Circulator aquastat for thermal post purge?

@LRCCBJ Thanks, that would be perfect.

Re water temps, we have cast iron rads with a massive amount of radiation area and a large volume of water in the converted gravity systems (150 gallons or so). As a result, the cold start boilers start at 65 degrees and often end a 30-minute burn at 130 degrees or less. Yes, these are low temps relative to most other heating systems. Still, because there's so much cast iron and water inside the boiler, I figure there are at least 10,000 BTU's left in there after the burn. And since a typically heating cycle puts out 50,000-60,000 BTU, that's potentially 20% of the heat output left to be recovered. Will be interesting to see if we can get a measurable reduction in fuel consumption with post purge. I would think with our system, 5%-10% savings might be achievable.

@MikeAmann Nice job, Mike. I knew someone had already invented the wheel! I see you get about a 30 degree temperature drop during a 30 minute purge. I was expecting to need about a 30 minute purge here too.

Will be interested to hear what reduction you see in oil consumption.

OK, got it. Thanks, guys!

@hot_rod Thanks, yes, I have been alerted by you, @EdTheHeaterMan and many others that our two WGO-5's have probably been condensing flue gases for the last 30 years. I have done everything I can to get the water temps up, with higher "swing" settings on the Tstats and an overnight setback so we have at least one longer boiler run in the morning. Yet miraculously, despite the decades of cold water abuse, the heat exchangers are in remarkably good shape, and there are maybe only one or two pins missing in one of the boilers.

We've discussed this extensively in other threads, so I won't reinvent the wheel here. Suffice it to say that when the boilers need replacing, I will ensure that the installer installs a bypass, as should have been done with these but was not. Also, in a (very belated) effort to improve the current situation, I've installed triac-based "wave chopper" controllers on the Taco 007 circulators to reduce the flow rate from 19 gpm to about 12 gpm, thus slowing the flood of low-temp water through the boiler and increasing the delta T from 12 to 20 degrees. So far the controllers are working well.

Here's the thread where our low water temps were discussed a while back:

https://forum.heatinghelp.com/discussion/193617/weil-mclain-wgo-5-breaks-all-the-rules#latest

@hot_rod Since the post purge by definition only happens after the burner has shut off, at that point there are no combustion gases being produced to condense, so I'm not sure what the concern would be.

The only effect I can think of on the boiler is that it will cool and contract slightly faster than without the purge, but the rate of temperature change will still be a lot less than the rate of change when the burner fires up and the rapid thermal cycling begins on the "up side" of the cycle. Also as I mentioned, I've slowed the flow rate as much as I can with the new triac controllers.

There are the usual barometric dampers, but not vent dampers, so we do lose heat up the chimney after shutdown.

@EdTheHeaterMan talked about purging a cold start cast iron boiler like mine earlier in @MikeAmann 's thread on the same subject and said there should be no ill effects, and I'm just following his advice (I think!)

@EdTheHeaterMan Yes, I remember seeing an old thread from here where someone questioned how EK could advertise such high cycle efficiencies, and Roger patiently explained that the high efficiency of EK's was due to (1) low heat exchanger mass, (2) low water volume inside the HX, and (3) post purge.

We can't reduce our HX mass or water volume until we get new smaller boilers, but we can do the post purge. I'll be interested to see the results. We've kept good records of annual oil consumption for 10 years, so if the purge reduces our oil consumption, it should show up in our numbers. Based on some back-of-the-envelope math, I think a 5% reduction is possible.

@EBEBRATT-Ed Aw Ed, there you go ruining my Sunday. 😉

Yes, we have relatively low water temps, but we still have 700+ pounds of hot cast iron and 80+ pounds of hot water sitting in the boiler after shutdown. That's around 800 pounds of thermal mass with a lot of stored energy.

Based on average temperatures, I figure there's about 10,000 useful BTU's left in that thermal mass after burner shutoff. For an average boiler run time, that's more than 10% of the useful BTU output of the boiler. So if post purge can keep half of that 10% from going up the chimney, that's a 5% energy savings.

