Considering a Tankless Combo Unit

[adefeatedman]

Hello all:

The pump on my boiler went out yesterday and the pilot on my hot water heater has been going out every couple of days. My wife and I are considering an expansive upgrade to make sure everything will be working during the winter season (we are expecting our first born in January).

I live about 30 minutes north of NYC. Some details on our house, we have natural gas fuel, it's a 3000 sq.ft colonial with no basement. There are three full baths in the house. The house is currently divided into three zones with radiant baseboard heating. The entire upstairs (4 bedrooms) has its own zone with about 88 linear feet of baseboards. The main area downstairs has its own zone with about 67 linear feet of baseboards and the fifth bedroom and third bathroom in the corner of the house has its own zone with about 13 linear feet of baseboards (this area we rarely use).

The kicker and motivating factor is our utility room is very small and shared with the washer and dryer. A few pictures of the space issue is attached to the post. Saving money on fuel costs is always a bonus, but the added space we would gain by being able to ditch the hot water heater and furnace for a wall mounted unit would be a great benefit on laundry day and would definitely increase the home's value down the road.

My question is, will a combo unit be sufficient for my usage? If so, what recommendations do you all have for me?

Thank you and any help is greatly appreciated.

-Jacob

 

[Dana]

First, since you have a heating history on the place, run a fuel-use based heat load calculation, to put a firm upper bound on the size needed for space heating.

The problem with tankless combis is that they are best suited to houses with low domestic hot water needs, and high heat loads. I suspect you have only a small to moderate heat load, and with three full bathrooms, a higher than average hot water load.

Run the heat load numbers first, and measure up the radiation (zone by zone- it matters ). The 67' & 88' zones will probably be fine with a right sized condensing boiler, but you may want to do something with the 13' zone. But the total radiation and total load numbers will be necessary to ball-park the peak & average space heating water temps required. Most wall hung combi boilers big enough to serve three bathrooms won't modulate low enough on the space heating side to avoid short-cycling itself into low efficiency and a shorter lifespan.

I strongly suspect you'd do just fine with a tank-type condensing combi, with a ~175- 00,000 BTU/hr modulating burner (half that of tankless combi that would come anywhere near serving 3 bathrooms), or at most an 130KBTU/hr HTP Versa. That would free up the space currently occupied by the boiler. The inherent thermal mass of a tank type combi keeps it from short cycling itself to death on zone calls, even though the minimum firing rate is higher than your average winter load (or any individual zone's load.)

The other possibility would be a right-sized wall hung condensing boiler (probably -50-60,000 BTU/hr, since even at 180F entering water temp the 88' of baseboard delivers only about 45,000 BTU/hr, the whole 168' can only deliver 84,000 BTU/hr but most heating systems have more than 1.5x the radiation needed.) and an indirect fired water heater running as a zone off the boiler. With the indirect given priority by the zone controller it'll get the full output of the boiler. Even with a 50K condensing boiler behind it a 50-60 gallon indirect will have faster recovery than most standalone 50 gallon water heaters such as the one in your picture.