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Boiler Summer/Winter hookup - oil consumption estimate?

amin1992 Member Posts: 38
edited March 2020 in Oil Heating
Hi there guys. I bought a home this fall with an old oil boiler that supplies our heat (hot water baseboard) as well as the domestic hot water via an internal domestic coil. The aquastat is currently set to 155F/185F with the diff at 25. This has done just fine heating the house and giving us very hot water.

We've yet to use the system while not heating the home so I have been a bit nervous about how the hot water will be in the winter. With the advice of our boiler tech, and on a warm 75F day last week, I changed the aquastat to 135F/185F and kept diff to 25. The boiler fell down to around 140F and I took a shower.

Within 5 or so minutes, it went cold despite turning the shower to the hottest temp possible. Bummer! Went down to check and the boiler was burning away, but the temp didn't look to be climbing so thinking it couldn't keep up.

Once things warm up again, I'll mess around with the settings. But worst case, I feel like I need to run the boiler at 155F/185F all summer...

So to my question. How does oil consumption for a summer/winter boiler compare between the seasons if the temperature settings stay the same? Since the boiler wouldn't have any calls for heat, only domestic hot water, I'm assuming the usage would be less. But worried it might consume almost as much oil...

This past winter, we used around 100 gallons of heating oil per month.

Thank you for the help!


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,377
    Tankless coil is old technology. Better would be a small indirect, where the boiler fires to maintain the tank, and not every time someone opens a hot water faucet.
    Tankless coils may require maintenance over time as they tend to lime up, reducing heat transfer.
    I wouldn't lower the aquastat until you're into summer. At this time of the year, the cold water entering the boiler is still very cold, so the boiler needs to be hotter.
    Plus, most tankless coils are only rated for about 3 gpm max.
    With no faucets leaking (and you don't say how many people/ages) in the home, or what climate you live in, so it's hard to guess at consumption.
    Still better or equal to an electric water heater, especially with oil prices below $2.00/gallon.
    Looking thru my database, I'd guess 75-150 gallons for the entire summer (Phila, PA)
  • GroundUp
    GroundUp Member Posts: 1,748
    Fuel usage is a direct derivative of BTU usage. Heating the house as well as the DHW all winter you say was roughly 100 gallons a month, so heating the DHW only will be considerably less fuel used as it's not using fuel to heat the home as well. Actual usage numbers would require exact heat loads of both space heating and DHW, but for a rough ballpark let's say you use 100 gallons of 110 degree domestic water per day and your incoming city or well water is 55 degrees. That makes a 55 degree rise to overcome, which is 45,815 BTU per day (this is just an example, your exact numbers will differ). If your boiler is 75% efficient, that would require 61,068 BTU worth of fuel input. There are approximately 139,000 BTU in a gallon of oil, so using the above example you'd be using approximately .44 gallons of oil per day to heat domestic water. There will be some standby losses, etc from the boiler so again these aren't exact numbers but a rough ballpark.
  • amin1992
    amin1992 Member Posts: 38
    Hey Steve, thanks for your advice. Some day we'll replace this boiler and install an indirect tank, but for now it's chugging along so it doesn't make financial sense to replace now.

    We did have the coil cleaned out about 6 months ago due to hard water.

    Good point on the cold water. I didn't even think about that, just because it was 75F outside doesnt mean the ground water has warmed up much.

    Our showerhead is 2.5 GPM so that shouldn't be an issue at least.

    Only 2 people in the home and we never shower back to back luckily. I shower morning, she showers at night.

    And wow, 150 gallons for the whole summer would be awesome. I'm actually right outside of Philly! Funny.

    Thanks again
  • amin1992
    amin1992 Member Posts: 38
    Groundup, thank you for sharing this with me. It definitely helps my planning.
  • PRR
    PRR Member Posts: 76
    We had oil/steam and hot water. We didn't change any setting seasonally. Oil consumption in summer was *much* lower than winter. We didn't try to track it. The hot water was usually endless.

