Snow Melt Mavens, I need your advice

RayWohlfarth

I came across this project from a long time ago. It was a snow melt system. When we started the boiler it ran for an hour and a half with the bypass valve closed. There was little change in the water temperature. Ever impatient Ray partially closed the bypass valve and allowed the water temperature in the boiler to raise to 160 degrees F. The loop heated quickly in like a half hour. My question to you is: Did throttling the bypass valve actually shorten the loop heating time or was it just a coincidence. The boiler still provided the same Btus whether the valve was closed or not. I would love your thoughts.

mattmia2

Pressure from closing the bypass some opened a stuck thermostatic mixing valve?

Jamie Hall

Interesting, @RayWohlfarth . And, I think, more complicated than it looks…

But the first question is… is there also a pump in the boiler, or is that one circulator the only one? If that one circulator is the only one, then a big part of the explanation is that with the bypass cracked open a part of the low from the pump was going through the bypass from the return to the feed, and only part of it going through the boiler. And also, if that is the only pump, then the BTUh power output to the slab will be very nearly the same in both cases — unless the boiler has a remarkably high head loos. With the bypass partly open, you are getting a 60 degree rise through the boiler, whereas with it closed you got a 20 degree rise — suggesting less flow though the boiler (that old hydronic equation just can't be fiddled!).

I don't think it made a whole lot of difference in the loop heating time.

ratio

ISTR @DanHolohan talking about that very thing. When I read the explanation, it made perfect sense, but I've lost it since then.

mattmia2

Wait if the bypass is completely open, why would any of the water want to go through the loop (or the boiler depending on where the circulator(s) is)? The bypass is less resistance to the circulator than the loop if the circulator is on the boiler. The bypass is less resistance than the boiler if the circulator is on the loop. I assume the boiler was cycling on high limit.

DanHolohan

This was something that came up when the Dead Men began converting gravity systems to pumped systems. By bypassing most of the cool water returning from the system the boiler was able to come up to high temperature (and not condense), and that speeded up the heating out there in the system.

RayWohlfarth

@Jamie Hall It just had the one pump.

@mattmia2 I didnt want to open the valve all the way. I didnt trust the low water cutoff.

It was an old cast iron sectional boiler. It was a strange thing. Thanks everyone.

@DanHolohan Thanks for still teaching me something new

hot_rod

what type of boiler? What was the SwT with the valve closed, and delta?

Was the system under load ? Melting snow during the adjustment?

With the bypass closed the boiler must have been seeing cool return? 30 degrees or lower?

GroundUp

Is Ray absolutely certain that the change occurred after the valve was partially closed? If the valve began as closed and then was partially opened, this type of reaction makes perfect sense and is exactly what Dan described above. It's very common with low temp systems and non-condensing boilers to utilize a bypass this way and they behave completely opposite of what you describe here. Perhaps the ball in your valve was machined or installed incorrectly, where a perpendicular handle was open and a parallel handle was closed (I have seen this several times in my career)?

ScottSecor

@DanHolohan I suspect many of those older gravity systems also had a tankless coil or side-arm style for dhw needs. Bypass might be more important on boilers that also provide dhw from a practical standpoint.

DanHolohan

I agree, Scott. Thanks.

RayWohlfarth

@Hotrod It was a very old cast iron sectional boiler with an atmospheric burner. It was not melting snow. I wanted to see if the loop would heat, the owners complaint. It was just weird that this happened Not sure if it was due to the valve partially closed. I still think about this job 20 some years later

@GroundUp I have the patience of a corgi and was looking at some way to get outta there. It ran with no significant temperature rise on the return until I partially closed the valve. Weird huh. It could have been coincidence

Thanks

PC7060

https://forum.heatinghelp.com/discussion/comment/1857294#Comment_1857294

Ray’s mascot packed up and ready for next job

RayWohlfarth

@PC7060 Thanks for that