Radiant Heat Question

[alfrescoguy]

Hello Folks, My new house is under construction and I'm planning a separate 40 gal water heater to feed my radiant heated garage floors. The plan is for the 40 to sit next to the 75 gal that feeds the house in the basement.

My question is do you recommend connecting them for any reason (maybe with a shutoff valve) so the water in the garage floor gets circulated, used, then freshened during the winter months when I'm running it, or do I have separate systems with the 40/radiant heat then being a closed system where the water really never gets refreshed?

 

[yeldogt]

Did you do a load calculation? A typical 40g water heater only has a 26k burner .. at 80%. I'm assuming you have NG.

What is heating the house ? 75g is big ...... how about a polaris with a loop for the garage?

Edit: I would not connect --- the whole idea of an open loop system is bad IMO.

 

[raspy]

To answer your question: There is no need to connect the two water heaters mentioned. Closed loop radiant does not need to be "freshened". Keep the two systems separate. Or, if you wish, you could use domestic PEX in the radiant system, along with all domestic compatible pumps and equipment, and run domestic water through it with only the 75 gallon water heater doing both jobs. However, that is not the best method.

Remember: It's not the gallons that heats the floor, it's the BTU input to the heater. The number of gallons means nothing, what size is the burner?

 

[alfrescoguy]

Thanks for the info! Yes, the std 40 gal is spec'd NG (don't know BTUs, but assume 26K), I doubt any load calc was done, and I think the builder already bought the heater.

Sounds like 100% voting to keep 2 systems separate. Thx! The 75 is anticipated to supply my hot water consuming family in our force air house. Guess part of me was thinking the radiant garage water might get sediment or funky, but then algae can't grow where air and light can't get to it.

I will check to see if it's not too late to look at Polaris as an option. What model would you recommend to heat a 4 car in the midwest?

 

[slowTA]

Also, with 2 separate systems you can add antifreeze in case you ever decide to shut off the garage heat during the winter.

 

[engineer2]

My plumber friend told me according to code, a radiant heating system must be separate from potable water. Perhaps a plumber here can post as to the logic behind this.

How many sq ft? How well insulated? What type of winter climate?
Your water heater data plate probably lists BTU consumption and an EF rating.

From that you get BTU output.
Approximate BTU consumption of the system is easy to calculate from pump gpm and inlet/outlet temperature differential.
From that you get the amount of BTUs going into your garage. Figure around 10,000 to 12,000 BTU needed per car space as a very rough estimate for cold midwest winters.

 

[HoosierBuddy]

You would definitely not want the systems to be connected. In fact, I think a plumber would back me up that the radiant system would need a backflow preventer in the makeup water to prevent even a minute amount of the water from that system backfeeding into the potable water.

I think the fear is tepid water in the radiant system could be a breeding ground for the micro-organisms that can cause legionaries disease which could (in theory) be aspirated if someone was exposed to them in a shower.

I haven't looked at your home plans but I'm a bit baffled by a 75 gallon NG water heater for domestic hot water. OK. Maybe this is a mcMansion and has a spa tub or two somewhere? If so...why is he cutting corners by specifiying a water heater to handle the radiant duties?

Has he even considered a boiler that can handle the radiant AND can provide separate hot water?

Something is not adding up for me. You didn't say...so I guess I'll ask...is the 75 gallon water heater ELECTRIC? Please tell me my suspicion is baseless.

Phil

 

[raspy]

engineer,

Calculating consumption based on in and out temps and flow rates will give you somewhat meaningless numbers.

When a slab system first starts out, with a cold slab, it could easily absorb 100 BTU. per foot, but that has nothing to do with the heat load of the building. When up to temp, a well insulated home might have a heat load of 10 BTU per foot. A multi-zone system can have a lower overall BTU delivery than a single zone system, and should have.

This is where designing and control strategy gets more complicated. You have the mass of the slab, the specific heat of the material times the quantity. A lot of energy is required to change the floor temperature. Then, as the floor temp rises, you have the increasing heat loss of the structure. As the temp comes up, you have a longer and longer conducting radius from the tubes to even out the floor temp, so you have a smaller temperature differential from hot supply to floor. Smaller differential means lower BTU delivery. As time goes by, the walls and other surfaces are also heated indirectly and the losses to the outside increase.

A modulating boiler is good for overcoming the mass and throttling back to meet the heat load. Lots of power in the beginning, and a cruise to meet the load. Just like accelerating a car up to speed on the highway and then throttling back to maintain a steady speed.

 

[engineer2]

Thank you for explaining the dynamics. I guess I should have said "at a given point in time..."

Mine consumes about 8,000-10,000 BTUs in steady state conditions. 500 sq ft basement floor. Been running for 14 years without a problem. Right now it's on the domestic NG 50 gal water heater, but it will be separated once I expand the system into the unfinished area of the basement.