Caculating Water Heater Size for Radiant

[kwright]

I originally had very new but used water heater lined up to use as the boiler for my radiant heat system (which is just about to be installed). Due to unforeseen circumstances, the planned water heater will not work for my install, so I have to go out and buy a new one.

Given several other constraints, a propane-fired conventional water heater is the plan (I'm not going to pursue tankless or electric right now).

My garage, which is 28'x36' will have all the standard stuff... good insulation in the walls and ceiling, two inches of sheet foam under the floor and around the inside of the poured wall, etc. The PEX is being installed by a professional, using a four loop system to cover the full 28'x36'. I live in Michigan, so the winters can be pretty rough.

The heating guy knew all along I was intending to install my own water heater - which he didn't have an issue with. However, now that I'm buying a new unit, I'm intending to ask him his opinion on how big of a heater he believes I need.

My question for the group here is, any thoughts or advice on how large of a water heater to get, given what I've shared regarding my system? Since I'm buying new, size is an open option.

I'm interested in having others opinions in my pocket before I talk to the heating guy. Note that I really don't think he's going to try to sell me anything, I just want others views on the appropriate size water heater to get.

Perhaps there is a way to calculate the size heater required, given the amount of PEX and such, though I haven't found any calculator of this type here. Unless I'm looking in the wrong place (entirely possible, I might add).

Thanks...

 

[Steve in Mi]

Makes me wonder how you figured/sized the first water heater to do the job if you are asking now how to figure what you need. Anyway, you can use the calculator here to get an idea of Btu's needed to heat the space and match that to the Btu output of water heaters - I think that could be a place to start you search.

http://www.heatershop.com/btu_calculator.htm

 

[ehjorten]

You need to calculate the heat loss of the building to understand how many btu's you need to heat it to a certain temp. If you have a professional installing the PEX tubing I would imagine that they could calculate (and should have calculated) the heat loss for you to install the tubing correctly.

 

[kwright]

Makes me wonder how you figured/sized the first water heater to do the job if you are asking now how to figure what you need. Anyway, you can use the calculator here to get an idea of Btu's needed to heat the space and match that to the Btu output of water heaters - I think that could be a place to start you search.

http://www.heatershop.com/btu_calculator.htm

The original water heater was a 49 gallon unit, which the contractor knew about. I'd asked him about the 49 gallon size and it's applicability given the garage design and size, and he indicated it would work fine.

We did not discuss details or specifics beyond this.

Thanks for everyone's input... I appreciate it.

 

[z28toz06]

Make sure you get a heater with sealed igniter so you don't have to worry about fumes igniting. Also it makes sense to get an ambient outside temp controller.

http://www.tekmarcontrols.com/consumer.html

 

[badgerboiler]

Heat loss is the first thing. Your insulation is marginal at best. Tube spacing is fine.

Size volume of your water heater has nothing to do with heating your slab. The btuh output is the only consideration after accepting the pathetic fuel efficiency.

Heaven forbid if your contractor tries to "sell you" something, but if he offers to install your water heater, design and install the close piping and controlls I would give him a chance.

It is not easy, but if done right will serve your purpose very well.

The standard for insulation in MI is 2" of closed cell polystyrene at least four feet from the perimeter in. Insulate the edges at least and go down two feet around the outside if the backhoe is still around.

MA