So this year I'm starting up my boiler for the first time. Its a very old hot water boiler (installed in 1968) which I plan to replace next year. My low limit was set to 120 and hi to 180, two zones and home is a Hi-ranch no basement. Initial pressure before start was like 12psi and once the water rose to about 160F pressure was at like 25psi. I dumped water from the drain to about 17psi. Heard some air get introduced... Turned up each thermostat and both pumps are running. When I turned down the thermostat to where it would not call for hot water and came back a few hours later the boiler maintained 180F. Is the boiler suppose to maintain 180 or 120 when not being called for heat?
Boiler resting question.
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Could be a few things....
On your call for heat- you probably had reached the high limit and the boiler turned off. Cast Iron boilers have a lot of mass- so once heated to 180F with no call for heat in a warm basement it will stay near 180F for quite a long time.
Old temp gauges can get sticky... it could have really been several deg under 180F... but the needle hadn't moved yet.
Here's an excellent read on aquastat hi/low/diff settings:
http://inspectapedia.com/heat/Aquastats.php
You may want to check your expansion tank- make sure it's not waterlogged.
On your call for heat- you probably had reached the high limit and the boiler turned off. Cast Iron boilers have a lot of mass- so once heated to 180F with no call for heat in a warm basement it will stay near 180F for quite a long time.
Old temp gauges can get sticky... it could have really been several deg under 180F... but the needle hadn't moved yet.
Here's an excellent read on aquastat hi/low/diff settings:
http://inspectapedia.com/heat/Aquastats.php
You may want to check your expansion tank- make sure it's not waterlogged.
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Setting the low-limit to 120F all but guarantees that you will have corrosive condensation on the heat exchanger plates, degrading heat transfer efficiency and hastening the demise of the boiler. If it's a gas-burner you'll be fine down to about 130-135F, but if it's an oil boiler the low limit should be no lower than 140F.
With most 1960s vintage aquastat controls burner should not be firing until the boiler reaches it's low-temp limit, then burns until it hits the high-limit where there is a call for heat from the thermostat or not. Cold starting gas boilers sometimes require a call for heat to fire the burner too. Once up to temp it takes hours for a 180F boiler to drop to 120F from standby losses alone. The boiler jackets are usually insulated to at least R2-R3 even back in 1968. In a 65F boiler room it'll initially be losing maybe a couple of degrees per hour to standby loss, but it will slow down to the 1 degree/hr range by the time it's below 130F.
Since you are planning to replace the beast, now is the time to run some load calculations to get a handle on what to replace it with. The easiest/quickest is to use fuel-use against heating degree-days to estimate the load at your 99% outside design temp using the old boiler as a measuring instrument. This is a napkin-math quick calculation, but surprisingly accurate. The method is outlined here. With a boiler that old derate the DOE output /input efficiency by 10% when running the calculation.
In high volume systems (cast iron radiators?) it's not surprising to see a pressure rise from 12psi to 25 psi over a temperature rise of 40-50F. The expansion tank needs to be sized for it, and pre-charged to the correct pressure. If it's a bladder-type expansion tank with a tire-inflator type valve on one end it'll spit water if the bladder is shot and leaking. But if that were the case the pressure rise would have likely been greater. Still, it's worth isolating the tank and setting the pre-charge of air to 12psi, and settting the system pressure to 12psi (when cold).
With most 1960s vintage aquastat controls burner should not be firing until the boiler reaches it's low-temp limit, then burns until it hits the high-limit where there is a call for heat from the thermostat or not. Cold starting gas boilers sometimes require a call for heat to fire the burner too. Once up to temp it takes hours for a 180F boiler to drop to 120F from standby losses alone. The boiler jackets are usually insulated to at least R2-R3 even back in 1968. In a 65F boiler room it'll initially be losing maybe a couple of degrees per hour to standby loss, but it will slow down to the 1 degree/hr range by the time it's below 130F.
Since you are planning to replace the beast, now is the time to run some load calculations to get a handle on what to replace it with. The easiest/quickest is to use fuel-use against heating degree-days to estimate the load at your 99% outside design temp using the old boiler as a measuring instrument. This is a napkin-math quick calculation, but surprisingly accurate. The method is outlined here. With a boiler that old derate the DOE output /input efficiency by 10% when running the calculation.
In high volume systems (cast iron radiators?) it's not surprising to see a pressure rise from 12psi to 25 psi over a temperature rise of 40-50F. The expansion tank needs to be sized for it, and pre-charged to the correct pressure. If it's a bladder-type expansion tank with a tire-inflator type valve on one end it'll spit water if the bladder is shot and leaking. But if that were the case the pressure rise would have likely been greater. Still, it's worth isolating the tank and setting the pre-charge of air to 12psi, and settting the system pressure to 12psi (when cold).
