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Rick
Rick, HVAC Supervisor
Category: HVAC
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Experience:  40+ yrs. experience as a licensed oil & gas technician.
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What size boiler do I need for a 4,000 sq/ft house in New England,

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What size boiler do I need for a 4,000 sq/ft house in New England, with a high-hot water usage?
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There is no way I can answer this question without doing a heat loss analysis. To do that I need a lot more info. I need to know the total length of all the outside walls. The dimensions (footprint) of the foundation. How much insulation in the floors walls and ceilings, How many floors, ceiling height, total square footage of the glass area of all the windows, are the windows thermopane or storm windows, how many exterior doors, and where in New England you live.
Customer: replied 4 years ago.

I understand that it is not that simple. For starters the house is in southern CT. The house is a cluster-mess. It started as a 2,000 square ft. with a Peerless WV3 - max 1.1gph. He has doubled the size of the house, and now has added the hot water load.


 


From what I gather, the outside perimeter of the house has doubled, and the building materials are practically the same.


 


There will be four adults and two children living in the house, and the boiler is going to be piped with an indirect.


 


I know that doesn't help too much. Do you know of any heat loss calculation freeware out there so I don't just tell him he needs a boiler fired at a little over 2.2 gph?

There is no free heat loss software that I'm aware of and I just went looking not to long ago. I can tell you this as a starting point. A well insulated 2000 sq ft house with more than the usual amount of glass on the MA North Shore would have a heat loss of about 50k. So if you double that and assuming decent insulation and the fact that design temp for Southern CT is about 10 degrees lower than NE MA. I'd say a 3 section oil fired Peerless would be enough for space heating BUT no where near enough for high demand indirect hot water. If you size the boiler based on the indirect's demand you should have plenty for space heating. For example a SuperStor SSU80 requires 212k btu's to meet it's speced recovery rate. An SSU 60 needs 174k. I wouldn't go with anything smaller than an SSU60 for high DHW demand and would really recommend an SSU 80. An SSU 60 specs out at 370 gallons first hour rating at 115 degrees.
Customer: replied 4 years ago.

Just so I have this right (please correct me where I'm wrong):


 


1.00 GPH #2 Oil has usable of 112,000BTU's (roughly)


 


A good guesstamate of heat needs: 150,000 BTU's


 


SSU80 Needs: 212,000 BTU's


 


Therefore we would need a boiler firing at about 3GPH???


 


Are you familiar with Buderus products at all? I was looking at Buderus G215 Boiler with a dual-coil indirect (both coils piped together): http://www.buderus.us/files/201001230218160.SM300-400TankBrochure.pdf


 

The amount of usable heat that you get out of a gallon of fuel depends on system efficiency so there is no rule of thumb. You don't focus on firing rates the focus is on the net output of the boiler. Not a big fan of Buderus, they make a good product but they're overpriced and not American made. Nothing to stop them from pulling out of the US like Vailant did a while back. You're link takes me to a solar water heater so there is no way to compare the spec to the SuperStor. I would stick with SuperStor, good track record, lifetime warranty and a very competitive price
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Customer: replied 4 years ago.

FYI: If you use the Solar tank like the one I sent you the link to, you can just pipe the supply from the boiler to the upper coil in the tank, pipe the outlet of the upper coil to the inlet of the lower coil, and then the outlet of the lower coil to the return on the boiler and you will get twice the bang for the buck. If you have 160 supply on a single coil you get 140 out, on a dual-coil that same 160 will come out 120.........

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..........helps in high-demand indirect situations.

The problem is you can't know ahead of time what the recovery rate is for this unit since it has no specs for boiler feed to both coils