Small Space - Combi or On Demand DHW

[Greg_19]

My apologies if this has already been covered at length - I do recall seeing a thread about a small space, but can't seem to find it when searching...

I'm taking on the rehab of a single floor 425 ft2 one bed/one bath house in the Blackstone Valley (MA/RI). I am looking to replace the dated electric baseboard and DHW tank and go propane fired to maximize the livable space. Clearly, maintaining living space is a major concern and I don't have much area to spare, so it makes sense to me to section off a small utility closet and wall mount what I can.

Having done a bit of reading here and elsewhere, finding a combi boiler with such low min output (roughly 20,ooo) seems to be the challenge. Would I be better off to find the appropriately sized boiler and run a separate unit DHW?

Any thoughts, recommendations, comments, or accusations of lunacy for actually wanting to live in 400 ft2 are welcome...

 

[Dana]

My apologies if this has already been covered at length - I do recall seeing a thread about a small space, but can't seem to find it when searching...

I'm taking on the rehab of a single floor 425 ft2 one bed/one bath house in the Blackstone Valley (MA/RI). I am looking to replace the dated electric baseboard and DHW tank and go propane fired to maximize the livable space. Clearly, maintaining living space is a major concern and I don't have much area to spare, so it makes sense to me to section off a small utility closet and wall mount what I can.

Having done a bit of reading here and elsewhere, finding a combi boiler with such low min output (roughly 20,ooo) seems to be the challenge. Would I be better off to find the appropriately sized boiler and run a separate unit DHW?

Any thoughts, recommendations, comments, or accusations of lunacy for actually wanting to live in 400 ft2 are welcome...

There is essentially no such thing as an "appropriately sized boiler" for the very low loads of n insulated 425' house, which is almost certain to come in under 10,000 BTU/hr (if the windows are closed, that is. ) The best you can do with fossil-burner is a tank type water heater set up as a combi with an external heat exchanger, using the thermal mass of the water to keep it from short cycling. (The HTP Phoenix Light Duty is an appropriate choice here.)

There is such a thing as an appropriately sized ducted or ductless air source mini-split heat pump, and that's what I'd recommend. If it's an open floor plan a single head 3/4 ton Mitsubishi or Fujitsu cold-climate mini-split will usually do it. If the room loads for doored-off rooms are substantial, a 3/4 ton mini-ducted Fujitsu is enough, if there is somewhere to run the ducts, either a crawlspace/basement or the top of a closet between two rooms ( but NOT in an attic, above the insulation.)

The operating cost of a right sized mini-split is about half the cost of heating with condensing propane, at MA style propane & electricity prices, or about 1/3 the cost of heating with electric baseboard. You'll have to do something else for hot water, but these things provide ultra-efficient ultra-quiet cooling as well.

With the kwh usage of few months of electric bills (going back to at least September) and a ZIP code it's possible to calculate the heat load of this place with reasonable accuracty.

 

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[Dana]

Regarding the ducted Fujitsu option, unlike most mini-duct cassettes the Fujitsu units can be mounted vertically, and can built into a "utility closet" or cabinet less than 8 square feet. These are pictures of a 1.5 ton Fujitsu installation heating/cooling a ~1300' house in California (sized for the cooling load), which is bigger than the 3/4- 1 ton version that would cover your heating/cooling loads:

But mounted in the top of a closet just below ceiling level or in a sealed basement/crawlspaceit need not take up any precious floor area at all.

The simpler solution with an open floor plan would be the standard high-wall coil or floor mount:

 

[Greg_19]

Thanks for the info Dana! The mini split seems like the way to go - and will save me a good chunk of change too it appears. Now onto reviewing DHW. Do you (or anyone) have an opinion there? Looking at Takagi, Rinnai, and Rheem and finding pros/cons on each.

 

[Dana]

A plain old electric tank water heater is often cheaper to run than a propane tankless even at New England style electric rates. What is your local propane & electricity pricing?

If propane-tankless, a condensing Rinnai is probably the safest bet. Takagi used to have a great reputation for reliability a decade ago, but scuttlebutt has it that the newer versions aren't living up to that rep. I don't really know about Rheem. For a 1-bathroom house you don't need to go large- 140-150,000 BTU/hr is enough to support a full flow shower with margin to spare.

If going with a mini-split stick with cold-climate versions only, and from a vendor well-supported regionally. That pretty much narrows it down to Mitsubishi (which has the regional training & design center in Southborough MA) and Fujitsu, though LG has some value-priced models that might be considered.

Be sure to mount the outdoor unit where it won't get clobbered by falling ice dams or roof avalanches (under protective eave or rake overhangs is good, or build out a small shed roof) , on a stand or bracket-mounted on the wall with the base pan above the historical snowpack depth. (At the head of the Blackstone in Worcester that's about 4' off the ground). You don't want to be one of those people digging up the mini-split after every Nor'Easter. With sufficiently deep & protective roof overhangs it can be lower, but don't install it flat on the deck at ground level as if it were merely an air conditioner.

 

[Dana]

BTW: If you're on the MA side of the line it's almost certain that National Grid is your electric utility, which means as an electric heating customer you would qualify for a $1000/ton rebate subsidy for installing a cold-climate mini-split if the equipment is listed on the NEEP cold climate heat pump spreadsheet.

That list is currently undergoing revision, but the 3/4 ton Fujitsu 9RLS3H and Mitsubishi FH09NA are both on prior revisions. In competitive bidding the all-in installed price for either of those would be less than $3.5K, maybe even less than $3K, depending on how hungry the installers are this winter/spring, so after rebate it should run $2-2.5K net out of pocket.