Converting House to Gas -- Combo boiler or traditional?

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valethor

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Hi -
I am on Long Island, NY. I am converting my house from oil heat to gas heat and I am hearing different things from the different companies I am quoting regarding whether I should do a combination boiler/hot water tankless unit or a traditional boiler + water heater.

My house is about 3000 square feet. We have four full bathrooms, but right now it's just my wife, myself, and our 18 month old.

I understand there are three options:
Direct water heater + boiler
Indirect water heater + boiler
Combination tankless unit

One plumber is suggesting the indirect option and says the tankless are much more difficult to service, require annual maintenance, and availability of parts can sometimes be an issue. The other plumber is suggesting that the combination tankless unit is the way to go and sells the "infinite water" pitch to me.

I understand a tankless is a little worse at simultaneous hot water (two showers at once, or dishwasher and shower at the same time, etc.)

What setup do you guys recommend?
 

Sylvan

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I like a separate hot water source instead of having a boiler running all year long to produce HW
 

Dana

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Combi boilers are really only a perfect fit for homes with moderate hot water needs and unusually high heat loads. That's probably not your house, assuming you have glass in the windows and doors that shut.

Any 2-bath house on L.I. would need a 199KBTU/hr combi to provide decent hot water performance, but those combis have minimum firing rates in the 18-20,000 BTU/hr range (sometimes higher) , which is probably WAY over the average wintertime heat load of any individual zone, and often more heat than some of zones' radiation can emit into the house at temperatures that yield condensing efficiency.

A reasonably tight 2x4 framed 3000' house on L.I. with clear glass storm windows over Levittown-vintage single panes would have a 99% design heat load of about 45,000BTU/hr @ +15F outside (the design temp for most of L.I.). A tight newer 2x6 framed house with low-E glass and some foundation insulation would have a heat load in the 28K-35K range. To get a handle on what YOUR 99% heat load is, run this math.

With the heat load in hand, then measure up your radiation, zone by zone. The zones with the least amount of radiation are important for figuring out how prone the system will be to short-cycling the combi boiler or boiler into an early grave or low efficiency. Then run this math to figure out the lowest firing rate needed to yield high combustion efficiency without short-cycling. If your radiation is cast iron radiators with some internal volume (not fin tube baseboards or tall fin-tube convectors) there is usually some amount of forgiveness due the thermal mass of the radiation itself, but if it's all fin-tube baseboard it's tough.

As an example, say you buy a big combi-boiler with a min-firing rate of 20,000 BTU/hr-in, yielding (at 95% efficiency) 19,000 BTU/hr, and the heat emitters are all fin-tube baseboard, and the house is broken up into a couple of zones (first floor + upstairs). A temps low enough to deliver that 95% efficiency typical fin-tube will only be emitting about 200 BTU/hr per foot, so for the zone to emit the full 19K perfectly with no cycling the zone would need about 19,000/200= 95 feet of baseboard. If the zone only has 65' of baseboard it's going to cycle, and it would need some amount of programmable tweaking on the boiler's controls to keep the numbers of cycles low while keeping efficiency high, which is at least sort of do-able. But if the zone has less than 50' of baseboard per zone with less than 100' for the entire house added up it's going to be a problem.

With mod-con boilers the minimum firing rates can go quite a bit lower. Many 50-80,000 BTU/hr boilers have minimum firing rates less than 8000 BTU/hr out, which only takes 40' of baseboard to balance at condensing temps.

With an indirect fired water heater zoned as the priority zone the recovery rate for a 50 gallon water heater is reasonably short- too short for it to create comfort issues for the heating zones that are being suppressed by giving 100% of the boiler output to the water heater for that period (that is, unless yours is a 6 person household all taking early AM showers within a one hour time window or something.) If the only kid in the house is 18months old you have a lot more procreating to do before that became an issue. :)

So DO run the fuel-used based heat load numbers first, then measure up the radiation. With those two numbers you can figure out roughly the water temp needed to deliver that load at design temp, and work backward from there to dial in the "outdoor reset" curve using the outdoor temperature sensor to reduce the water temp to the minimum needed during the other ~8675 hours in a year, without short-cycling the boiler.
 
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