High Set Point on Honeywell L7248 Aquastat


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Old 10-16-08, 09:12 AM
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High Set Point on Honeywell L7248 Aquastat

Hi,

I'm new to this site and forum, and I just upgraded the 44 yr. old Weil-McLain Boiler with a new WGO-3 model. I got it for a reasonable price on e-bay and had it installed by a plumber friend but when we got the plumbing taken care of we ran into wiring issues. We got them resolved last weekend and fired up the boiler, but the L7248 is only allowing the high temp setpoint to be adjusted down to 180 F, and I'd like to get down to 150 or lower. The manual says the normal set point range is from 130 to 220F. Is this a common issue, and recently reading the threads from Topgun, etc. is there a work around for this condition?

Hope someone can help.

Thanks, Rick
 
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Old 10-16-08, 06:50 PM
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Try to find another boiler for a good price. When you reset the aquastat to 150f the boiler will end up rusting out due to flue gas condensation issues. Leave it at 180f. The return water temperature on a cast iron boiler must be at least 130f or higher unles you did a boiler bypass. What type of radiation do you have and how many thermostats?
 
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Old 10-17-08, 09:24 AM
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Plasticman

The house has one zone with hot water baseboard, and a second and third zones with 1/2" Pex-Al-Pex, in the floor. The 3rd zone isn't active yet as we just wanted to get heat into the house, as it's that time of year. If I need to keep the temp that high I guess my alternative will be to install mixing valves in zones 2 and 3, because everything else I've read says that the Pex shouldn't be hotter than 110F in and should return at approx. 90 F. But the 44 year old Weil-McLain that I pulled out ran at 150F, so I still have 2 questions, why does this new one have to run so hot, and why can't I set it down lower than 180F? The aquastat manual says the range should be settable from 130 - 220F. So is this normal?????
 
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Old 10-17-08, 10:43 AM
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Flue Gas Condensation

The problem you face with running a boiler cooler than it should be is flue gas condensation. (google it, and there are plenty of threads here to search)... basically, what happens is that if the temp in the boiler is below the DEW POINT of the flue gas, those gases will condense into an ACID inside the boiler, flue pipes and chimney and eat away at them quickly.

It wasn't as much of a problem with older boilers due to their large flue passages, and inefficient design, but newer boilers will suffer terribly ... to the point of destruction. You don't want that.

Yes, install tempering valves for the radiant, and any sections that you feel you need lower temps for ...

Does your boiler make domestic hot water by means of an internal coil ? or do you have a separate water heater ?

It is possible that the reason the a'stat isn't letting you set that temp lower is because you have a LOW LIMIT set. The a'stat probably won't let you drop it to withing 20 of that low limit setting. But trust us, you don't want to run the boiler that cool... do, and you will have trouble down the road.

The boiler RETURN water temps should reach 135 within like 5 min or so on EVERY cycle, and then long enough to dry the condensate that does form as the boiler is heating up. You drop that temp to 150, and you will be lucky if the return ever even sees 130 ...
 
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Old 10-17-08, 12:50 PM
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Plasticman

OK, that makes more sense to me now as far as the setpoint.
I do have a separate electric hot water heater, so no tankless coil. Right now the aquastat is set up for cold start and the low setpoint cuts back in at 160. So it sounds like I should leave those settings alone and install either 1 or 2, 3 way mixing valves to feed the pex zones. Do the 110 feed and 90 return temps make sense to you for the pex? From what I've read so far the engineered wood floor that I will have for a finish floor is supposed to be no warmer than 85F. Does that make sense to you?
 
 

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