    If the boiler is insulated, let it sit hot all summer. But if you can't take a shower, suspect the coil is limed-up.

    In this "newer" house we got on-demand propane hot water. Older unit going goofy. We finally got H-D's cheapest electric tank. Much happier, and we do not notice the added electric cost. (Of course this depends how much you wash.) The gas/demand is still plumbed as backup or in case I have to defrost the sewer again.
  • SuperTech
    SuperTech Member Posts: 2,022
    Honestly you should consider an electric tank water heater if an indirect is out of your budget. By not keeping your boiler 140-155 degrees 24-7-365 your fuel savings will be dramatic. Tankless coils in a cast iron boiler are tremendous fuel hogs. It's a crude an inefficient way to produce domestic hot water.
  • pecmsg
    pecmsg Member Posts: 4,388
    edited March 2020
    Get a superstore tank and separate zone.
    SuperTech said:

    Honestly you should consider an electric tank water heater if an indirect is out of your budget. By not keeping your boiler 140-155 degrees 24-7-365 your fuel savings will be dramatic. Tankless coils in a cast iron boiler are tremendous fuel hogs. It's a crude an inefficient way to produce domestic hot water.

    Unless your electric rates are really low that would be my last choice. Oil / NG, LP then electric in that order.
  • SuperTech
    SuperTech Member Posts: 2,022
    I'm not sure about that, regardless of fuel, tankless coils suck.
    Of course an indirect tank is the best option, but I don't have the best electric rates and I preferred my electric water heater over the tankless coil.
  • bburd
    bburd Member Posts: 782
    I lived in Philadelphia for many years. Electric rates there are very high. I also lived with tankless coil systems for a long time.

    Most tankless coils were designed for aquastat settings of 160-180° low limit and 180-200° high limit, with a 10° differential. The boiler water must be significantly hotter than the hot tap temperature you want to transfer enough heat.

    If you have the usual Honeywell triple aquastat relay, it turns on the burner 10° below the low limit setting, and turns it off at that figure plus the differential. So at a low limit setting of 135°, your burner will turn on when the boiler water falls to 125°, and off at 150°. I am not surprised you had a cold shower.

    If I were you, I would turn the differential down to 10° and raise the low limit setting until you can get a consistently hot shower. You might be able to drop the low limit 10-20° in the summer.

  • bburd
    bburd Member Posts: 782
    *Be sure to set the high limit at least 20° above the low limit setting; and higher than that if you use a larger differential. You don’t want the settings to fight each other and keep the burner turned off.

  • amin1992
    amin1992 Member Posts: 38
    Thank you all so much for the suggestions and input. I will heed it all. it means a lot.
  • psb75
    psb75 Member Posts: 755
    Before you consider an electric water heater you should definitely look into an electric HEAT-PUMP water heater. There are often very good incentives and rebates available. These units pay for themselves in short time with their energy bill savings.
  • george_42
    george_42 Member Posts: 119
    Years ago I had 100000 btu oil fired boiler with coil. I put a meter on it to check oil consumption in summer and found that I used about one gallon of oil a day to keep boiler at 180 deg and make hot water with coil. I had 4 people in the house at that time. That should give you a pretty good idea of consumption. George
  • weedhopper
    weedhopper Member Posts: 59
    We used about 180 gallons to heat water late last spring thru fall. Three people in the house.
  • Cokomo
    Cokomo Member Posts: 16
    Our 30 year old Peerless boiler (77%) used 1.6 gals. daily when heat was off late April-November for dhw.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,736
    Here is an idea I used for a customer that had a 2-year-old electric water heater. A Philadelphia plumber installed this to save on their oil bill. By turning off the oil burner in the summer they saved about $300.00 a year when oil was at its highest several years ago. The resulting electric bill for the year was $400.00 higher using the electric water heater. The consumer had less hot water and paid $100.00 more for that privilege and also paid for the new water heater.

    I did not have the heart to charge them for a new indirect so I got creative.

