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Is historical usage data better than Manual-J for sizing boilers?

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Posted by dana on May 02, 2008 at 15:44:36:

I'm about 18-24 months out from replacing my boiler and have been tracking actual gas usage per degree day (base 65) as I tighten up & insulate my ca. 1923 bungalow. While Manual-J heat loss calc might be a reasonable & conservative way to estimate radiator/radiant-tubing requirements, my gut (and arithmetic) tells me that it overestimates the actual losses by quite a bit, which would lead to an oversized boiler. (I've been using the Slant/Fin freebie. By setting the design-day temp on the tool to the average temp for my last-months gas bill it's giving me loss rates more than 50% higher than the boiler's metered INPUT!! And the house isn't yet nearly as well insulated as the parameters I've entered into the tool- I was using it to calculate what I needed once I'm DONE!)

With old gas bills, weather data and a spreadsheet I've calculated that my actual -5F design-day requirements are about 80kBTU/hr (input) to my old system. After a bit of tightening (and prior to fully insulating)it would be about 70kBTU/hr, and conservative guesstimate post wall & basement-wall insulation & further tightening it would run about 55kBTU/hr in.

My boiler is a mid-1990s Burnham 206A (easily 2x oversized at this point) heating most of the house via a hydronic air handler, and a second zone of ~300' of staple-up radiant floor. It can't be all that efficient, and I expect as-run it's no better than 65% AFUE, no matter what the plate says. (It short-cycles a LOT with just the hydronic zone running.) Within the next year or I intend to retrofit radiant floor to the rest of the place (keeping the air handler for the AC), but intend to put a modulating/condensing boiler in place before commissioning it.

Bottom line, is it better to estimate the boiler size by a linear projection of what it would be assuming design-day temps based on actual history using the old system? If yes it would be about 1/3-2/3 the size of what the heat-loss software tells me(!).

Assuming I'll get another 15-20% improvement with the radiant floor & mod/con boiler, that puts me in the 40-45kBTU/hr range (about 2x what the Slant/Fin estimator give me!)- who besides Peerless makes 'em that small?

Last (and probably least) since I'll be running at least one more heating system with the Burnham 206A, will it boost it's efficiency if I turn down the high-limit to something like ~160F? (Currently the highest temp I've ever read on the gauge with the air handler running is ~180F, and the radiant zone needs nowhere NEAR that temp. The longest the air handler ever runs is about 45 minutes to bring it up to 68F after a weekend-away setback down to 55F.) I've toyed with the idea of hacking an outdoor reset into the system, but figured I'm better off in the short term taking simpler measures, saving the money for a mod/con boiler rather than monkeying around to get the last 3% of efficiency out of the existing system.

Apologies for the long-winded question, but I figure insufficient data-in might result in less-well considered info out.



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