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  1. #1
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    Oct 2018
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    How Inefficient is heating at Too Low Boiler Temp?

    I have had a bunch of posts in the past month or two about my HVAC system and my very high gas bills. I recently rewired the relays and found relays for 3 out of my 4 zones were not wired to the boiler. So unless zone 1 was running (which ran off of the Aquastat), zones 2, 3 and 4 were only getting water at the LO temperature on my triple Aquastat, which was 140F.

    This temperature was clearly not enough to heat those zones. So each of those zones were running longer and probably putting my boiler on the edge of condensing. Since rewiring and letting all zones get 180F water from the boiler, I notice the system has hardly had to run. Does this make sense? Maybe objects in the room are working better as heat sinks since they are now getting hit with much warmer air now when the hydroair is running?

    Also, I still have about 23 days until my next gas bill, but I am really curious; could this have been the cause of my high bills? I guess more specifically, my question is: is it really inefficient for a boiler to run a lot to maintain 140F for long heating cycles than it is to heat up to 180F and satisfy a heating call much quicker (and for a much longer time?), and as a result, get much fewer heating calls?

    Thanks

  2. #2
    Join Date
    Apr 2003
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    Maple Grove, MN
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    Good question, but I think your thinking might be backwards.

    I'm pretty sure boilers are more efficient the lower the water temperature is.

    Also, the longer heating cycles should be slightly more efficient than more shorter ones would be. In reality they pretty much equal each other out though.

    There might be something else at play with how your system was setup that you fixed that could have been making it use more fuel, but I'm not sure.

    Perhaps someone smarter will chime in soon to dispute what I was thinking and then I will learn something. Like I said, it was a good question.

    By the way, those zones don't have a different type of heating than the other one does, does it? Could there be a reason that they would have wanted those zones at a lower temperature?

    What I'm getting at is that I know in-floor heat (for example) requires a lower temperature than baseboard or radiator heat would, so hopefully you don't have something like that going on.

  3. #3
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    Oct 2018
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    Thread Starter
    Thanks, ammoniadog.

    I definitely think I have/had two issues at play; ineffective boiler temps and a leaky zone.

    Zone 4 is an extension on the house that is a 300sqft room heated by hydroair. I could feel cold air was leaking into this room but was struggling to find the source. This leak would mean the stat would call for heat (even when set to 52F) every 1-2hrs. So every time that zone called for heat, it was only getting 140F to heat the coil, which means it was pretty much blowing cool air while trying to heat the room, while also having the boiler cycling on a lot to maintain the Aquastats LO of 140F.

    Last weekend, I think I discovered the source of the leak. The air handler and feedstack are in an insulated but unconditioned closet that is near outside temperature. I think where the stack enter the ceiling to branch out across to the registers in the room, it was letting cold air leak into joists of the ceiling. This theory is further supported by the fact the room above this extension has a VERY cold floor. I am hoping this leak combined with a too low boiler temp were the culprit. My last gas bill was $250 over 30 days and I was only at the house for 2 of them. All other days stats were at 52F.

  4. #4
    Join Date
    Jan 2004
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    PA
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    Doubtful it was the low boiler temp. But a cold air leak will definitely increase the heating bill.
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    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Apr 2012
    Location
    Millsboro, DE
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    Lowering boiler temperature reduces energy consumption, so that's not it.

    When you said "leak", I was thinking "water leak" (boiler water going to "Mother Nature"), a very real possibility. You might want to bring in a qualified HVAC contractor to check that. "Don't try this at home" applies here: Well-meaning DIY'ers have been known scald themselves and/or destroy their boilers checking for water leaks; do hire a Pro! You said "Triple Aquastat", if you're using the boiler for domestic hot water - quite expensive when there's no space heating required - have the pro check for domestic hot water leaks too.

    Sources of infiltration (AIR leakage through the building envelope) can be ID'd by "Blower Door" testing.

    And I've seen cases of gas meters "over registering", and you might want to discuss that with your Utility. Gas leaks occur too (scary!). High gas pressure at the appliance will increase usage - bring in a Pro to check "manifold pressure".

    Bottom Line: You need a Pro ASAP, as there could be an unsafe condition.

