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Thread: Oil to Gas Boiler Conversion Run-Around

  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?

  9. #9
    In the Trades Tom Sawyer's Avatar
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    Yes it will and in fact it has a 5 to 1 turn down range so it can be tailored to demand very easily.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Let's stop here and smell the roses!

    The fact that your radiators COULD provide say that 90K BTU, doesn't mean that it NEEDS that much to keep the house warm, and even if it can modulate to 20% of max, may be oversized for much of the season. The only way to determine what will work best, is to run the calcs on what your really need. Is that model 110K BTU (guess because of the suffix)? If so, you might be able to use a much smaller unit, save money upfront, and all during its life. It would likely be more efficient that what you have, but that isn't the point...the goal is 'right-sized'. A mod-con moves towards that, but there's a reason they come in multiple sizes - the min, may still be too large.

    The advantage of a mod-con is that on a mild day, instead of firing maybe 6-minutes an hour at 100%, it can fire 60-minutes at 20%, and you'll be more comfortable and save money.

    Most people don't need a bigger boiler to run the indirect unless you use a lot of hot water all the time (like say a laundromat).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    DIY Member danboston's Avatar
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    I agree. The house was built in the mid 1950's when everything was oversized because oil was cheap. I am a little leary of sizing the boiler based solely on the total inear footage of the baseboard heating fins. I have had several HVAC contractors tell me that is how they size boilers.

    What is the best way to run the calcs? I have oil consumption per year for the last several years. Is it true that the amount of fuel used for just heating water for non-heating (hot water) use is generally less than 1/3 of the total amount of fuel used?

    By the way, the input range for the Heat Transfer Product (HTP) HTP EL-110 is 22,000 to 110,000, whereas, the range for the EL-80 is 16,000 to 80,000 btu.

    Tx, Dan

  12. #12
    In the Trades Tom Sawyer's Avatar
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    go with the 110. Your house probably had an oversized boiler but, it was built in the 50's and insulation was not a big priority then. You are going to need a few thousand btu's for the indirect also. Cost of the units is the same.

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    Do a search on some posts made by Dana, or send him a PM. He's good at this. One way to look at this a little subjectively is: was the old boiler able to keep the house warm on a real cold snap and during that time, how much was it running? Then, how big is it and about how efficient? Say it was 150K, but ran at 75% efficiency, that's a bit over 112K, then, say when it was really cold, ran 50% of the time. It was only putting out about 56K during that time. Running half the time. The more time the thing runs in condensing mode, the more efficient it is. As you tighten the house up and add insulation and better windows, you'll decrease the needs, too. SO, if the old one was marginal, and you've got lots of improvements planned, keep that in mind, too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  14. #14
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Retired is the closest.

    When converting a residential boiler from oil to natural gas the procedure never varies. You will, by the way, be cutting your carbon foot print in half, lowering serious pollutants such as SOx and NOx to sub-California standards and if done properly, will likely lower the operating cost of your hydronic heating system by 25% or more.

    Every proper boiler replacement starts with a heat load analysis. You can do the math on fuel consumption from one year or five, but the outdoor design temperature may not agree. A manual 'J' takes into account important factors such as climate and most importantly, building materials. Radiation should be measured as a factor in sizing the appliance, particularly as fin-tube was regularly sized for 180F average water temperature. If the heat load indicates that the radiation is close to design, you will need a boiler that can produce 190F (some condensing boilers don’t). Fortunately, this condition is rare.

    If you don't perform a manual 'J' you don't know what your doing.

    High efficiency condensing boilers save fuel in several ways.

    First, they are sealed combustion and direct vent, eliminating chimneys drafting conditioned air to the outside and uncontrolled cold combustion air from entering the house. Typical condensing boiler stack temperatures are 120F instead of 320F.

    Second, all condensing boilers now come with outdoor reset built in (if you install the outdoor sensor) and modulate the burner to maintain the minimum boiler temperature needed to maintain the temperature at the thermostat. In most systems this will have the boiler in a condensing mode for the majority of the heating season and save more fuel than any setback thermostat while creating more comfort.

