How to add thermal purge to a boiler

How are you keeping the zone valves open?

I like the label on everything. this way when someone else needs to work on this, it will be idiot proof. But I remember someone saying that we are making better idiots now a days. And Mike, If you are a avid follower of the Holohan writings, you will know that putting arrows on the outside of the pipes is useless... the arrows need to bo on the inside of the pipes for the water to see, then and know which way to go.

Nice job, @MikeAmann . That's certainly one way to do it.

However, if one is going to replace the aquastat relay on a single-zone system, the Beckett AquaSmart has the thermal purge built in. It's time-based rather than temperature-based, but it works well in my experience.

And for replacing those super-expensive R8182 controllers, Beckett makes a combination AquaSmart/GeniSys package. I put one on my boiler some time ago:

https://forum.heatinghelp.com/discussion/176551/nice-job-beckett

RascalOrnery said:

Can someone help me understand how allowing the boiler to be drawn down to such low temps won't cause problems

Flue Gas Condensation will always happen on a cold start boiler. It is part of the physics of the combustion process and the cold metal that causes the flue gas to condense. You just don't want it to be there for extended operation. Let's say that the burner operates for 8 minutes before it gets up to the high limit on a small water content boiler system. for the first 2 to 3 minutes that cast iron will have condensation droplets on the surface. For the last 5 minutes of burning the flue gas temperature will be hot enough to burn off that condensation and the cast iron surface will be dry. Very hot and very dry. Then the burner stops as a result of the high limit setting even if there is still a call for heat.

That cast iron will have only the draft of the chimney drawing cold air thru the boiler past the hot cast iron. No chance of flue gas condensation here, there is no flue gas without combustion. Then the boiler temperature drops and the burner starts again. The boiler temperature is only 10° below the limit setting and therefore no condensation. This cycle can happen more than once but no more condensation.

Eventually the call for heat is completed and the circulator can stop and leave the 170° to 200° water there while the chimney draft pulls air across the heat exchanger or you can operate the circulator and put a portion of that energy in the house, and less of it in the air that is going up the chimney. Either way there is no flue gas when the burner is off, so no condensation at any temperature above 80° in the boiler. That won’t happen with just air until the boiler reaches temperatures in the upper 40°s. I don't see that happening in the winter… Do you?

Bottom line. Flue gas condensation gets burned off pretty fast on a properly designed system. Purging heat into the home or DHW is only going to save $$$ and will not have ill effects on the boiler.

That does make sense, but then it seems to me to negate the whole rationale behind 'primary/secondary piping' or 'bypass loops' to "protect the boiler" , I can understand protecting the boiler from thermal shock but flue gas condensation is also brought up in the various resources I've read.

As long as the boiler setpoint/HL is above condensation temperature why would it ever matter if there's condensation? It will always be "burnt off" right? So I'm confused why it's regularly mentioned as a danger.

I'm also understanding all of this from a 'non condensing boiler' standpoint.

Are cold start boilers also very low water volumes usually?

It appears that no one answered @RascalOrnery's query. "As long as the boiler setpoint/HL is above condensation temperature why would it ever matter if there's condensation? It will always be "burnt off" right? So I'm confused why it's regularly mentioned as a danger."

If you replace an old, large water content, coal converted boiler that operated by gravity (no circulators) with large cast iron radiators, It would not be uncommon for that system to have over 100 gallons of water in it. Now remove the boiler and replace it with a new smaller water content steel or cast iron boiler that has less than 5 gallons of water content, your system may have over 80 gallons of total water content. In that situation with the boiler supply connected to the old supply main and the boiler return connected to the old system return main, you could be circulating anywhere from 70° to 125° water for as long as 45 minutes before the call for heat is satisfied. you may never get the system water temperature above 130° in the fringe seasons. That would be unacceptable. The flue gasses would condense for the entire cycle because the water on the other side of the cast iron or steel would keep the combustion side of the metal beliow the flue gas condensation temperature for more than 3/4 of the burner operating time.

That is why there are bypass and primary secondary piping arrangements for large water volume system. That allows the boiler temperature to get hot enough to eliminate Flue Gas Condensation.

Back the the post thermal purge, no matter what the system water temperature is, the boiler water temperature above 130° on shut down will allow heat to be wasted up the chimney. Thermal purging into the last zone that calls for heat will have a savings. Any heat that does not go up the chimney will be money in the home. It's just that simple, when ever you can take heat from an idol cold-start boiler, and put it in the DHW tank or a room in the home. that is heat not wasted.