  6. #6
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    Apr 2012
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    Millsboro, DE
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    Another thing: If your gas and water meters are accessible, check them yourself; why wait a month?

  7. #7
    Join Date
    Oct 2018
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    Thread Starter
    Quote Originally Posted by ferd1942 View Post
    Lowering boiler temperature reduces energy consumption, so that's not it.

    When you said "leak", I was thinking "water leak" (boiler water going to "Mother Nature"), a very real possibility. You might want to bring in a qualified HVAC contractor to check that. "Don't try this at home" applies here: Well-meaning DIY'ers have been known scald themselves and/or destroy their boilers checking for water leaks; do hire a Pro! You said "Triple Aquastat", if you're using the boiler for domestic hot water - quite expensive when there's no space heating required - have the pro check for domestic hot water leaks too.

    Sources of infiltration (AIR leakage through the building envelope) can be ID'd by "Blower Door" testing.

    And I've seen cases of gas meters "over registering", and you might want to discuss that with your Utility. Gas leaks occur too (scary!). High gas pressure at the appliance will increase usage - bring in a Pro to check "manifold pressure".

    Bottom Line: You need a Pro ASAP, as there could be an unsafe condition.

    Thanks for the input. Sorry for my poor choice of wording above; by leak I was referring to cold air leak. As far as I can tell, there are no water leaks. In November, I had a pro flush the system with fresh antifreeze. I would like to think they would have picked up on if there is a leak but maybe not?

    I did call the gas company back in Dec and a tech came out to inspect my meter. He looked at the meter for 3 seconds, said its fine then looked at my boiler and said it looks fine. Tested CO output, which he said was fine. Then got a phone call and had to run to a new job. So in all, I don't really think he did anything.

    Is it normal to hear pretty loud hissing when gas is flowing through the meter? Or could that maybe be a sign the pressure is too high.

  8. #8
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    Aug 2002
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    Southold, NY
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    Quote Originally Posted by phil178821 View Post
    Thanks for the input. Sorry for my poor choice of wording above; by leak I was referring to cold air leak. As far as I can tell, there are no water leaks. In November, I had a pro flush the system with fresh antifreeze. I would like to think they would have picked up on if there is a leak but maybe not?

    I did call the gas company back in Dec and a tech came out to inspect my meter. He looked at the meter for 3 seconds, said its fine then looked at my boiler and said it looks fine. Tested CO output, which he said was fine. Then got a phone call and had to run to a new job. So in all, I don't really think he did anything.

    Is it normal to hear pretty loud hissing when gas is flowing through the meter? Or could that maybe be a sign the pressure is too high.
    Why antifreeze?
    What Type?
    what freeze point?

  9. #9
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    Oct 2018
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    Why antifreeze?
    What Type?
    what freeze point?
    We purchased the house back in October and it already had antifreeze in the system. It was recommended I have it replaced since it was 5 years since the last flushing. The specifics are in my notes back at the house (I won't be there until Friday) but I know it was a propylene glycol mix. I am guessing they use the antifreeze because the boiler is located in a utility closet that is only accessible from the outside and plumbing for the system travels through an unconditioned basement and crawl space.

  10. #10
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    Aug 2002
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    Southold, NY
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    The reason I ask is Glycol slows heat transfer so the boiler now works harder, not a bad thing but adds to operating costs. The lower the freeze protection the more it slows heat transfer.

  11. #11
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    Jan 2015
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    Bellevue, Washington, United States
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    In my experience high gas bills on a home heated with a boiler are most often caused by a water leak. Do you have hydronic pipes under the concrete? If so turn off the fill valve and see if the water pressure drops after a few minutes

  12. #12
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    Oct 2018
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    Thread Starter
    Quote Originally Posted by James Colver View Post
    In my experience high gas bills on a home heated with a boiler are most often caused by a water leak. Do you have hydronic pipes under the concrete? If so turn off the fill valve and see if the water pressure drops after a few minutes
    Thanks; I will definitely check that out when I get to the house this weekend. As far as I know, no pipes go under concrete. There is a 20 foot run from boiler room to crawl space that I cannot easily see. This area is slab that they either frame on top of to allow the pipe run or they dug away some slab to lay the pipework down and possibly did cement over it.

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