Circulator Post Purge - Can return temp get too low?

[Eric F]

Hi everyone. I have a Burnham ES2 boiler.

The boiler lets you configure circulator post purge/overrun time for 0 to 10 minutes. This keeps the circulators running after the call to heat has stopped, which seems like a generally sensible way to extract maximum heat from the boiler.

The setting is done by time, so I am wondering if this puts me at risk of pushing too-cold return water back to the boiler. You'd think that the boiler would have some mechanism to turn off the circulator when the return temps got too low, but the documentation doesn't mention anything. (Documentation snippet here)

Any advice? I'd set this to the maximum 10 minutes if return temps were not a concern.

Thanks!

 

[Dana]

What are the odds that your radiation would drop all the way to the 110F minimum return water temp in a 10 minute time frame?

A: Near zero.

A': It depends on the temperature the system was at the end of the call for heat, and the thermal mass of the radiation.

A' ' : During the post-burn purge session it doesn't really matter what the return water temperature is, since there is no combustion happening, thus no possibility of condensation on the heat exchanger plates. This is a cold-start boiler. When a new call for heat occurs the boiler controls don't power up the circulators until the boiler has reached some minimum temperature.

If you're really concerned, measure the returning water temperature entering the during the purge session, and measure how fast the temperature comes back up to 110F upon a new call for heat.

The biggest factor in how long the purge cycle runs is how much electrical power is being consumed. At the beginning of the purge the boiler's temperature is typically several tens of degrees warmer than the radiation. At the end of the purge ideally it would be much closer. But once the difference between the boiler temp and raditaion temp is under 10F you're probably using more energy in the circulator pump than you're scavenging from the boiler.

 

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[Eric F]

Thanks Dana, that answer was super-helpful.

I hadn't thought about the condensation/temp issue being connected to the burners firing but that makes sense. I also had thought that the temperature drop as the water travels across the radiators would be higher.

I'd like to figure out a way to easily measure the return temperature, that'd be the best way to know what's really happening. Unfortunately the system was installed with a temp/pressure gauge on the supply side, but not the return side.

 

[Dana]

There are strap-on pipe thermometers, but simply painting a patch of pipe (or giving it on wrap of hockey tape) and reading it's temperature with a $50 box store pistol-grip infra-red thermometer is good enough.

The temp/pressure gauges are usually measuring the boiler temp. Did they install one on the output piping?

The temperature drop across the radiators gets smaller as the radiators cool, but once the boiler is already pretty close to the return water temp there is no heat left to purge from the boiler. In a typical system at the end of a burn the boiler might be 170F, the output water temp might be 165-170F and the return water 145-150F. The amount of heat being purged from the boiler is quite small once the boiler has cooled to within 5-10F of the return water temp. But parking the boiler at 150F dramatically reduces the standby loss compared to parking it at 170F.

 

[Eric F]

Thanks again. I will get a strap-on and check the actual return temperature under different conditions and report back.