Heat calc

[nate379]

I have never had an issue with my house staying warm, but have a question.

I was b/sing with the neighbor this afternoon in the garage and somehow got on topic about his boiler when I saw his pressure gauge and it was at 0 psi.

Anyhow, our houses were built by the same company. His is about 1200sq ft with 525 sq ft garage and mine is 1400 with 625 sq foot garage.

Both slab heat, more or less same system.

Only thing is his place has a 120,000 BTU boiler and mine is only 90,000BTU. His place was built nearly a year after mine, so not simple as wrong boiler in wrong house.

Just kind of wondering if there would be any rhyme or reason to that?

 

[danski0224]

Probably what the heating company got the best deal on at the time.

Not enough money in most residential new construction to "do it right".

 

[nate379]

I hear you there.

Had drywallers in yesterday to patch 2 holes in the wall from the plumber (had a vent stack leaking). Even talked to them about taking the time to do it right and I'd slip them each a $20.

Work was "ok" but not to my standards. The texture wasn't close to matching and mud job was "eh". Bad enough that I re-did it last night and got much better results... and I'm just average joe that does drywall a few times a year.

 

[redsky49]

The square footage of a house - essentially its "footprint" - has little or no bearing on the heating requirements. Either house could be correctly sized, or incorrectly sized as the case may be. You will only know for certain by performing a proper heat loss calculation.

In addition, the actual operating efficiencies of the two boilers could also come into play. I suspect that the boiler capacity numbers you are using may be the input capacities. Boilers will vary from about 78% (older units) to approaching 96-97% efficiency. This alone could account for the differences between the two boilers.

Also, in most heating systems there is a fudge factor included to account for system "take up" (the ability to quickly heat up a cold building), system deterioration over time, etc. This discretionary number varies between designers and applications. Generally it ranges from 10-20%. As opposed to cooling units, there is no benefit to slightly undersizing the heating system.

Boilers also are available in incremental sizes that may cause the installer to select a boiler that exceeds that calculated load, simply because the next smaller unit would be way undersized. You generally can't select a boiler at the precise calculated load point.

Other factors may include the form factor of the boiler, whether it's a cast iron sectional type, a high efficiency condensing boiler, if there are plans to use the boiler for domestic hot water, future expansion of the heated space, etc. Flue requirements may also come into the decision.

A lot goes into the correct boiler selection.

As always, offered only as opinion