We'll see…

@fentonc Thanks for that report. I expect some overshoot, which is fine. But we have ecobee thermostats which do an excellent job at adapting to over/undershoot and correcting on subsequent cycles as needed. Will be interesting to see if the purge results in the ecobees reducing the run times, and by how much.

@hot_rod Thanks for your thoughts. Answering your questions:

1. Yes, we have both oversized boilers and oversized radiation. During the polar vortex event in Boston area last Feb with 0 degree temps and high wind chill, our boilers ran only 40% duty cycle. And average water temp at rads was probably around 120 degrees.
2. Yes, most of season boilers are running 20% duty cycle or less. Typical run is cold start, 30 minute burn, with SWT about 130 degrees at burner shutoff, then 4+ hours idle.
3. Even without purge, the boilers always start with cold water because there is so much idle time (typically 4-5 hours) between burns. So purge is not going to make the start condition any different. Boilers always start cold now, and they will start cold after I install the purge.
4. The boiler and flue do not show any signs of condensing. Even with the low RWT, the flue gas temp is 400 degrees F, and we're burning 1.2 GPH. So that is a lot of hot gas, at 400 degrees, which is NOT condensing.
5. The flue pipe is standard galv sheet metal, with a short run to the adjacent brick furnace. If the flue gases were condensing in the flue pipe, we'd be getting drips from the loosely fitted joints, which we don't.
6. We had the chimney inspected last year. It has a clay liner, and the chimney guy said it was in good condition, no repair or cleaning. No soot. It's 100 years old.

I think it's worth repeating that our flue gas temperature is 400 degrees, and that's measured after the boiler in the flue. So the gas temperature inside the boiler is over 400 degrees, which makes it impossible for condensing to be happening in the boilers. These are 30 year old boilers and the heat exchangers show no evidence of condensation. There are maybe 2 pins pissing in one of them. Otherwise they're fine.

Also, the relatively short run of flue pipe means the 400 degree exiting gas is not cooling fast enough to condense inside the flue pipe. And the clay-lined chimney is in good condition.

So I keep returning to the point that the 400 degree flue gas makes it virtually impossible for condensing to be happening inside the boiler where gas temps are even higher. And there is not evidence of condensing downstream in the flue pipe, or even in the chimney.

I know it seems hard for people to wrap their heads around this, but these boilers have been running with stone-cold water temps for 30 years without condensing, because the 400 degree flue gas is still 400 degree flue gas whether the RWT is 70 or 140, and the 400 degree gas prevents condensation on the hot side of the heat exchanger.

@Robert O'Brien If I had a nickel for everyone here who told me "this is a problem," I'd be rich now. 🙂

I've been discussing these boilers (our two WGO-5's) here for several years and am grateful for all the knowledge and good advice the pros here have shared. For some history, you might want to check out one of my earlier posts that explains our setup.

https://forum.heatinghelp.com/discussion/193617/weil-mclain-wgo-5-breaks-all-the-rules#latest

I respect and appreciate all the good advice I've been given freely. However, one ongoing mystery is why all the pros keep telling me "your boilers must be condensing, and this is a big problem" when there is no evidence that's actually happening.

These boilers have been running cold start with low water temps for 30 years, long before I even moved in 15 years ago. The boilers run 1000+ cycles per season, averaging 30 minutes per run, with starting water temp 65 and ending at 130. So that's 30,000+ cycles that each boiler has run in its lifetime.

Every year we have the heat exchangers cleaned, and every year I inspect them myself alongside the tech. And every year, they're fine. No evidence of condensing, either there or in the flue. After 30 years, there are only maybe 2 pins missing in one of the boilers. They could easily run another 10 years, IMO.

Someday I'm going to invite all the pros here for pizza at my house, so we can all tear these "mystery" WGO-5's apart and find that sneaky condensation in the 30-year-old boilers that have run 30,000+ cycles and show no signs of problems.

I'm also thinking maybe I should invite a Weil-Mclain rep out to admire their miracle boilers, since they seem to be doing fine for 30 years in conditions that everyone says should have destroyed them. 🙂