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Thread: Oil to Gas Boiler Conversion Run-Around

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  1. #1
    DIY Member danboston's Avatar
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    Default Oil to Gas Boiler Conversion Run-Around

    I am new to the forum and it looks great. I am looking to replace my 50 year old oil boiler with a gas boiler that will give high efficiency and will be low maintenance. I live in a 50-year old ranch with full basement in an area just north of Boston, Massachusetts. The total square footage of the ground floor is 1,700 ft2 and the basement is 1,500 ft2. The house is currently heated with a tankless American Standard oil boiler (Arcoliner 3B J3 series) and a forced hot water Mono Flo baseboard fin system on two zones. We currently run out of hot water quickly if two showers are taken back-to-back or if the forced hot water heating system kicks on when in the shower. The house had no insulation when we moved in back in 1993 and at the time had treated cellulose blown into the walls and attic. This year we are planning to remodel the kitchen and family room and will have two walls measuring approximately 40 feet total that will be mostly large windows. I am currently refinishing the basement and it will be fully insulated (floors, walls and ceiling) to R-13 or better. On average we keep the temperature in the house at around 58 F when we are not home and at night (pretty chilly). We burned about 720 gallons of oil in 2010. I tried the NORA Fuel Savings Analysis (FSA) calculator and it gave me a very low design day heat load of ~30,000 btu/hr when I input oil usage of 720 gal/yr with a temp of -2 F as the design day temp.

    I received a quote from a local HVAC contractor that specified a gas-fired Heat Transfer Products (HTP) Elite HTPEL-110 High Efficiency (94%) heating boiler and a 45-gallon HTP SSU-45 indirect water heater (Super-Stor). As far as I can tell he only use the total lineal footage of the baseboard heating system to figure out how to size the boiler. I asked him if he used the Manual J method to calculate the heat load and did not get a straight answer. Another contractor told me that the new high-efficiency boilers are over-rated and that you are lucky to get 90% efficiency – and therefore, why worry about 5% when you can get one of the regular 85% gas-fired boilers installed for much less up-front cost. He also told me that since I want to install a self-heated 70-gallon Jacuzzi in the basement, that I will need to upgrade from a proposed 45-gallon Super-Stor to a 120-gallon Super-Stor. The first hour rating of the 45-gallon HTP SSU-45 indirect water heater (Super-Stor) is 200 gallons, therefore, that should be OK, right? When I asked how he will size the boiler, he told me that he will base it on the existing oil boiler size.

    I am committed to switching to gas since I am convinced that oil prices will continue to rise at a much greater clip than natural gas. I would like an efficient gas-fired boiler to save dollars in the long-run and an indirect water heater so we do not run out of hot water, can heat both the ground floor and basement , and can safely fill a 70-gal Jacuzzi with warm water in one shot. However, I am also concerned about incorrect sizing and/or configuration of a high-efficiency gas-fired boiler with an indirect water heater that will consequently break down and need continual maintenance. Can you help me with the heat load calc, decide how to find a good HVAC contractor, size and configure an appropriate system? What do you recommend? I really appreciate it.
    Last edited by danboston; 04-02-2011 at 10:22 AM.

  2. #2
    Plumber jimbo's Avatar
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    You need to find a better contractor. Some things which jump out at me: R13 is not enough for walls and ceilings. 70 gallon Jacuzzi! How do you plan to heat that? Will it take any hot water from your domestic supply? Do you want that much humidity in the basement???
    I hail from Newbury ( in a former life!!). I don't see how 17K btu is enough to heat 3200 sq ft. Fortunately, I now am in San Diego, where there has not been a snow storm, hurricane, tornado, tidal wave, flood, or earthquake , of significance, in recorded history! I am staying put, unless the zoomies from japan get out of control!!!!!!!!