    Normal Indirect Job

    Creative Indirect Job

    By using the coil in the boiler to feed an electric water heater, disconnect the 240V. feed, wire the lower electric water heater thermostat to operate the boiler the same way you might wire the indirect and using a nonferrous pump, the customer had a virtual indirect water tank. Basically the heat exchanger is in a different location, with the same result. The result is plenty of DHW and 12 years later they purchased an indirect when the electric water heater failed.

    The customer felt that the money spent on the electric water heater was not wasted, the amount of available DHW was more than adequate and the summer fuel usage remained lower than the original summer usage from before the electric water heater was purchased.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,377
    edited March 2020
    Our long lost Wall favorite, @icesailor used to propose a similar concept many times, piped differently as I don't know where you found 2 extra ports on a standard water heater.

    Here's his quote from one of the threads:
    Tanks for the tanks:

    "If you are using a water heater for the storage tank, like an electric water heater, pipe the hot and cold to the water heater full sized like you were going to connect it as an electric water heater. That usually means 3/4" in and out. Between the cold water inlet valve and the cold water inlet to the tank, install a 1/2" connection. Remove the drain from the bottom of the tank and replace it with a 6" brass nipple, a 3/4" brass tee, a 3" brass nipple and ell and whatever brass nipples you want to use. Buy a Taco 006BT bronze threaded circulator. The extra nipple and ell is to offset the circulator away from the bottom cover and make it easier to access the wiring. They now offer a stainless steel one but be sure it is threaded. Install the circulator with the arrow pointing up. Pipe this to the "cold" side of the tankless. Pipe the "hot" side to the top of the water heater where you left the 1/2" connection. I install valves and unions and a 1/2" check valve above the circulator but it is an option for you but not for me..


    There are many ways to do this. I'll give you the third world way. You can modify it with knowledge.

    Remove the lower cover. Remove both wires from the lower thermostat. It is going to be a "switch". Use a "Cord Whip" or for a really third world connection, if you have a dead extension cord, and the male end is OK, cut it off. Connect the white wire to the appropriate neutral in the circulator, the black wire through the water heater thermostat and the other end back to the "hot" wires on the circulator. Make sure you connect the green ground wire to the provided green screw on the circulator. Set the thermostat on the water heater to whatever you want. 130 degrees seems nice. Change the boiler controls. Set the "Low Limit/Circulator" to 140 degrees. Set the "High Limit to 160 or 170 degrees.

    You are good to go.

    If the water in the tank needs to be hotter, turn up the temp, on the tank. If the house doesn't heat up as fast or enough, turn up the high limit. That will only be a problem in the coldest months.

    You will notice that the water will be hotter in the winter when the heat is cycling. In the summer it will be cooler because the boiler will be on the operating control and the boiler only will start when the water heater circulator calls.

    You do not need any other controls to make this work. You can go nuts and try to re-design the world but this has worked for me and my customer for a lot of years.

    As far as specific hot water storage tanks, when I built the house I live in now, I had to have it plumbed by others. Who did a fine job. I told them to use an electric hot water heater for a storage tank. They lost their mind. I shoe=wed up one day and found a "John Wood" hot water storage tank installed. Piped as per their instructions. Which I find, don't work as well as the way I have written. I changed it to the way I have described. I just replaced the tank after 10 years. I bought an AO Smith regular 50 gallon electric hot water storage tank. It is the same as the John Wood tank without the 1" plugs in the element holes. In 40 years of doing this, I have never had a leak through the elements.

    Electric water heaters are the cheapest insulated water heater tank you can buy.

    You can get all nuts and fancy and use the internal tank wiring but you must understand how the wiring and controls work. The easy way is the one I describe. The thermostat is a switch to control the circulator. The boiler control reacts to the drop in temperature. There is no physical, electrical connection between the two. It is already set up to do so.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,736
    edited March 2020
    relief valve opening at or near the top. I used a Tee at the relief valve with an extra-long temperature sensor.

    at the bottom, there is a drain valve. I used a tee there.

    on heating help .com, I used an illustration to show that the location of the heat exchanger changes from the tank to the boiler.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics