What should I do? Convert from oil to NG?

[smyke]

Hello,

I had a thread going in the Water Heater section and some questions had been answered there but per Dana's recommendation I am starting a new thread here to help me figure out what I need to do.

I just purchased a house knowing that the heating will need to be updated.
Electric company did an audit yesterday and found the existing boiler to be 83% efficient (Utica POBT-5 that is rated at 161700BTU).
They did not do a blower door test since I have vermiculite insulation in the attic. Its all covered with plywood but apparently the test cannot be performed for obvious reasons. I didn't know that. If I get the vermiculite tested and if it is asbestos free then they can come out and test. We'll see.

House is a single story ranch with the mentioned attic and a basement that contains 2 single-car garages (separate doors). There is no insulation on the ceiling in the basement.
There are 2 heating zones in the house. One (older section built in 1955) covers about 1060 sq ft (living room, 3 bedrooms, 2 baths and a hallway) with 950" (80') of 1" baseboard. Second zone (addition built around 1975) covers 560 sq ft (kitchen and dining room) with 479" (40') of 3/4" baseboard.
Lady that has been living there since day one liked to keep it pretty warm from what I understand. She had used almost 1000 gallons of oil over the last year. I am attaching the delivery info I received from the oil company.
There is an electric water heater (AO Smith ECT52) that has the incoming water preheated by the boiler.

My initial plan was to convert to natural gas (line at the street, gas company will hook up the house for free) and using a Bosch Combi unit but I am open to other options.
For cooling I was thinking about mini-split system that heats also and use that as supplement to oil or vice versa.
We may just go one winter with everything as is and go from there.

I had one quote done for Bosch (they didnt even give me any other options) for $9000. Their estimate was 52000BTU.

I am in the process of calculating the heat loss with the Slant/Fin but its a bit of a PIA to use.

Any and all help will be greatly appreciated.

Mike

 

[Dana]

Your mid-winter K-factors on the heating bills are 6.5-7 heating degree days per gallon, and those are really the only ones that count, since the others are too heavily skewed by idling losses an hot water heating. Assuming the ridiculously oversized beastie boiler was actually getting 83%(not really, but let's worst-case it), and your K-factor is as high as 7 keeping the place at 70F instead of 77F or whatever (also a stretch) the heat load math goes like this.

Heat output per gallon: 139,000 BTU/gallon x 0.83 eff/= 115,360 BTU/gallon.

7 degree-days/gallon is 1/7= 0.143 gallons per degree day.

That's 115,360 x 0.143= 16,497 BTU per degree-day.

With 24 hours in a day, that becomes 16,497/24= 687 BTU per degree-hour.

Since the heating degree-days in the K-factor are base-65F, the presumptive heating/cooling balance point is about 65F. Her real balance point might have been higher if she kept it at 75F or something, yours might be as low as 60F if you keep it at 68F, but probably not. Being consistent with the model we'll assume 65F. If your outside design temp is +10F, thats' (65F -10F= ) 55F heating degrees.

687 BTU/F-hr x 55F = 37,785 BTU/hr

And that's a worst case, skewing everything to the high side of reality. Reality is probably more like 30-32K.

The smallest oil boilers out there run about 60K, out, which makes them nearly 2x oversized for the load. With high-mass radiation it won't short-cycle, but it'll still lose some efficiency to the oversizing unless you install heat purge controls, etc.

There are many modulating condensing boilers that will deliver that, and still modulate down to under 15,000 BTU/hr (or even under 10,000 BTU/hr -check out HTP's UFT-80, (HTP is built in MA, with good local support) which can modulate between 8000-80,000 BTU/hr- in.) But propane is still pricey in MA compared to natural gas or mini-splits. If the gas company will hook you up for free, a smallest-of-the line mod-con with a min-fire less than 1/3 of your design heat load is the right way to go. Your average wintertime temp is about 30F, which means your average wintertime load is going to be less than (65F-30F) x 687 BTU per degree-hour= 24,000 BTU/hr (and probably more like 21-22K.). Low-mass combi-models modulate very much below that, which means it would be cycling rather than modulating most of the season. What model Bosch were they suggesting, and what is it's min-fire input or output?

A ~30K load is also within range of high efficiency mini-splits. If the floor plan isn't very open and it's all doored off you may have to go with somewhat lower efficiency ducted mini-splits, but even those are WAY cheaper to heat with than condensing propane. The 1.5 ton mini-ducted Fujitsu 18RLFCD is good for about 20,000 BTU/hr and could handle a bunch of doored off bedroom areas as a single zone. A 1-ton -12RLS3H wall-blob unit (or it's floor model cousin) is good for about 13500 BTU/hr @ +10F, and the the 1.25 ton 15RLS3H is good for about 16,000 BTU/hr @+10F. Wall / floor units are fine for more open areas, and if you have a basement to run the ducts in it's pretty easy to install mini-ducted units. Some combination of 2 or 3 mini-splits would be able to cover 100% of your heating design load, with margin. (Mitsubishi has pretty good wall-units, but the mini-duct series is a bit marginal at low temp. They have bigger-deal air handler units for low temp heating, but they're a lot more expensive.)

Even if you're only installing mini-splits sized for the cooling load, if you pay the up-charge for a heat-pump version it'll still carry the lion's share of the heating load, and if you get something with an HSPF in the 12s or higher it'l be cheaper to heat with than condensing natural gas whenever it's above 35F (which is most of the heating season hours in Woburn.)

Slant Fin's somewhat crude I=B=R style load calculation tool regularly overshoots reality by 25-35% but forge ahead anyway, see what it comes up with.

BTW: There's no such thing as 1" baseboard or 3/4" baseboard- are you measuring the pipe, or the depth of the baseboard? If it's baseboard and it's only proud of the wall by that much, they're probably set-into the wall a bit. If they're covered in sheet metal they are fin-tube convectors- pop a cover and measure the size of the convector fins.

It's more worthwhile insulating the WALLS of the basement than the ceiling, and that's triply-true if the boiler is in the basement. If you insulate between the boiler room and the conditioned space of the house you guarantee that the jacket and distribution losses are really lost, rather than accruing to the fully conditioned space. When you get to that point, check back, (or search the remodeling forum on this site for details on how to dot that on the cheap, without creating a mold farm.) In the garage areas it's well-worth air-sealing and insulating the ceiling plane, and the partition walls to the basement area.

 

[Sponsor]

 

[smyke]

They are quoting me Combi 151. A bit too big I guess.
When I said 1" baseboard I meant the pipe diameter.

Slant/Fin is giving me 4566BTU/Hr. Not really sure I did it right.

I attached the picture of the floorplan.
An idea of heating with the heat-pump is very interesting since I wanted to use that equipment for cooling already. If I went that route what should I do with domestic hot water?

 

[Dana]

They are quoting me Combi 151. A bit too big I guess.
When I said 1" baseboard I meant the pipe diameter.

Slant/Fin is giving me 4566BTU/Hr. Not really sure I did it right.

I attached the picture of the floorplan.
An idea of heating with the heat-pump is very interesting since I wanted to use that equipment for cooling already. If I went that route what should I do with domestic hot water?

The min-fire input to the Greenstar 151 is 36,000BTU/hr in, so it's min-fire output is about 34K, which is approximately your whole-house heat load @ 10F, which means it will literally never modulate in heating mode. You'd still need the max-firing rate to be able to run more than one shower or fill a tub at anything like a reasonable rate too. It's a great boiler, for somebody else's house (somebody with 2x your heat load.)

A heat load of 4566 BTU/hr is the wrong order of magnitude. You either did something wrong, or it's a typo.

If that was meant to be 45,660 BTU/hr you might have done it right. Discounting it by 25% for the typical over shoot delivered by that tool would comein at about 34,000 BTU/hr, still a credible number.

Without running the room by room heating and cooling loads (which you should do), and not knowing where the basement is, I could see this layout being served by a 3 mini-split solution, a 3/4 ton wall-coil for the dining/kitchen zone, another 3/4 tonner in the living zone, and a 1-ton or 1.5 ton mini-duct cassette serving the bedrooms & bath zone. Bath 1 would not need a duct register if the rest of the place is heated. Bath 2 might, depending on what the load numbers are. The bulk of the load in that zone is Bedrooms 2 & 3, which have 2x the window are and more than 2x the exterior wall area. Depending on the ceiling height it might even be possible to mount the cassette in the top of a closet or something and route the ducts in soffits under the current ceiling level (but don't punch holes in the ceiling and mount it in the attic no matter what, since that adds to the actual load, and ruins the pressure boundary integrity.)

 

[Dana]

Regarding hot water if going with a mini-split solution, a second generation heat pump water heater with an EF of 3 or better will dehumidify the basement and cost about the same as heating hot water with natural gas. Recovery times are long though, so it may be worth going for an 80 gallon version if more than 2 people are showering back to back, or you're a tub-bathing family. If you're primarily a showering family it may still be worth installing a drainwater heat recovery heat exchanger to both extend apparent-capacity as well as yielding higher efficiency.

It occurs to me that to really REALLY worst-case it, rather than using the winter average K-factor, of ~7, if you take the absolute worst K-factor of 6.5 (the Valentine's Day fill-up) it would make the implied heat load 37,785 x (7/6.5)= 40,692 BTU/hr, not that it changes the rest of the discussion in a meaningful way. At 4x oversizing the as-used efficiency was probably about 70% , which would also knock that ~41K load number back down to ~34K.

Reasearchers at the Brookhaven National Laboratories developed some boiler models based on lab testing, from which (amongst other things) a reasonably accurate heat load could be inferred at any oversizing level based on fuel use. This model is incorporated in to the FSA calculator downloadable from the NORA website. Using that tool, selecting Boston as the the location, entering boiler specs of 161,000 BTU/hr and 83% efficiency, and entering a K-factor of 7 and clicking "Adjust Heat Load" button it it comes up with 33,500 BTU/hr @ -2F. (Why they used the 99.9th percentile temperature bin is beyond me). Entering a K-factor of 6 and re-adjusting, it comes up with a heat load of 39,100 BTU/hr @ -2F.

Taking that 39K @ -2F number (which is worse than worst-case, but they only allow whole numbers in the K-factor, couldn't enter 6.5, only 6 or 7) and interpolating to a design temp of +10F it's an implied heat load of 32,100 BTU/hr @ +10F. Using the K-factor 7's 33.5K @ -2F number it interpolates to 27,500 BTU/hr @ +10F. That's the range you're looking at.