    Third, a condensing boiler is low-mass with virtually no standby losses. This is in stark contrast to you old oil boiler, which had to first heat several hundred pounds of cast iron just to keep the chimney hot. Add the tankless coil and you have a recipe for waste (and an obscure fuel usage number, which will be of no use in sizing your new boiler).

    Finally, in regard to high efficiency boilers “AFUE” is only marginally useful, as a condensing boiler will be at steady state efficiency in a matter of a few minutes (86% minimum combustion efficiency regardless of return water temperature) and a cast iron boiler may take a half our or more to hit peak efficiency, never reaching 86%.

    From this information, you can see that choosing a high efficiency condensing boiler is really easy. The question then remains, which one and who should install it. If an experienced professional sizes your condensing boiler by a dedicated heat load program (dedicated to hydronics) you will then have the most accurate heat load estimate that can be had, taking into account existing radiation, insulation, windows and the quality of construction.

    When choosing a contractor to replace your old boiler, eliminating those who can't show you a sample heat load analysis, generated by dedicated software, will make your list, if not your search, much shorter.

    A condensing boiler that is more than 20% over-sized will tend to bump off the bottom instead of modulating flame down to a nice steady (and nearly perpetual) burn, it will be on and off like your old boiler, killing efficiency and comfort at once.

    As to sizing an indirect-fired water heater; each must be sized to the domestic load, which is first determined by the largest tub. The indirect should be very close to the size of this tub, as the tub filler will likely draw the tank down in a matter of minutes. This is a few gallons of output for the average condensing boiler and too much for the biggest residential tankless water heater. No boiler up-sizing is necessary or recommended when matching boiler to indirect.

    Finally, R13 is certainly more than enough for a basement in most climates but care should be taken to choose insulation that will resist moisture. I prefer XPS but if you use fiberglass do not install a vapor barrier, which is erroneously prescribed in some codes.

    Ventilation will be a concern as with any basement bath, but a Jacuzzi is not a hot tub and reasonable care with exhaust fan selection will serve nicely.

    Don't be discouraged in your quest for high efficiency boilers. When I installed my American made condensing boiler in 1987 no one could help me...now everybody is an expert. Go figure.
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    Last edited by BadgerBoilerMN; 03-21-2011 at 06:03 AM.

  15. #15
    DIY Member danboston's Avatar
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    I had another contractor come by yesterday. He was a veteran who has been in business for over 40 years doing only boilers. He told me that since we have a Mono-Flo baseboard heating system with aluminum fins (not cast iron), then he does not recommend a gas-fired high-efficiency mod-con boiler. The main reason being that the return water ends up being too hot and therefore the boiler will never have a chance to enter into condensing mode. If we had cast-iron fins, or a radiant heating system, then he said that is a much better situation for a mod-con boiler since the return water ends up being much cooler. Because the gas company wants to charge me $6,200 to move the gas main 100 feet to the front of my house, he recommended I stick with oil. He recomended a Burnham MPO-IQ115 high-efficiency (87%) 3-pass oil-fired boiler. This is a cast iron water boiler with a heating capacity of 98,000 btu/hr and a weather sensitive control that adjusts the boiler temperature up or down based on outdoor air temperature.

    For an inderect water heater, he recommended the Burnham Alliance AL50SL. This is a 50-gallon, seamless hydrastone-lined tank that has a first hour rating of 225 gal/hr and continuous draw rating of 171 gal/hr with a 6 gpm flow rate. He said that would work fine to fill a 70-gal jacuzzi and still be able to supply hot water for other uses.

    Again, the house (ranch) is well insulated with all new windows, including the basement. We have 150 total feet of Mono-Flo baseboard heating with aluminum fins. The square footage of the ground floor is 1,700 ft2 and the basement area is 1,500 ft2. I plan to heat the basement as well (the basement has dri-core panels on the floor, R-13 foamboard on the walls, and R-30 fiberglass batts in the ceiling).

    He initially wanted to install the next higher-sized boiler (129,000 btu/hr), but when told about the insulation, he said that the lower boiler with a 98,000 btu/hr would be fine.

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