Is my boiler too small?

Discussion in 'Boiler Forum' started by dermin19, Nov 25, 2010.

  1. dermin19

    dermin19 New Member

    Messages:
    2
    Location:
    Denver, CO
    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?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,258
    Location:
    New England
    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.
  3. dermin19

    dermin19 New Member

    Messages:
    2
    Location:
    Denver, CO
    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.
  4. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    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.
  5. rmelo99

    rmelo99 Network Engineer

    Messages:
    349
    Location:
    Connecticut
    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....
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,258
    Location:
    New England
    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.
  7. Dana

    Dana In the trades

    Messages:
    2,969
    Location:
    01609
    Insufficient pump flow would be another reason why it couldn't deliver enough heat, and that would show up as a larger difference in temp between outgoing & incoming water.

    It's also possible that it's outdoor-reset was programmed to the wrong curve, or it's outdoor temp sensor is mis-installed/defective giving a too-high reading, lowering the output temp of the boiler.

    Sizing a hydronic boiler by measuring the radiation size is just as inaccurate as doing it by square feet of living space. But once the heat load is known by a proper heat loss calculation you'd need to measure the radiation to determine the peak water temperature, and select the outdoor reset curve accordingly.

    At full fire with 180F outgoing water the best the Purefire 110 is going to deliver is ~ 85% efficiency, which would mean with a 110K input you'd get 93-94K out. ( At lower output temps-or more accurately, lower RETURN temps, that rises slowly until you get down to ~140F out below which the efficiency climbs quickly into the 90s.) Mind you, 93K still 3X the heat load my ~2200 square foot 1920s home with storm windows over & double-hung windows has at 4F. You'd have to be practically un-insulated &/or have a window flung wide open to have a true heat load over 90KBTU/hr at 4F in a mid-sized house, but I suppose anything is possible.

    Still, if it held the line at 61F you only need a ~10% improvement in the thermal & pressure envelope of the house to make it work. That could be as simple as fixing any major air leaks do you have any open fireplace flues? Mail slots? Recessed lights cutting into the attic? Assuming it's a full basement, is the foundation sill & band joist all cob-webby (a sign of air leakage.) Foundation sills & band joists are the single largest most commonly overlooked infiltration point- it's HUGE! How about door sweeps & weather stripping on exterior doors?

    Air sealing is your single-most cost effective improvement- find an insulation contractor who specisalizes in it after you've nailed down the most obvious stuff. Treat all the big holes first, but then concentrate on leaks into the attic, and leaks into the basment. The stack-effect is a huge driver of air infiltration- if you fix it at both the top and at the bottom, you can get away with a lot of leakage in-between (but fix those anyway.)

    If you have single-pane windows, storm windows (exterior or interior) are cost effective, for far less cash outlay than even bottom-of-the-line replacement windows, and if the original windows are reasonably tight, with storms they'll even OUT PERFORM bottom of the line replacement windows.

    A lot of 1950s houses had R8 "econobatt" insulation in 2x4 studwalls, leaving about a 2" gap of empty space inside the wall cavites. Better ones had R11 full-depth batts, but over time those can sag/fall leaving gaps & voids in the insulation. In almost all timber-frame construction it's possible to retrofit blown cellulose into cavites without completely ripping the house apart, and it's well worth it from both a comfort and economic point of view. You could blow rock wool or fiberglass in there as well, but cellulose does a measurably better job of slowing down air movement through the walls, and it usually about the same cost or even somewhat cheaper. Ideally they would be able to "dense-pack" the material to 3lbs+ per cubic foot density, at which point it will have reduce the air flow potential by ~98% below that of 1950s style R11 batts. If yours has R8 econobatts it would be great news, since it's easier/cheaper retrofit blown insulation into a partially empty wall cavity that a "dense-packing" tube fits into more readily. Dense-packed you'd be in the ~ R13 range, but unlike low-density batts, it would stil PERFORM at R13 when it's 0F out, whereas R11 batts lose nearly half their R-value at temps that low due to convection currents in the insulation itself.
  8. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,261
    Location:
    Maine
    I'd bet money that it is not correctly piped.
  9. david_griffin

    david_griffin New Member

    Messages:
    25
    Location:
    Denver, CO
    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.
  10. david_griffin

    david_griffin New Member

    Messages:
    25
    Location:
    Denver, CO
    i would add another question. What is the temperature of the stack pipe ? Probably flue gases are veeeerrryyy hot.
  11. tk03

    tk03 New Member

    Messages:
    56
    Location:
    Harrisburg, pa
    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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