Recommended oil furnace temperature settings?

Discussion in 'HVAC Heating & Cooling' started by spicyjeff, Mar 24, 2008.

  1. spicyjeff

    spicyjeff New Member

    Messages:
    1
    We have an oil furnace controlled by a Honeywell box with two temperature dials on it. One appears to the max and the other the minimum temperature for water in the coil of the boiler. Is there are recommended setting for either of these for maximum efficiency it this heater is for forced hot water heat only? As we use an indirect water heater for service hot water that regulates temperature on its own. Thanks for any input!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    Unless the boiler is smart enough to raise the supply temperature by zone, you probably want 180-degrees going into the indirect to get reasonable recovery rate. Most radiators are desgined for that temperature, too. To keep the thing from short cycling (it works best if it runs for awhile), you might want to try something like 200-160. See what the pros have to say. The new, smarter controls will adjust the boiler temperature for the needs, and might only run say 140-degrees on a mild day to keep them on longer rather than cycyling on and off at a higher temp. You don't have that option. They combine this with a variable burner, but I don't think they've been able to do that for an oil burner, only gas.
  3. amazer

    amazer New Member

    Messages:
    2
    Location:
    southern NH
    Adjusting boiler temperature for summer

    This same question has occured to me, as we have a similar oil boiler with probably the exact Honeywell temperature control unit bolted to it. My heating guy suggested that we set it 170-190 degrees in winter and set the thrid dial to a spread of 15 degrees (not sure exactly why, since the spread seems to be 20 degrees, but this 3rd dial possibly controls something else).

    On very cold days (below 15 degrees or so), I boost these numbers to 180-200, since our house doesn't have quite as much running baseboad as it should.

    BUT, here's my question: since we have an indirect SuperStor hot water tank, which runs as a zone off the boiler, what's a good temperature to set the boiler during the warm months? I've lowered it to 140-160, which still provides us with ample hot water... but I'm not sure whther this is the most efficient temperature setting.

    Should I consider lowering the numbers even more (130-150?), or would that be inherently wasteful of oil somehow?

    Any advice/insight is appreciated!

    amazer in southern NH
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    My boiler control treats the indirect as a priority zone and automatically bumps the supply temp up to the highest it normally runs (195) while supplying heat to the indirect. Once that is satisfied, it adjusts the supply temp to the other branches as required.

    The baseboard heaters will provide more heat as your supply temp rises. Ideally, an outside reset system would compare the outside temps to the call for heat and adjust the boiler operating temp, but that is usually on newer boilers.

    One consideration is if the boiler is designed to handle cold starts. Some heat exchangers can't handle a burst of cold water into them. If yours can, and it has a priority zone override, then you could turn the summer boiler temps way down, essentially enough so it doesn't turn on except when the indirect calls for heat. maintaining standby temps is wasteful, but go as low as you can and not cause damage in the summer. IF the boiler doesn't have a priority circuit to override the normal setting when the indirect comes on, you'll get a very slow recovery rate on the indirect if the input drops below 180 (the normal design temp). Over time, at a lower setting, it will still heat the water, but depending on use, it would appear as a much smaller tank. If you never run out of hot water, it either treats it as a priority zone and overrides the settings, or you aren't using enough hot water at a time to dilute the tank and cool things off enough to notice.
  5. amazer

    amazer New Member

    Messages:
    2
    Location:
    southern NH
    Interesting!

    Thanks, Jim, for your comments. I don't know whether our SuperStor is treated as a priority zone and/or prompts the boiler to heat up higher than my current 140-160 range (I set it for 170-190 in winter).

    It's quite possible that we don't use enough hot water at any one time to deplete the supply.

    In any event, it is more economical to set the boiler a bit cooler in the summer, isn't it? The technical support rep at the company that manufacturers the SuperStor suggested I keep the boiler at 170-190 even in the summer, but that seems wasteful to me.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    Depending on the school of thought you adhere to, it's a good idea to set the WH thermostat to 140 or so. This requires a tempering valve on the outlet to protect you and your family, but does two things: maximizes the hot water available and is hot enough to kill many nasty bacteria.

    It depends on the boiler design and control circuits. Mine monitors the outside temperature, the return temp, and a call for heat, then adjusts the supply temp to meet the circumstances. Depending on how cold it is outside, it might only run to 120-degrees, or on a really cold day might go up to as high as 190. It has a modulating burner that can throttle down to 20% of maximum and can approach the high 90% efficiency. If your boiler can handle the cold starts, in between calls for heat, it might cool off to near ambient. You probably want to keep the temp at least a bit higher than your WH temp setting. Some only have a very small supply of water that it keeps hot, so it really isn't using all that much.

    See what some others have to say and I'd consider calling the tech people at the boiler manufacturer's place.

    What some people have done here is add an electric WH after the indirect. Since it is supplied with heated water, in the winter it almost never runs, and only runs as the water stored in it cools off. In the summer, shut the boiler off and rely on the electric. Now, you'd have to do a trade-off of the costs and space to install this verses the energy to run the boiler.

    Since the SuperStor is quite well insulated, you might consider a timer, and only turn the boiler on for the normal use times. Don't know how well the thing would like the cool-down, heating cycles.
    Last edited: May 1, 2008
  7. BigLou

    BigLou New Member

    Messages:
    138
    spiceyjeff

    I would advise against playing with temp setting by more then a few degrees.
    while lower water temps give you higher efficiency (this is how manufactuers lie to you about efficiency ratings) most if not all oil boilers will be destroyed by lowering the temp to much. you need the return water temp above 150 or so to avoid condensation.

    Oil condensate is very corrosive and will destroy the firebox in the boiler as well as the flues connected to it. Basically you want to make sure the flue gasses are exeting the highest point of the chimney above 150 degrees on the coldest day of the year. If not the condensate will greatly reduce the life of both the boiler and the flue even a stainless steel one.

    What might work is installing a smaller nozzle or even burner to derate the BTU rating of the unit some what. Its been my observation that 90%+ of heating equipment is oversized.

    Its all an interconnected system so derating the system may require changing or adjusting the circulator pumps for a slower flow rate for better heat transfer. There is nothing wrong with running an oil system at 170 or so and getting less BTU's from the burner you just need to run the unit a little longer which is a good thing

    Lou
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