Advice on new gas boiler install


Old 11-30-11, 09:45 AM
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Advice on new gas boiler install

Hello -

I'm looking at replacing my 40 year old oil boiler with an efficient gas boiler. The house itself is 100 years old, with cast iron radiators (as well as a few in-wall radiators).

I'm awaiting bids from a few companies, but I had a few questions for the pros here as well:

1) We're thinking of doing a remodel at some point in the future, which would add some square footage (~25% more), but it's probably 3-4 years down the road. How should I size the boiler to account for this (most have recommended Burnham alpine). Since it's a modulating boiler, do I risk wasting a lot of energy on having a boiler that is too big, considering that I don't want to end up with one that is too small in the future?

2) I have a 5 year old water heater. Our current monthly gas bill is about $50 (hot water, gas stove). I understand an indirect tank is more efficient, but I'm wondering if the cost savings are there, considering the indirect might add $2-3k to the cost (while I'm probably only spending ~$500 a year on hot water currently). Is it problematic to do an indirect tank later, once my current water tank has completed its lifetime?

3) I would like to do a heat loss calculation as explained in the sticky, but I didn't see a link for how to do that well (or how to ensure my contractor does it well). Does anyone have advice on that?

4) I noticed a few people mount these boilers on the wall. What are the pros/cons of that?

5) Anything else I should be aware of / wary of when reviewing the bids?

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Old 11-30-11, 07:38 PM
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Hi and welcome. Great that you are thinking and asking these kinds of questions. Two overriding considerations: a) if you don't HAVE to replace the boiler in the winter, don't. Contractors are busy fixing no-heat calls and other crises. Have the work done in spring-fall (although summer can be busy for them with no-AC calls, too). b) before doing anything with boiler sizing or replacement, tighten up the home by insulating and air sealing. Especially for a 100-year old house, you can easily cut the heat loss by half -- and thus the heating requirement and $$ spent heating it. That means getting a good energy audit that includes a blower-door test and infrared imaging to guide insulation and air sealing improvements. It's some of the best $200-400 (or so) you can spend on your house.

As to the questions.

1) Despite some claims to the contrary, it is absolutely possible to oversize a modulating-condensing boiler. The bigger they are, the higher the low-end output. When they bottom on minimum modulation, they short cycle, which is inefficient and adds wear and tear. For a mere 25% increase in footprint, I wouldn't worry about it. You could also "model" it in heat loss software, by adding another room or two. Or three. And if you build it right, the heating requirement will be small and in the noise of the slop in boiler sizing and heat loss calculation. I've added 20% area to my house over the years, and along with insulation and air sealing of the rest of the house, the overall heat loss is nearly half what it was in the late 20th century....

2) I am biased towards indirects, and if you will be replacing the water heater anyway, I'd go ahead. Or at least have your boiler's piping set up with supply and return stubs to accommodate a future indirect. Indirects fired off a modcon can be very efficient. And it gives your boiler something to do in the summer, too, rather than just sit in a humid basement.

3) You can get the free Slantfin heat loss software here:

As above, you can use the software to look at a potential addition, and also to estimate what adding insulation and air sealing will do to reduce heat loss of your existing building.

There are also modest cost programs out there whose names I can't remember.

4) Walls are nice for getting the boiler off the floor and out of the way. Especially if your basement ever floods (e.g., if you live near a river).

5) Take your time. Shop the installer, not the brand of boiler. Generally speaking the major manufacturers produce good modcons, and both they and the contractor will stand behind them. A good contractor will do a heat loss, or charge you for it and credit the charge if you hire him for the job. A good contractor will be very familiar with the brands he sells, and likely have attended factory training. Ask for pictures of their installs. The piping and wiring should be neat, clean, and laid out with ease of maintenance in mind. A good contractor will see your cast iron radiators and say that they are an outstanding match for a modcon boiler because of their high thermal mass and high-volume water content. A really perceptive contractor will also note the potential need for a two-temperature system to provide lower-temp water to the radiators, and higher-temp water to the copper-fin-tube in-wall convectors. But that will depend on a variety of factors that he will discuss.

Ask more questions, post your contractor impressions, etc. Plenty of good folks here to help.
Old 12-01-11, 05:34 PM
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Just to add to what else was stated here is the Alpine allows you to turn down the fan speed in heat and DHW separately so it actually reduces the input (creates a smaller boiler). Then when the addition goes in, turn the fan speed back up to what you want so you get more heat. My guess 80% of the mod/cons installed could be reduced by 30 - 50%.
Old 12-12-11, 10:14 AM
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Thanks so much for the feedback. I've been waiting to get the bids in, now I have them (I got 3 bids from 3 companies that had good ratings in my area).

To respond to your questions/suggestions:
b) before doing anything with boiler sizing or replacement, tighten up the home by insulating and air sealing
We did a full house insulation and air sealing last year, with one of the companies now bidding the boiler install, we did a blower door test, infra-red cam, the works. It definitely made a difference - the attic is actually cold now in the winter. We didn't insulate the walls, but did floors and ceilings and crawl spaces were done.

I'm now trying to compare the bids. It's tricky, as they all use slightly different language to describe what is included. As all 3 bids are for the same boiler (Burham Alpine 150). While the bids have some commonalities, I've isolated the following terms which one or two but not all three use - can someone explain what these are, and if they are all 'optional' or always included (in which case, perhaps some bids simply didn't list them):
low water cut-off
spiro air elimination system
new circulators
boiler drains
flow check air vent
water feeder/backflow valve
acquastat relay

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