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Thread: Is my boiler too small?

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    DIY Junior Member dermin19's Avatar
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    Default Is my boiler too small?

    Hi, thanks in advance for any help. We recently had a plumber install a high efficiency peerless purfire 110000 BTU boiler. Last night it got down to 4, and we were only able to maintain a temperature of 61 when we had it set to 65. We asked our plumber how he figured the size of the boiler and he said he measured the baseboards. Our house was built in the 50s and is probably not the most efficient, but we can't really afford new windows. Any suggestions?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    What temperature was the boiler running at? The amount of heat you can get out of it is somewhat dependent on the temperature the outlet is set to. Were all of the baseboards getting hot water coming in? Do you have thermometers on the outlet and inlet sides to the boiler? Do you know what they read? Was the boiler actually firing continuously?

    Were you able to keep the house warm last winter? What size boiler did you take out? Measuring the baseboard length isn't a good way to determine what the house needs, but it is an approximation of how much heat a boiler could output in the house.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member dermin19's Avatar
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    Thanks for the response. The Boiler we took out was really old, but its output was 300,000 btu. The temp was set to 160 F, but this morning we changed it to 180 F, and have still been having the same problem. We have tried to set it at 70 F, but have not been able to get up to that point. During the day the temp got up to 27 and we were still only able to mainatain 66. As far as we know all of the baseboards are warm and are receiving hot water. We recently purchased the home, and were not here last winter. We have a temp gauge on the outlet which reads 180. We do not know if it was firing continuously last night but seems to been firing continuously today. Thanks again for any help.

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    DIY Senior Member zl700's Avatar
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    If the boiler never shuts off, the water temp never gets to 180 degrees, and the home never reaches 70 degrees, all three situations combined means its too small.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

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    Network Engineer rmelo99's Avatar
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    I'd tend to agree with the others. However using the size of your old boiler as a measurement should be out. Just b/c it was 300k btu doesn't mean it wasn't 2x or 3x oversized. I prob did the job, just not efficient at all. In addition a 300k boiler at 80% eff is only giving you 240k of heat, as an example.

    I'm guessing you got a high eff boiler 93%, 95%?

    You mention you weren't in the house last winter, so does that mean this is your first winter in the house? If so there could be other issues that you are not familiar with. Perhaps it always had problems keeping up with single degree temp days. Not enough baseboard radiation in the rooms? Could be a whole host of things.

    Bottom line is there are more accurate ways to determine the size of your boiler. The measuring the number of feet of baseboards is an old school rule of thumb. It makes the assumptions that someone accuratley measured each rooms heat loss and sized the radiators for that heat loss(very unlikley given the age of your home).

    I would have your installer check that the system is functioning/working as it should. Then if all that checks out I would revisit the size of the unit. Many installers that use the kind of sizing your guy did end up with OVERSIZED units in their customers houses.

    Can you give us more details on your system, boiler type, fuel type, radiators type, piping pics, diagrams. We can help out more if we have more info.

    Without any more info this is prob the best we can help....

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The boiler could be fine, but you don't have enough baseboard for your heat losses. The choices there are add more, or tighten the house up so it doesn't leak as much. If you have an attic, adding insulation there is often quick and easy and may be enough. But, air leaks can overcome many heating systems ability to keep the house warm.

    Do you have a return water temperature gauge? If so, what does that read? That would give an idication of how much heat was extracted.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member david_griffin's Avatar
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    Quote Originally Posted by zl700 View Post
    If the boiler never shuts off, the water temp never gets to 180 degrees, and the home never reaches 70 degrees, all three situations combined means its too small.
    i would add another question. What is the temperature of the stack pipe ? Probably flue gases are veeeerrryyy hot.

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    DIY Member tk03's Avatar
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    The most important question here is was the boiler running all the time and what was the water temperature? If the boiler was short cycling it was producing more heat than the radiation could get rid of. Then start looking at adjustments and flow. Increase flow and get more heat to the home. Do not over-pump.
    Someone asked for pic's, that may help.

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    DIY Junior Member david_griffin's Avatar
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    Quote Originally Posted by dermin19 View Post
    Hi, thanks in advance for any help. We recently had a plumber install a high efficiency peerless purfire 110000 BTU boiler. Last night it got down to 4, and we were only able to maintain a temperature of 61 when we had it set to 65. We asked our plumber how he figured the size of the boiler and he said he measured the baseboards. Our house was built in the 50s and is probably not the most efficient, but we can't really afford new windows. Any suggestions?
    Your heating device is too small. House insulation is poor and probably you have poor windows. This is normal you will never heat it to desired temperature. Ask your plumber to refund money.

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