How cool can you safely let your boiler get between burns?

If I run my boiler for 6 hour/day, and it gets down to ~100f at the end of the resting period, will I damage it long-term? Is that too cool (IE, will I get condensation & then corrosion in the heat exchanger)?

I have a Maxim M175, and last year I let it idle during the shoulder season (pellet boilers do a lot better with that than wood boilers), but even with the idle feed turned all the way down it still makes a bit more soot in the burn chamber than I'd like. I've been running it only at night for the last week or so, which works well, but I'm worried about how cool it gets in between runs. Any comments?

Clarkbug said:

^ Listen to him, not me.

Are you talking water temps or temps of the boiler itself when you mentioned 100 Deg. F?

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Water temps. I don't have any storage, until I turn on the heat in the house, there's no load anyway. When I start it, I let it run until the boiler is up to ~165 before I give it any load. This takes maybe ~30 minutes, after which it runs hard for ~4 hours. The temp of the water in the boiler (90 gallons) sinks to ~100-110 between runs. Probably because I have a water to air heat exchanger in my plenum, and even with the blower off, there will still be losses due to convection. For me, storage is not an option - especially since once it gets truly cold out, the boiler doesn't idle for any significant time anyway. My alternatives for this time of year are:

1) Run for 5 hours a day (like I am now) with ~30 minutes to get it up from 100-110 to 170-180
2) Run 24 hours a day, with ~30-50% idle
3) Burn propane (I also have a propane furnace)

danjayh said:

Water temps. I don't have any storage, until I turn on the heat in the house, there's no load anyway. When I start it, I let it run until the boiler is up to ~165 before I give it any load. This takes maybe ~30 minutes, after which it runs hard for ~4 hours. The temp of the water in the boiler (90 gallons) sinks to ~100-110 between runs. Probably because I have a water to air heat exchanger in my plenum, and even with the blower off, there will still be losses due to convection. For me, storage is not an option - especially since once it gets truly cold out, the boiler doesn't idle for any significant time anyway (and since with a pellet boiler you can load enough fuel to last several days, the cost/benefit just doesn't balance). My alternatives for this time of year are:

1) Run for 5 hours a day (like I am now) with ~30 minutes to get it up from 100-110 to 170-180
2) Run 24 hours a day, with ~30-50% idle
3) Burn propane (I also have a propane furnace)

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In retrospect, I probably should have gone with one of the European models that just puts out the fire when idle, but I got my maxim used for ~1/3 of the MSRP ... so I couldn't pass it up.

That being said, I think concerns about condensation are probably pretty similar for what they'd be for a regular wood boiler - so is it OK to let the water in a traditional wood boiler get down to ~105f between burns?

harttj said:

I have the same boiler and have done what u are doing without issue. The time below condensation temperature is minimal at startup. I normally just let it idle if it stays in the 40's during the day. Usually one call for heat plus DHW keeps it pretty clean.

Tim

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Very strange ... even if my heat kicks on several times a day, mine still gets full of creosote. If I want it to stay clean, I would say that it needs to idle less than ~1/3 of the time and be in 'normal' or 'high' ~2/3 of the time. If I let it idle all day with only one-two calls for heat (so idle maybe 21-22 hours, on for maybe 2-3), it would be a disgusting mess after only a day or two.

harttj said:

Sounds like you need more air. If the burn pot and ash are not white turn up the air. Usually having the air at one yellow is a good starting point for me on normal. Idle can be tricky. Corn tends to idle cleaner than pellets.

If it is full of creosote after 1 day burning way rich all around.

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Creosote only forms on idle. If it's loaded enough to burn in high/normal, it's clean as a whistle. I think hot air going up my chimney is pulling in fresh are through the fan assembly while idling, causing the fuel to smolder. During idle, the boiler emits a small, slow, continuous stream of smoke from the chimney. I really have no idea how to fix that.

Another piece of evidence for this: I've never had my M175 go out during idle, even on a day that was much warmer than anticipated (55 -60) degrees where it idled pretty much all day (it was a huge mess inside after that day). I have my idle air + pellet feed turned all the way down. I have my normal and high feeds all the way down, and the air on the first yellow.

I really wish Central Boiler would release the firmware source code. I would like to modify it so that it intentionally puts the fire out (only feeds the lower auger and runs the water up to 10-15 degrees higher than the setpoint on 'high' burn mode) either at certain times of the day and/or after a certain idle timeout. I find their controller so %^$* frustratingly bad (it runs the propane way too long when igniting) that I've several times thought of figuring out what all the signals going in/out are and rolling my own controller. It also irritates me that it uses propane for the ignition at all - it should be electric like the Euro models, and putting the fire out during idles should be the standard operating procedure.