heat loss, emitter output, and reset data -- early results

Rob_35

Just sharing some data, FWIW. Mike T.'s "Eurocave" results it ain't but that certainly was a source of inspiration. I've been collecting some very simple data to inform the tweaking of my reset curve. Recently got a new boiler and doing full ODR with a tekmar 260. Single-family home, copper fin-tube throughout. No indoor feedback sensor at this time. Just plain programmable stats. Two zones.

Attached is a plot of the fin-tube baseboard output (BTU/hr) at the supply temperatures set by the tekmar 260 for the corresponding outdoor temperature (presently using a 5F ODT, and 175F supply temp at the ODT). I modeled the emitter BTU/hr output at various supply temps using the Slantfin engineering data sheet (simple curve fit to the output data they provide, and used the total baseboard in the house to get a whole-house output). Thanks Noel for pointing me to that sheet and the ensuing discussion.

Also shown is the whole-house heat loss (BTU/hr) determined using Slantfin's HE2 software at OTs from 65F to -5F (the ODT is +5F; just pushed it lower to see what the heat loss is).

It hasn't been below 29F yet, so the output curve is not yet well-populated.

What's interesting to me, so far:

1) currently using the fin-tube terminal unit setting. That curve has a bit of a bump in it at the low end (see http://www.tekmarcontrols.com/literature/acrobat/e003.pdf to compare emitter types and curves). I assume that's to make up for the presumed lack of output that fin-tube has at lower water temps. You can see the bump in the data between about 37-46F. It looks like the curve is becoming more linear (as it should) as it gets colder. Experience so far suggests I could probably pick a flatter curve (like the ones for high- or low-mass radiant) and still perform fine (see 3 below) at these OTs.

2) the flat-line at >47F OT is the min supply temp setting. The default is 140F, but I've got it at 115F and the house heats fine at those outdoor and supply temps, even though it's fin-tube.

3) So far, the output is averaging about 3-4k BTU/hr greater than the loss. This agrees well with observations. The stats are satisfying so we are not at constant circulation (though it takes a while to get a stat to satisfy -- e.g., recovering from a minor night setback the temps rise maybe 1F/hr). Also, the loads are still pretty light and solar gain, people, cooking, computers, etc. are obviously a good part of the "unmodeled heat output" at these temps.

Anyway, just sharing. Most of this kind of stuff I learned about here on The Wall. Thanks. Maybe you guys take this stuff for granted, but to a newly-enlightened HO like me, this is pretty cool. The goal for the heating season is to see how the house does when the emitter output is lowered to more closely match the loss. I tried to do a thorough job on the heat loss using HE2. I realize there's a fair bit of room in the Manual J heat loss, but it's probably the best I'm going to do for now. I guess one check will be if I can drop output below the heat loss yet still heat the house. Right now, though, it looks pretty good.

Comments and suggestions appreciated.

One immediate question is whether adding the indoor sensor makes this whole exercise moot?

Thanks.