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Thread: Boiler and Water heater sizing and pricing help

  1. #1
    DIY Junior Member Swilmans's Avatar
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    Default Boiler and Water heater sizing and pricing help

    Hello all!

    I have just purchased a house that was built in 1957 and is in need of a new hot water heater and boiler. As of right now it has an oil burner and hot water tank. I am converting over to gas and was wondering what sizes and types I should be looking into. Ive has estimates from 2 companies but haven't really received a lot of information from them with the estimates other than prices and efficiency. The home is 950sqft, the attic has been re insulated and the home has all new windows. I also have an oversized wood burning stove that I plan on using a lot. This is my first home and first time with renovations so if more information is needed please let me know. The estimates were basically-

    98% efficiency- $5000 material $2500 labor

    85% efficency- $4000 material $2000 labor

    75% efficiency- $3000 material $2000 labor

    Also a permit for the gas conversion is needed and they are adding $1750 for the permit and 2 pressure tests (there is gas already in the basement run for the stove)

    Any advice would be greatly appreciated
    Last edited by Swilmans; 08-06-2013 at 06:56 PM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Ask any of them if they did a manual-j heat load analysis for your house. The vast majority of guys just do a quick WAG on it, and almost always oversize the thing, often by a factor of as much as 5x. This is a recipe for disaster.

    Without knowing what size units they are proposing, and whether they actually did any sort of valid heat load, those numbers are almost entirely useless. If you read some of the other threads here, you'll get an idea what's involved. If I was to give it a WAG, I'd guess you probably could get by with pretty much the smallest boiler you can get, which is in the area of 50KBTU or so. A mod-con that can modulate down to a small number most of the time would be able to meet those efficiency numbers for at least part of the year. One that is oversized will take a MAJOR efficiency hit, and not provide the comfort you want.

    The LAST thing you want is to copy the size of what you have, since in 1957, they were almost always way oversized. Nor, do you want them to measure how much baseboard heater you have and say you need such and such a boiler. You also cannot just say you need x BTU per sq foot. NONE of those are a valid means of sizing your boiler! Nor is getting a bigger one to heat the water tank.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    A typical 1000' house on Long Island built in the 1950s with at least R11s in the walls would have a heat load well under 20,000BTU/hr, and even if it has NO wall insulation at all it would be under 35,000BTU/hr so it doesn't take a Manual-J to size the boiler- the very smallest of the line of ANY of the major boiler vendors would handle the load, and may already be oversized by 2x or more.

    A $7500 total cost for a 98% efficiency boiler would be CHEAP in my neighborhood (central MA).

    A 75% efficiency burner wouldn't be code-legal to install in my neighborhood (unless it's a conversion burner for the existing boiler, which should be well under the quoted $5K installed price.)

    An 85% efficiency gas burner would likely be a forced-draft 3 plate cast iron thing of about 60-70,000BTU/hr input, and likely 3x oversized for the load, but as long as you have enough radiation it wouldn't short-cycle.

    If you have enough radiation to get enough heat into the rooms with 140F water when it's +15F outside (the 99% outside design temp for nearly all L.I. locations) you may be better off with a condensing gas hot water heater (A.O. Smith Vertex, or the smallest Polaris) with a plate-type heat exchanger for the space heating loop, since the combined cost would be less than a boiler + indirect HW heater. Cheaper still would be the Bradford White Combi-2, since it has the heat exchanger built-in but it would run ~80% efficiency rather than ~95% for a Vertex or Polaris.

    How much radiation do you have, and what type? If you're heating mostly with wood, the Combi-2 might be your best bet, since your heat loads are pretty low.

    If you don't have air conditioning a 2-3 head 1.5-2 ton ductless heat pump and a force-draft gas HW heater might be a better solution, but that's a whole other nut to crack.

    Do you have a fuel-use history on the place from last winter, and were you burning wood then? (If yes, how much of each type of fuel?)

  4. #4
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    good , right quality properly sized and legally done high efficiency boiler will start in 25++K range.

  5. #5
    DIY Senior Member dj2's Avatar
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    Ask them for a price of removing the oil and possibly the oil tank.

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