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

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Posted by dana on May 06, 2008 at 17:00:39:

In Reply to: Re: Is historical usage data better than Manual-J for sizing boilers? posted by HeatPro on May 02, 2008 at 16:12:57:

(apologies for the null-post)

I've tried to get the heatpro program to work, seems to keep calling software not actually installed on my machine, but I'll get there eventually...

Adding mass to the radiant retrofit would be cumbersome since I won't be ripping up the floors. The intention was staple-up/extruded plates for lowest-possible operating temps.

Decommissioning the forced air (at least for heating) seemed like a good bet since even the best sealed & balanced ducts generate pressure boundary differentials increasing air infiltration into the building's thermal envelope- they're inherently less-efficient.

In the current configuration the basement stays so warm (~64F) that the floor is ALREADY 62F or higher, but I daresay it's not the most efficient method. With insulated basement walls (will be ~R20 polyiso + 1/2" gypsum for fire barrier) it's likely to be even warmer next heating-season even after I insulate the ducts and boiler plumbing runs. The 300 square foot radiant floor zone has radiant barrier + R19 batting, but that part of the basement still feels warm (probably since it's an addition with foundiong walls insulated by insulated concrete forms to R15 or so.)

Even prior to heat-loss modeling it's pretty clear that the structure won't need more than 50,000BTU/hour on design day based on the measured kBTU/degree-day performance of the existing oversized kludge, and most of the time much much less. Which modulating boilers with the lowest modulation settings should be looking at? Should I be looking at sealed combustion modulating water heaters instead? (Rinnai, Tagaki, et al ?)

Also, in lieu of an outdoor reset, for the spring I've kicked back the aquastat on the air handler to turn on at 140F (had been 180F), and lowered the overtemp-limit aquastat on the Burnham 206A to 180F (the lowest it'll go without swapping aquastats.) Now when that zone calls for heat the boiler's temp gauge rises slowly from 140F to 180F over several minutes, the burners turn off (air handler stays on) and relights when the temp falls to 140F. There's no forced draft, only an automatic draft-damper, so it's not wasting a purge cycle or anything, just ramping up slowly and turning off the burners, ramping down to 140F then ramping up again.T his isn't the cleanest hack (and far from ideal) but I'm assuming there will be some efficiency-gain(?), since the average temp is more than 20F lower. (Is there a rule of thumb about how much to expect by lowering the temp?)

I haven't measured the return water temp directly (but I will), and my gut tells me the temperature cycling isn't abusing the heat exchangers TOO much, and that since the boiler temp stays over 140F there isn't much potential destructive condensation(?).

When the smaller radiant floor zone kicks on, the burners cycle 90-120 seconds on, gets up to ~180F, then 20+ minutes off, pretty much as-before.



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