Sizing For New Propane Boiler

derekw

Hello,
My wife and I are considering replacing an older oil-fired boiler with one using propane (no natural gas where we live in Maine). I used a Weil-McLain worksheet to compute the heat loss for our home: between 75,000 and 80,000 btu/hr, depending on construction assumptions. We have Slant/Fin baseboard heating which I measured, and which should produce (approximately) 105,000 btu/hr@ 180 degree water temp; 50,000 @ 130 degree temp.
The original Burnham oil boiler installed in the home was rated for 190,000 btu/hr. A local propane contractor has suggested installing a Bradford-White Brute 199 combi condensing boiler (199Mbh). The company also makes a Brute 140.
Given the heat loss assessments for the house, I asked the contractor if the Brute 199 was possibly oversized for the home’s needs. His response was that this boiler could modulate output as far as 10:1 down from full power. Again, given the heat loss analysis, would the Brute 140 seem to be a better fit?
Any perspective you could offer is appreciated!
Thank you.

Jamie Hall

Any boiler should be sized to accommodate the expected heating load (note that for steam, this is the capacity of the radiation, not the structure). Especially in the case of a mod/con boiler feeding a hot water heating system, there's no point in having the maximum output of the boiler exceeding the heat loss of the structure, never mind the heating capacity of the installed radiation.

So... you have an expected heat loss of around 80,000 BTU/hr. You have installed radiation capable of handling 105,000 BTU/hr at 180 -- the maximum any hot water system should ever run at.

Therefore...

There's no point in selecting a boiler capable of producing more heat than that at full power. From where I sit, even the Brute 140 is too big. The Brute 199 is way too big.

derekw

Thanks very much for your feedback Jamie!

hot_rod

In reality, you have to size to the larger load. If for some odd reason they demand 5- 6 or more gpm of DHW then the boiler needs to size to that. In a combi or tankless you need the high BTU input to generate continuous hot water. With either you don't have the luxury of a tank to buffer the high flow dump loads.

It is getting more common to see DHW loads exceed the heating loads, smaller homes, more energy efficient homes. On the hot water side, larger and more shower heads, body sprays, large tubs with impatient bathers, etc. I suppose if a consumer is expecting and willing to pay for large flow endless DHW, someone will provide it.

I'm a fan of combi boilers, knowing when it makes more sense to have separates is the key.

IF you have a very load heat load, and want a lot of endless DHW the boiler is going the be oversized for heating all of the time, and really oversized on the more common low load days.

Until wehave wider or endless turndown ratios, this will be the case.