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    The mod-con boilers efficiency is during an optimum burn. You won't get an optimum burn if it short-cycles because it is oversized. If you make the indirect a priority zone, all of the heat from the boiler goes to the indirect while it is calling for heat, and if you match the input BTU and temperature settings, you should be able to get that first hour rating. Note, though, that when filling a big tub like the one you have, with a decent valve, it won't take anywhere near an hour, and you are more limited by how much is already hot, since there's not all that much time to heat incoming water. That first hour rating (as I understand it) is closer to a constant use value (like say in a shower) than dumping it quickly all at one time. So, 200g/60min = 3.3gallons per minute, or enough to run a typical shower constantly for an hour and not run out. Since you can only get about 75-80% of a tanks contents when dumped quickly before it starts to cool off, a 60-80 gallon indirect should fill the tub (since not all of it is full hot and some cold).

    There are some that are much more into hard numbers that will probably stop by, but it seems that you may be able to use a fairly small mod-con. The key is that it can modulate to a typical day's needs, but still have enough to meet your maximum day's needs. If 18K btu is your max, a small 60K mod-con boiler (I like and own a Buderus) should work with the ability to modulate down to about 20% max, discounting the heat loss, would bring the min to about 11K or so, and the max way over your max needs. It would likely not give the full first-hour rating on an indirect, since most of them spec a larger boiler.

    You want to be able to meet your design day, but get the most efficiency if you can modulate to a more normal usage which allows it to maintain max efficiency. Doing what those two vendors suggested would likely saddle you with a way oversized unit, and poor efficiency.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Member danboston's Avatar
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    Thanks for the responses, maybe I tried to bite off more than I can chew by throwing everything out at once. Let's take it one step at a time...

    As I mentioned, I tried the NORA Fuel Savings Analysis (FSA) calculator and it gave me a VERY LOW design day heat (DDH) load of 17,705 btu/hr when I input oil usage of 800 gal/yr (assuming that we would burn more oil if we targeted a warmer indoor temp of 66 F - 67 F on a regular basis – except at night). It came back with a WARNING message indicating that the DDH load was unusually low. Only when I bumped up the gallons of oil used to 960 gallons did it not give me a warning message. This yielded a DDH of ABOUT 25,000 btu/hr and an efficiency of 53.4%. Does this make sense given the size of the house, the oil burner (old boiler with tankless coil), oil usage, indoor temperature, linear footage of baseboard heating, etc. ?? I figure we should get that straight before we talk about sizing a boiler and an indirect water heater...

    Tx, Dan

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    I'd look at the winter months, then the summer months and subtract the oil usage from the summer months used to just keep water warm and try to come up with the oil usage for just space heating in the winter. If you average it out over the whole year, you've got 6-7 months or more when the boiler is running just to make hot water, and not to keep the house warm.

    That number does seem low, especially since you mention you don't have the greatest insulation. But, if you went with what you have, you'd almost certainly be oversized.

    One quick sanity check is to try to recall how much the boiler actually ran on the coldest day of the winter. Ideally, it would be at or near 100%. I'd guess yours turns on and off. A properly sized one would run (burner on) at or near 100% on that design day max need. That would provide you best comfort and economy. Cycles hurt economy and longevity.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    DIY Member danboston's Avatar
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    The NORA FSA calculator gave me a design day heat load of ~30,000 btu/hr

    Tx, Dan
    Last edited by danboston; 04-02-2011 at 10:24 AM.

  7. #7
    In the Trades Tom Sawyer's Avatar
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    I think you are overcomplicating things. You have 150' of baseboard with a maximum capaciity of 90,000 btu @ 180 degree water temperature. The plus minus range here is pretty small. Anything that will deliver between 70 and 90,000 is going to work fine and be efficient.

  8. #8
    DIY Member danboston's Avatar
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    So, a gas-fired HTP Elite HTPEL-110 High Efficiency (94%) heating boiler and a 45-gallon HTP SSU-45 indirect water heater (Super-Stor) would do the job in an efficient manner? Is that a mod-con boiler?

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