Converting from oil to gas

Discussion in 'Tankless Water Heater Forum' started by allisonj33, Jul 28, 2014.

  1. allisonj33

    allisonj33 New Member

    Jul 28, 2014
    Hi. We live in an approximately 80 year old brick home a block from the ocean in South Jersey. We have oil heat and have a tight utility room that houses the boiler and hot water heater as well as washer/dryer. There are only 2 people living in the house most of the time, but as many as 8 a few times a year. The house is brick and approximately 2000+ sq. feet. There are 3 1/2 bathrooms.

    We want to switch from oil to gas to reduce the monthly bill and so we can get a gas stove. There is no problem getting gas to the house, and we presently have an above ground oil tank after having 2 underground tanks removed. The question is what to get instead of the present setup. We have received 3 quotes. 2 of them proposed a combination boiler/tankless water heater. We like the idea of that because it minimizes how much space the units will take up. One proposed a Bosch combination heat and domestic hot water wall hung high efficiency boiler with a capacity of 151,000 btus and AFUE rating up to 95% for $14,425. The other proposed a Quietside S-Line series with a capacity of 160,000 btus and AFUE rating up to 92% for $12,700. Those quotes included removal of the oil tank and all labor, materials, and permits.

    The quote we received today suggested a Weil McLain boiler with 125,000 btus and AFUE rating up to 97% with a separate tankless Rinnai 7.5 gpm for $8,995. This quote came from someone who was not sold on the combo units and did not want us to be a guinea pig. His charge did not include removal of the oil tank.

    We want to maximize the space in the utility room so we can add some storage. We do not want to do that at the expense of getting a lousy setup though. What we like about the current setup is that we can have all the showers, the dishwasher, and washing machine going at the same time and still have plenty of hot water. We understand that will not be possible, at least with the combination units. We would appreciate any feedback on the combination units versus separate tankless water heater and boiler. If we do decide to go with the combination unit, any thoughts or experiences with the Bosch versus the Quietside for the proposed prices would be helpful.

    Thank you.
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Nov 29, 2010
    My two cents. Swapping to nat gas may work out in the short term but look for gas prices to rise substantially in the next few months. Anyhow, if you're dead set on gas go with two separate units. Boiler and indirect are a wize choice. I don't like combination units because if one component craps out, so does the other.
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  4. allisonj33

    allisonj33 New Member

    Jul 28, 2014
    Do you have any experience with how often the combo units go versus the individual?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    From an efficiency standpoint, a boiler and an indirect WH are probably your best bet. All of your proposals are probably way oversized, and therefore will cost more to buy, and to operate. An indirect can typically be smaller than a standalone tank because you can run it hotter (with a tempering valve), they are very well insulated (low standby losses), and they have the full capacity of the boiler to reheat the water if they do get drawn down. Do not size the boiler for the WH capacity...size it for the house, and a typical house in that area probably only needs in the order of 40K BTU, but the only way you'd know is to do some analysis. Do NOT use the existing sized boiler as a guideline, as most of them tend to be 2-3 or more times oversized. If you have some of your old oil bills and stamped with the K-factor, you can look up the heating degree days for that period, and calculate how much energy you used for that period. That is a very good basis for the size boiler you need.

    Nobody knows the future costs of NG verses oil, but today, per BTU, NG is a lot cheaper, and if fracking is not outlawed, is likely to continue to be a good value.
  6. Dana

    Dana In the trades

    Jan 14, 2009
    Gas pricing would have to more than DOUBLE to break even with oil as a heating fuel, which isn't too likely. (Are you expecting $3/therm gas? How about $2/gallon oil?)

    Combi units are rarely the right solution- the firing rates at the low end are often higher than the design condition heat load for the house. A right-sized boiler for the heat load, with a right-sized indirect-fired tank for the hot water load (typically sized for the biggest tub you have to fill), is almost always a better solution.

    Getting to the optimal solution requires figuring out what the actual heat load is, how much radiation you have on the system, and how much hot water you need. Ideally you'd run an ACCA Manual-J type heat load calculation, but if you have an oil boiler, with a zip code and a mid to late winter fill-up bill (and the EXACT dates between fill-ups, to look up heating degree day data for that period) you can be pretty close to the ball park.

    If you have a K-factor stamped on a mid-late winter fill, the K-factor and the zip code is sufficient, no fill-up dates necessary. (The units of an oil burner K-factor is in degree-days/gallon, no need to look up the degree-days, since it's embedded in the number.)

    Almost all oil boilers are oversized for the heating loads of even modestly insulated houses in southern NJ, but there are exceptions to prove the rule. If the boiler had an embedded hot water heating coil (even if it's no currently used for that), it was most likely sized for the hot water load, not the heating load. The instantaneous heat draw for domestic hot water is usually several time the space heating load, and 3x-5x oversizing is common, but comes with severe efficiency consequences.
    Last edited: Jul 30, 2014

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