I have a Weil-McClain Hot water boiler with domestic coil. The unit is circa 1970's. I recently installed an instant on demand hot water system for domestic use.

My question is how do I fix the boiler so that it only fires when the thermostat calls for heat as opposed to maintaining a temperature range for domestic use, which I now no longer require?

 

Change the control to an L7248C aquastat which is cold start.

 

Correct me if I am wrong, but I think that this boiler was not designed for cold start. Also your system may not be piped in such a way.
You may have problems with this.

Would it be better to use a modulating control that would lower the boiler to only 140 minimum?

 

Any residential cast iron boiler can be cold started. I will agree it should have a minimum of a boiler bypass but would anybody install a cast iron boiler with a domestic hot water coil without a boiler bypass. I am being presumptuous as I know there are many cast iron boilers installed without boiler bypasses. Boiler bypass extends the hot water draw and keeps the circulator from short cycling on larger water volume systems.
I know most people look at boiler bypasses for large water volume systems only but i like them on all systems. You can adjust them as needed with flow but the idea of running cooler water in the system on warmer days is attractive. It is an easy inexpensive way of improving comfort in the home. More people are doing this with Outdoor Reset today and outdoor reset is a great idea. But add odr to a cast iron boiler and you need boiler protection.

 

I have a 64 year old Weil-Mclain boiler that I unhooked the domestic coil from winter 2008 and installed an electric 50 gallon water heater. During the winter I keep the boiler temp set at 180F and when heating season ends, I lower the setting to 110F. The boiler fires about once a day and has used about 15 gallons of oil in the last 3 months. My electric bill has averaged about $25 more per month. So far I feel I made a good decision and have not run out of hot water once in a year and a half.

 

Why would you fire your boiler during the summer, if there is no need for heat or DHW then you are just wasting fuel.

Put an outdoor reset control on it, and set the warm weather shut down so that it will all be automatic and you will not need to worry about adjusting between summer and winter temps.

 

Last year I shut the boiler off for the summer to save fuel, but when I restarted it in October my house filled with smoke and steam and I had to have the fire dept come in and remove the smoke. My boiler tech said it would be best to keep it warm due to it's age. Apparently it leaked some water into the jacket.

 

A 64 year old boiler I would be very careful what I did to it. You may have some seepage at he section gaskets and/or coil plate gasket. the advice in this situation would be a good idea. Normally I would say replace the coil plate gasket and shut it off but not on a 64 year old boiler. You may encounter more trouble than you want trying to get the coil out to change the gasket. Changing the section gaskets would be out of the question. At that point just change the boiler. BTW what AFUE do you have......48%?????

 

Not sure of the AFUE, can't find the specs. I have a few reasons I have not replaced the boiler.
1. Cost
2. Only used 600 gal of heating oil last year, so my heating bill was only 1300 for the year and felt it would take me forever to recoup the 6-10 thousand dollars to replace it. My slant fin heatloss estimate is 52000 and I have a 2200 sf home
3. Simple design and works good
4. I don't have gas in my house and the install cost is high and not sure I want to stay with oil.
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I read this forum all the time and all you guys have valid arguments for what is the best replacement. I am just unsure of which way to go...super efficient with possible high maint costs or traditional design that may be more simple and less costly to maintain.

 

If you're staying with oil, the 3-pass boilers like Burnham MPO and Biasi B10 are kind of middle ground between high tech and traditional. They should last quite a long time and not be maintenance or tech headaches.