Boiler questions

Paul Riddle

My house has a Weil Mclain 68 series oil-fired hot water boiler. Not sure of its exact age, but guessing 10-15 years. It is set up with a tankless coil, but the coil is not in use (previous owners installed a dedicated oil-fired water heater). The boiler has a Honeywell 3-way aquastat, low limit set to 160, high limit 180, differential 15. That's all fine, but here's where things get weird: The aquastat is not wired to the circulator. Instead, the circulator is directly controlled by a line-voltage thermostat. The thermostat is effectively a switch that turns the circulator on or off, completely independent of the boiler. The thermostat terminals on the aquastat are jumpered together. So, the aquastat sees a constant call for heat and maintains the boiler temperature at the high limit of 180. The low limit is never used at all. Questions:

1. Would it make sense to rewire this setup to use a low-voltage thermostat and run the circulator off the aquastat?

2. Can this boiler (Weil McLain 68 series) be operated "cold start" (burner runs only when thermostat calls for heat) without risk of damage from condensation? If not, what would be a safe low limit setting for the aquastat?

3. Is there a safe way to shut down the boiler completely for the summer?

Thanks for any opinions,
Paul

Mark Hunt

Sounds like


the former owners had issues with insufficient hot water. The coil is probably so calcified that the flow through it was not enough for the demand.

I am guessing that the current wiring configuration was done to keep the boiler hot for domestic water.

I would have a contractor come in and replace the aquastat relay and run the boiler as a cold start.

Depending on what style convectors you have, cast iron vs. copper fin, you may need to have the near boiler piping changed.

Mark H

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Dale

Safe?

Your third question would seem to get a yes, just shut off the 120 boiler elect supply which should feed power to the oil burner. If your system is tight leaving deareated water in it all summer won't be unsafe. If you want tighter temperature control a 24 volt stat gives it, the 120 will work OK. I think the 180 set temp was for the old hot water load. You could do a heat loss for your house and find out what the max design temp would need to be, a well insulated house might only be 160. As to the low temp and cold start I don't know, however alot of oil guys say keeping it at 130 as a low limit is a good idea, I would go with whatever boiler temp and piping strategy WM recommends. Most systems benefit from constant circulation in the winter, let the stat control the burner, Go to an indoor outdoor reset if you want to max savings.

Guy_5

Cold Start

I agree with Mark, get a contractor in there to re-wire the system so that it will run properly. While you're at it, it would be worth it for you to convert to a 24volt thermostat so that you can install a programmable one.
I have always recommended maintaining a 100 degree minimum boiler temperature, especially on tight pin type boilers such as yours. Cold starts create a white crusty (sulphur?) build up that blocks and/or corrodes the pins within the flue passages.
i have that same boiler in my own home and have no complaint with it. (Hey, it came with the house!)

John@Reliable_4

I agree with Mark on all, BUT don't make it a cold start!

I work mostly on oil systems and see what happens to "cold start" boilers and it's not pretty. Take advantage of the aquastat that you have. From what you said I don’t think it would be hard or costly to make the system run the way you want. The benefits of leaving the boiler on during the summer far out weigh what will happen. The cost of oil during the non-heating season will be pennies a day, but can extend the life of boiler 50%. Hope this helps John@Reliable

Leo

WM Cold Start

This particuler boiler will plug itself if left off all summer. I get frustrated seeing these guys cold start and even more frustrated seeing one off that has a maintaing control on it. I have unplugged a lot of these and the cost of the service call far out weighs any savings in oil. Do like the others have advised and get it put back on a low voltage stat.

Good Luck,

Leo

ScottMP

I come across this alot of times

Would there be any trouble in turning the low limit of the controll down as low as it will go ?

Scott

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John@Reliable_4

Scott, I set them at 120 Lo & 180 Hi w/ 25 diff

and come service time still clean heat exchanger, with no scale on pins. John@Reliable