on the wrong side of the wood panel, and because it uses 5/16" PEX it requires multiple runs of tube, and thus lots of extra hardware.

But I would very much appreciate any comments on the relative merits of any of those heat dissipation options, or any others.

Thanks again for the help.

KC_Jones

The calculator you used is to do a whole house not a room. The calculations you got from that assume that room has all exposed wall, ceiling, and floor this will dramatically skew the numbers high. That is in addition to the padding that is most likely built into that program.

You need a calculator that can do inside walls and rooms that have ceilings within the building envelope.

Your numbers are definitely way high.

clifton_clowers

You are correct. The online Slant/Fin calculator put it at about 4000 BTU/Hr.