Temperature discrepancy in boiler with L8124A aquastat + Adding ODR


Old 12-05-15, 05:37 AM
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Temperature discrepancy in boiler with L8124A aquastat + Adding ODR

Hello all,

First of all, this is my first post, but I've been reading tons of really helpful posts on here recently, so thanks to everyone for all the information and help already.

I've got an old Well-Mcclain P-566 boiler with a Honeywell L8142A aquastat in a 2600 square foot house with tons of slant fin radiation. The radiation seemed way excessive even on moderately cold days (big temperature swings, short circulator run times, tons of popping and cracking, etc) so I started looking into lowering the boiler temperature. I figured an ODR was the way to go, but as a first experiment I reconfigured the aquastat to 160/140/15 (I have indirect DWH as well).

The results have been really encouraging. The heat already feels more comfortable, the house still heats fine with the weather we're having (certainly not the coldest days of the year yet, but nights below freezing). Since the boiler's days are numbered anyway, I was thinking I would just stick with it this way and manually ride the HI temp setting on the aquastat as necessary to give us a little more heat as it gets colder. Then I'd invest in some smart control system with the next install. However I felt like the boiler was short-cycling a bit, so I started looking into smarter controls that I could add now on the cheap that would have a good chance of payback in the coming season or two before the system is likely to be replaced.

I came very close to buying the Intellicon HW+ which seems like a good solution for the short cycling alone, but I ended up ordering a Tekmar 256 which seems to have very similar dynamic offset functionality with the added advantages of real ODR. Plus as you can probably already tell, I'm not against the idea of having some settings to play with.

The 256 will be here next week and in the meantime I've just been monitoring things trying to learn as much as I can about the nuances of the system (we just bought this house at the very end of last winter, and since then have done a ton of insulation work, so it's really like a different system).

Long intro, but here's the point:

With the aquastat HI set to 160, I've been measuring the send and return temps from boiler with a nice Fluke IR thermometer, and surprisingly it seems to be turning ON with send temps in the 160s and OFF with temps in the low 180s. The return temp when it just turned off was in the mid 150s (zoned system, not all running right now).

Is this discrepancy between internal and external temperatures at all typical with this aquastat control? Is there some common reason the temperatures would be this different? And when I go to install the Tekmar 256, should I trust the temperatures I'm measuring on the pipes, or what the old aquastat is telling me? My biggest concern is condensation in the boiler. As I've said, it's an old boiler, but I'm hoping to squeeze another season or two out of it, which the tech that serviced it this fall said seems fairly likely. While it seems clear that its current control strategy is really wasteful, I don't want to trash the boiler in the process of trying to improve it. I was pretty confident that I knew where things needed to be set before, but now that the temperature inside is a big question mark, less so.

Any advice is much appreciated.

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Old 12-05-15, 06:12 AM
Join Date: Jun 2012
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I was in a very similar situation as you. Oversize boiler, plenty of slant fin, short cycles etc. I wound up installing the HW+ and was also able to down fire my oil burner as per the boilers specs. The HW+ immediately showed me an almost 20 degree difference from what the aquastat was set to. I verified the temps with an ir gun as well. Being the aquastat is repeatable, although off, I set the water temp according to the HW+. The results were night and day with much longer run times and a very substantial savings in fuel oil.
Old 12-05-15, 06:21 PM
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Interesting and good to hear. Thanks.

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