hlv
Harry Voorhees
I'm planning to replace my 33-year old natural gas Teledyne-Lars boiler with a high efficiency unit, to take advantage of Massachusetts' early boiler replacement program rebate, and because it doesn't seem worth throwing money at it to fix the noise (flashing?) issues which have worsened over the last couple years. Since the standalone gas hot water heater is also vented to an unlined chimney, it would need to be replaced too.
This forum has been immensely helpful in understanding boiler sizing, and I have a couple questions regarding my heat loss calculation and options that have been proposed.
My design day heat loss, computed from actual gas usage (minus a bit for DHW, cooking and drying) and degree day data is 48,000 BTU/hr. This is based on 77% efficiency measurement of the boiler stack pipe during my last energy audit. Would that be the right efficiently number to use for this calculation or should it be discounted somehow?
The load estimate corresponds pretty well to my energy-modeled design heat loss of 46,000 BTU/hr for this 2-story house with unfinished attic and cellar, 2200 sf of conditioned space, cellulose in walls and attic floor, fiberglass batts sort of held against the cellar ceiling by wire supports, and caulked interior trim joints. Someday I might add a third (short) zone to the attic, which may require 4,000 BTU/hr more. Maybe a mini-split would be a better choice there for year-round conditioning, or maybe the increased heat load can be offset by air sealing improvements (currently about 8 air changes per hour at ACH50). So overall needs might be 48-52K BTU.
Here are three options that were quoted:
* Lochnivar-Knight WHN055 (11-55K BTU input) or WHN085 (17-85K BTU input) with 40-gallon indirect (model depending on heat loss calc)
* Buderus GB142/24 (23-83K BTU input) with 36-gallon indirect
* Navian CH-210 combi unit (20-175K BTU input)
Dana, jadnashua, and BoilerBadger have emphasized the importance of not over-sizing a boiler, and preferring low minimum output, but I'm wondering how close should one cut it? For example, at 51K max output is the smaller Loch-Knight cutting it too close? What about on a design day morning when hot water becomes a priority? My friend's REM/Rate program spit out something like "Required: 50K BTU, Specified: 80K BTU". Why such a plus up? That seems like a lot for piping losses (which I think are already accounted for in the estimate based on actual usage).
The Navien combi unit is a nice package and would save about $2K over a separate tank setup. Is there reason to believe that the combi unit would have less longevity or require more service than the bulkier option? A slightly smaller model is available, although I would be concerned about supporting 2 showers with winter water temps (3.6 and 4.0 GPM @ 77F rise respectively).
Any answers or suggestions would be appreciated.
This forum has been immensely helpful in understanding boiler sizing, and I have a couple questions regarding my heat loss calculation and options that have been proposed.
My design day heat loss, computed from actual gas usage (minus a bit for DHW, cooking and drying) and degree day data is 48,000 BTU/hr. This is based on 77% efficiency measurement of the boiler stack pipe during my last energy audit. Would that be the right efficiently number to use for this calculation or should it be discounted somehow?
The load estimate corresponds pretty well to my energy-modeled design heat loss of 46,000 BTU/hr for this 2-story house with unfinished attic and cellar, 2200 sf of conditioned space, cellulose in walls and attic floor, fiberglass batts sort of held against the cellar ceiling by wire supports, and caulked interior trim joints. Someday I might add a third (short) zone to the attic, which may require 4,000 BTU/hr more. Maybe a mini-split would be a better choice there for year-round conditioning, or maybe the increased heat load can be offset by air sealing improvements (currently about 8 air changes per hour at ACH50). So overall needs might be 48-52K BTU.
Here are three options that were quoted:
* Lochnivar-Knight WHN055 (11-55K BTU input) or WHN085 (17-85K BTU input) with 40-gallon indirect (model depending on heat loss calc)
* Buderus GB142/24 (23-83K BTU input) with 36-gallon indirect
* Navian CH-210 combi unit (20-175K BTU input)
Dana, jadnashua, and BoilerBadger have emphasized the importance of not over-sizing a boiler, and preferring low minimum output, but I'm wondering how close should one cut it? For example, at 51K max output is the smaller Loch-Knight cutting it too close? What about on a design day morning when hot water becomes a priority? My friend's REM/Rate program spit out something like "Required: 50K BTU, Specified: 80K BTU". Why such a plus up? That seems like a lot for piping losses (which I think are already accounted for in the estimate based on actual usage).
The Navien combi unit is a nice package and would save about $2K over a separate tank setup. Is there reason to believe that the combi unit would have less longevity or require more service than the bulkier option? A slightly smaller model is available, although I would be concerned about supporting 2 showers with winter water temps (3.6 and 4.0 GPM @ 77F rise respectively).
Any answers or suggestions would be appreciated.
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