Power vent, direct vent or natural draft chimney

Discussion in 'Boiler Forum' started by Islander, Aug 14, 2012.

  1. Islander

    Islander New Member

    Messages:
    2
    Location:
    RI
    I have a oil boiler with a power vent. I usually cannot get through a heating season without a service call or two even though the system is cleaned/tuned up every fall. The technician says my trouble is the power vent and I should install a chimney. I do not intend on putting up a chimney and cannot understand that the natural draft chimney is better than a power vent. I assume that the power vent provides the same draft all of the time and that a consistent draft would be a good thing. The same tech says that the direct vent is better than a power vent but the chimney is the best. I'm disgusted with my situation and I have been considering installing a propane mod/con system. Thanks
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    Two of the three (power vent and chimney) rely on internal air for combustion, a direct vented system utilizes outside air for combustion. A direct vented system just by that one fact, will be more efficient since you aren't throwing away internal, already heated air, outside and having to heat the new air coming in to replace it.

    A power vented system has numerous additional safety devices in it that all must be working to allow it to run. A natural vent up a chimney wouldn't know if the chimney ended up being plugged, and was dumping all of the combustion products into the house (you might not either after it killed you!?). Even on a chimney, you might add a damper to help efficiency, and that, being mechanical and in a hostile environment, can fail.

    Not to say that a direct vented (closed combustion) system doesn't have safety features, but of the choices, they can be the most efficient.

    Not sure if they make a mod/con oil burner, but if you have NG available, that likely would be your best choice, should you decide to make the switch. You can do some searching here and see some of the recommendations - the first thing would be to get (probably pay for) a good manual-j (heat load) analysis. The existing boiler is almost certainly oversized, probably by at least 2x and maybe lots more. Any new system should be properly sized for maximum efficiency and comfort. Ideally, on the coldest day, the boiler should run continuously. And, with a mod-con, it has the ability to run continuously on more days than a fixed output device. This allows it to attain its best comfort and efficiency.
  3. Dana

    Dana In the trades

    Messages:
    3,020
    Location:
    01609
    All oil boilers are oversized for an average sized home in RI which leads to reduced efficiency. A smallest-of-the line propane mod-con would usually work though, but at an uptick in price/MMBTU on the fuel. Even with the reduced efficiency from oversizing, it'll usually still be cheaper to heat with a direct-vent triple-pass oil burner than with a propane mod-con, but an appropriatly sized mod-con would be more comfortable. At last season's prices heating with propane was comparable in cost to heating with electric baseboards (unless the island you live on is Block Island, or some other oil-fired island grid still unconnected to the mainland power grid.)

    A mid-winter oil bill with a K-factor stamped on it would give a very good indication of what the design-condition load is without doing a full Manual-J type calc. The smallest oil burners out there have output north of 50KBTU/hr, but a typical 2500' house in RI with any insulation at all and at least storm windows over single-panes will have a heat load less than 40KBTU/hr, at +10F (the 99% outside design temp for most of RI) and in newer-better construction it could easily be less than 30KBTU/hr.

    If the heat load analysis (Manual-J or fuel-use calc) indicates a heat load under 36KBTU/hr, and the house has a reasonably open floor plan, before spending the money on a boiler replacement, you may consider heating with ductless air source heat pumps (mini-split or multi-split), which would would have a similar up-front cost but would cost less than half as much to run as an oil boiler, based on the 5 year average RI pricing for oil & electricity. (Obviously Block Islanders won't be going this route.) Like mod-cons, ductless heat pumps modulate output with load, running quietly & continuously (most of the interior heads are quieter than a refrigerator at their lower blower speed ranges), and have none of the wind-chill comfort or temperature swing issues of hot air furnaces. A ductless heat pump would also give you high-efficiency air conditioning too. Most units operate just fine down to 0F, and many of the better ones have specified output down to -5F. Some have specified output all the way down to -13F.
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