How linear is heat loss and delta T?
Extreme cold in New England this weekend allowed for a heat loss test in our ADU. Primarily used in summer here in coastal Rhode Island (Zone 5), it only has resistance baseboard heaters for winter heat. Would like to upgrade to a heat pump. Working on a Manual J now too, but I’m looking for multiple data points on sizing. I don’t have a big electric usage history to work on since the ADU hasn’t been consistently used in winter in the past.
The baseboard heaters total 15.75 feet of length. At 250 Watts per foot, it is just over 3900 watts total. At 3.41 BTU per watt, looking at a total heating capacity of 13,427 BTU/hr, if I’ve done my math right…
The heaters kept the 70 degree set temp until the outdoor temps hit 16 degrees (4pm on Friday), then lost ground until Saturday at 8 am when the indoor temp bottomed out at 60 degrees when it was -8 degrees F. Sunny but cold day. Building regained 70 degrees again when outdoor temp hit 14 degrees around 5pm.
It looks like 15 degrees is the balance point for the resistance heat. That’s a 55 degree Delta T. 99% design temp here is +8 degrees F, or a Delta T of 62, which is about 13% higher than where the resistance heat lost ground.
Is it reasonable to just add 13% to the BTU rating of my existing heaters to get target BTU capacity of a new heat pump? That would bump up the heat needs to 15,000 BTU/hr at +8 F.
Is this a decent way of looking at actual experienced heat loss?
Thanks in advance!
Andrew
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Replies
That’s reasonable. You can keep some baseboards as a free backup.
The assumption of a Manual J is that heat loss is linear, so you've basically done the best possible test. I would offer however two caveats:
1. The house has some heat capacity, it doesn't respond immediately to changes in outside temperature.
2. Saturday was clear and sunny, you probably had some solar gain going on.
Based on those two factors I would say your balance point is probably higher than 15F. Were you able to track what was happening on Saturday night after the sun went down?
I'm in Newport County, East Bay. My recording thermometer said it bottomed out at 6F on Friday, quite a bit warmer than the rest of the state.
Good point about the house's heat capacity and the solar gain. Also, it got cold really fast on Friday afternoon, so the building heat loss may have lagged a bit more because of that.
I logged the indoor temps through Sunday, and the heaters held the set temp no problem after the sun went down. The other data point I have is the prior two mornings had lows around 20-22 F, and the heaters were able to keep up at those temps.
So maybe I should be thinking the balance point is somewhere between 15 and 20 F. That would mean the existing heat would be 13-24% undersized, meaning the upper limit would be 16,650 BTU/hr. All of these #s seem to point to a heat pump in the 12,000 BTU nominal size range, since it looks like max capacities at 5F on the NEEP site are around 16,500 for a few of the "1 ton" ccASHP's.
I'm in the East Bay too. Really far east, if you know the area. Beautiful part of the world. We got down to -8 F on Sat morning according to my outdoor thermometer (and my hands and feet, since I had to work outside all day). But I didn't set up a logger for proof...
Thanks again
Andrew
I have to admit, I went back and re-read my thermometer and the low as -6F, not 6F!
I'm over by Quicksand Pond.
OF course it is a great thought experiment to say 'how much envelope improvement do I need to get that 13 percent?'
Since even really good heat pumps are not that efficient at very very low temps it is not that wasteful to keep the baseboard as backup
Not oversizing will be more comfortable in the AC season.
To really get good info, I would take these measurements over multiple days to help average out variations that could otherwise confuse your data. As DC mentioned, there is a lag time to temperature changes, and you also have varying conditions outdoors. If you had a week or very stable outdoor temperatures, that would be ideal, by Mr Murphy is sure to make the weather wacky if he knows you're trying to make accurate measurements this week! The longer the time period over which you take measurements, the more accurate your results will be, since the little variations will start to average out somewhat.
I would absolutely keep the baseboards as backup heat since you already have them, and electric baseboards are essentially zero maintenance to keep in place while inactive, and they have no standby losses or need for a fuel supply. There is no downside to keeping them in place for backup purposes.
Oversizing a heatpump isn't usually as much of a problem as oversizing a conventional central air conditioner, because heat pumps tend to be much better about modulating. That doesn't mean put in something double the size you need, but it does mean if you oversize a little you're pretty safe. This is especially true for minisplit systems.
Bill