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Author Topic: Gas boiler tries to fire despite OWB supplying high temp water  (Read 3927 times)

wreckit87

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Gas boiler tries to fire despite OWB supplying high temp water
« on: October 18, 2017, 09:56:29 AM »

So I piped an OWB into a guy's existing system yesterday. Boiler loop heats a 20 plate, Taco 007 draws hot water from the plate, through a mixer, into a Lennox Q90-100 LP boiler which serves an 8 loop radiant system. Now, I've done this dozens of times before and left the wiring up to someone smart, it works out fine but I've never seen a boiler like this Lennox that has no setpoint control. The only aquastat is a high limit, and when operating the boiler puts out 120 degree water and short cycles every 90 seconds or so. I turned the mixing valve up to 130 thinking it'd cut out the gas burner while the stat continued to spin the pump, which it did once the supply water had reached temp. Problem is though, that the inducer fan never shuts off. And every 5 minutes or so, the igniter would light up and gas valve open, fire the burner for 3 seconds and then shut down again.

Given the insulation quality and size of the loops, I want to run 100ish degree water through the mixer instead of 130. At that, the gas burner will want to run all the time. Dude wants to keep the Lennox in operation if possible as a backup and to keep the OWB loop from freezing up if need be, so adding another stat to power the pump only was all my feeble mind could produce. He wants to do the electrical himself, so I was wondering if any of you gentlemen could explain this to me so I have some insight on what to tell him. I'm at a loss. Thanks in advance
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hondaracer2oo4

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RSI

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #2 on: October 18, 2017, 02:39:44 PM »

What would happen if you put a strap on aquastat on the OWB pipe and connect in parallel with the high limit?
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wreckit87

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #3 on: October 18, 2017, 04:17:02 PM »

Yeah, I read that entire manual and can't find anything pertaining to the issue at hand. I thought about the aquastat thing RSI, but was thinking it needed to be connected to the control board somehow, like to prevent the burner from firing as long as the OWB temp is above 100 or whatever? Electricity isn't for me- I just know the piping side of things. How would one tie the aquastat in with the high limit/ what would that accomplish? Please excuse my ignorance but does the HL also run the setpoint or is that a separate sensor elsewhere?
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RSI

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #4 on: October 18, 2017, 07:01:18 PM »

I just glanced through the manual and it looks like the high limit controls the temp. I think it says it is adjustable up to 190.
I would think if connections are accessible, you could connect to the output of the high limit control. You would need to know if it opens or closes when it hits the set point.
If it closes then just connect aquastat in parallel. If it opens then you would need to connect in series.

This should make it think that it is always up to full operating temperature whenever the owb is hot and go back to normal operation when the owb pipe is cold.
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wreckit87

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #5 on: October 18, 2017, 09:30:19 PM »

I just glanced through the manual and it looks like the high limit controls the temp. I think it says it is adjustable up to 190.
I would think if connections are accessible, you could connect to the output of the high limit control. You would need to know if it opens or closes when it hits the set point.
If it closes then just connect aquastat in parallel. If it opens then you would need to connect in series.

This should make it think that it is always up to full operating temperature whenever the owb is hot and go back to normal operation when the owb pipe is cold.

Hmmmm... I'm wondering what the point of a high limit is if it doubles as the setpoint? Wouldn't it go out on HL every time it reached temp? That's how others I've played with have worked anyway. The HL is more of a redundancy thing in case the setpoint control fails. I think. My other concern is that the HL aquastat on this boiler was set at 160. Setpoint according to the temp gauge was 120 right on the nuts with a 10 degree differential. I don't see how the HL stat can operate a setpoint 40 degrees less than its own setpoint but still serve as a HL. Probably just ignorance at its best here, but I'd really like to understand it
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RSI

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #6 on: October 18, 2017, 11:07:34 PM »

I was just looking at the manual an may be worth trying to just turn off the economy mode and turn temp down.
It looks like the high limit is just the water temp control.
Turning off economy mode will just make it run a fixed 10 degree differential.
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wreckit87

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #7 on: October 20, 2017, 07:45:52 AM »

Would the "thermal targeting" feature be dropping it to 120 although it's set at 160, because it's warm outside? Seems like an outdoor reset type deal, but there is no outdoor sensor OR economy mode switch. This is an older Lennox, not the Dunkirk that manual shows. There is exactly 1 switch on the entire unit, and it's a reset on the control board. No economy anything. I don't know man
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FrozenMongrel

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #8 on: October 20, 2017, 11:07:33 AM »

Is there way to run a 120v relay to only kick power on to the unit when the incoming OWB water temp is below a set point? I don't know if the power can safely be disconnected that way with the gas boiler, but an aquastat on the incoming line run to a 120v relay like you would run a circulator from would allow you to have either an outlet switched on/off depending on incoming temp or the unit itself if it's hardwired in.
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RSI

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Re: Gas boiler tries to fire despite OWB supplying high temp water
« Reply #9 on: October 20, 2017, 05:37:35 PM »

That may work but I assume it does some sort of cool down cycle after the burner turns off. If it is running and the power trips off it may not be good for it.
Finding a way to make it think it is at full operating temp seems like a better option.
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