Weil McClain Cast Iron Boiler Reset Temperature Settings

Chuck_21

Hi,

I have a Weil McClain Model 68 oil fired boiler with (4) Zone Valves and No Tankless. The old literature has the boiler temp at 160 deg F.

The room thermostats all feed back to a Taco EXP control panel with Outdoor reset. The boiler aquastat is a HW7224U and can be set up for low and high limits or just high limit.

(2) Thermostats are setback each day and evening 5 degrees to 65 Degree F. The other (2) will control at 55 degree F with one of them on occasion reset manually to 70 deg F.

My questions are the following:

Aquastat

Should the low limit be set or should I allow the boiler to cold start?
If so, what temperature?

Low Limit is currently set at 160 degree F.

High limit is currently set at 195 deg F.


Outdoor Reset

What Temps should the reset control be set at?

Boiler start temp is currently set at 160 deg F.

Design temp is currently set at 190 deg F.

Any help would be appreciated.

Thanks

Ken_40

Ansers:

Without a tankless, the low limit is moot/un-needed.

Cold starts harm all boilers. With a cast iron boiler like the 68, it will not be a factor of any material consequence; especially if you have the fire-side cleaned annually.

I would reduce the high limit to 160 for now. If some nasty cold day appears in dead of winter that leaves the home chilly, I would raise that limit ten degrees, and continue doing so after 24 hours, if the outdoor temps continue to result in a "cool house."

The lwower the outdoor reset temp "trigger point," the less fuel but cooler home you'll have; until it gets truly cold out - when the outdoor reset trigger point will always be "made."

I see no dangerous options available. Mess around as much as required to keep comfy and whatever it takes to lower the boiler water temps. The only danbger point I can see is running the boiler for long periods (colder days, not fall and spring-type temps) at 140 degrees or below. The boiler is not designed to condense, despite it doing exactly that normally on minimally cool days, which is why we try and make the boiler run at over 140 during cold weather - so as to prevent condensation; a 140 degree and below phenomenon; which if done for long periods can accelerate internal corrosion, and can result in un-reversable boiler-block damage

Ron Schroeder

Low Limit

Yes without the tankless the LL is mute but if this boiler has set for years at 160 and you let it get too low it probably will leak at the gaskets. I would keep it at 130 minimum and if I saw signs of leaking at the boiler crank it up 10 at atime until it stops.

Joe Mattiello

The boiler aquastat for tankless coil application has a high and low limit. I’m not sure if your existing control will allow you to bypass the low limit, and run as a cold start boiler. It would make economical sense to run the boiler from ambient when there is a load as opposed to running at a typical minimum 140 degrees continuously. A good friend of mine is running her boiler the same inefficient way; she is a single girl that may shower once a day, and maybe rinse out a dish, or wine glass. Oil prices here in RI are 2.05 per gal today. If you’re using the Taco PC700-2 outdoor reset control with PID technology the reset curve for a non condensing boiler with a cast iron heat exchanger should be at 140 degrees or higher. Obviously, you will run more efficiently with a condensing type boiler, but simply firing the boiler on demand only, will save on annual operating costs. The high limit should be set to what ever temperature is required to offset the heat loss based on outdoor temperature. Theoretically, the boiler should only see high limit at the typical coldest day, or when you have a Domestic hot water demand, which always takes precedence and fires the boiler to high.

Ron Schroeder

Joe Mattiello

Joe I have the utmost respect for you and your knowledge however after nearly 30 years in the field I can tell you that almost without exception a WM 68 series after having run at 160* for years will leak from the section gaskets once they cool to room temperature. This in turn will allow oxygen to enter the system and create an early death to the boiler. For that reason I can not recogmend allowing that boiler to go to cold start unlees one wants to go to the expence of tearing down the boiler and installing new gaskets.

Chuck_20

Boilers been shut down all summer

The boiler has been shutdown all summer.

The boiler pretty much has run continuous for about 15 years. The tankless hasn't been turned on for 6 years and is valved off.

In the 15 years we've shut the boiler down two summers for piping modifications. No leaks have occurred on either shutdown.

A new electronic aquastat is installed that can be set with or without the low limit.

Obviously the cost savings would be greater with a cold start boiler unless it sprang a leak.

Ron Schroeder

Chuck

If you are not having issues with water leakage on shut down then cold start is a good thing no problems but not usually the case. I like the Taco outdoor set back I usually set by the recogmended settings in the manuel and it seems to work out fine.

Chuck_20

Thanks for all the responses

The old manual list 160 degrees F as the operating temperature.

I was just trying to find out if I could lower the temp without running into any problems with this Model boiler. Even it was 140 Deg F.

Our local supplier said that the 140 deg F temp is what they want as a minimum return water temp.

Robert O'Brien

Don't

cold start any vertical oil fired pin type boiler.

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Chuck_21

Confused

If cold start is not good for this boiler, what is the minimum temperature I can put the Low Limit?

What are some of the reasons for not cold starting the boiler?

Ken_40

Cold start is fine...

We have hundreds of "cold start" c.i. oil boilers installed and service most of them long after installation. Cold starts are fine and harmless.