Help with oil burner (furnace) aquastat


Old 01-08-18, 05:03 PM
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Help with oil burner (furnace) aquastat

I've got an old oil furnace. It's hot water (not steam) and radiators (hydronic?). There's a Honeywell manual reset aquastat model L4006E which directly controls the burner. Since I've lived here it's been set to about 160 degrees. At a temperature less than that ( about 125-130 or so) the relief valve opens and water drips onto the floor. At 160, the manual reset button pops and the burner shuts off (circulator keeps running) and the burner remains off until you press reset. Sometimes this causes us to have no heat for awhile until we realize it's popped. So I bought a Honeywell L8148A aquastat relay and installed it, keeping the old one installed as a backup (thank goodness). After installation, I found the lowest temperature the L8148A goes to is 180, much higher than my relief valve.

In the meantime, I set the old manual reset aquastat to 130 degrees, and so far (I did this an hour ago) the burner is cycling on and off as it's supposed to! It's the manual reset aquastat that's doing the cycling. Don't get me wrong, I'm happy it's doing this, but is it a fluke and actually malfunctioning, or is it doing what it was designed to do?

Please feel free to ask any other questions if I didn't explain something correctly. I'm not a plumber or HVAC person, I just don't have enough money to pay someone to fix it correctly.
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Old 01-08-18, 10:40 PM
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Welcome to the forums.

I moved your thread to the boiler forum. A furnace is hot air. A boiler is hot water.

I'm not the pro in this forum but it's not your aquastat that is causing problems. You should be able to run the boiler up to 180f with no problems. It sounds like you have either a defective water fill valve or you have lost the air charge in the pressure tank.

The pressure tanks lose their charge with age as well as the internal bladder can rupture rendering the tank useless.

Good info posted at the top of this forum.........
Old 01-09-18, 02:12 AM
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Thank you for moving this to the proper place. I read the thread you suggested. I previously knew nothing about how the expansion tank works, now I've got a general idea and I understand that without the bladder filled with air, the pressure in the system can rise too much. Sounds like the air in the bladder compresses and allows higher temps with lower system pressure. I'm pretty sure I know where the expansion tank is.... And it's inaccessible without removing some drywall from the first floor.

So a couple follow-up questions. Can anyone think of a way to narrow down whether the problem is the expansion tank or the relief valve without getting to the expansion tank? (I'll also look at what I can see of the tank and try to locate the air fill point)

Second, (until I get the tank or valve issue fixed) I'm still trying to figure out if my manual reset aquastat is doing what's it's supposed to do or if it's an a malfunction.

It looks like when I get the pressure issue fixed, I can begin using the new aquastat I installed in the correct manner and the manual reset one can be what I Invisioned it to be; a safety device if the other one fails for one reason or another.

Thanks for the help!
Old 01-09-18, 03:59 AM
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The pressure in your heating system and the water temperature are not dependent on each other. The cold fill pressure is set at a pressure that will "hold up " the water in the system to the highest radiator. For example; 10 pounds pressure will hold the water up in the system to 23.1 feet (10 x 2.31 feet/pound = 23.1 feet. Each 1 pound pressure holds up a column of water 2.31 feet and so on. THAT IS IT.. Once the cold fill pressure is established and the burner is turned on the water will heat up and expand. The expansion tank is there to give the expanded water someplace to go without raising the system pressure too high and opening the relief valve. Remember, water can not be compressed. So if the top of the highest radiator is say 30 feet above the pressure gauge then 30 feet divided by 2.31 = 13 pounds the minimum starting pressure, and add about 2 pounds for safety. The manual reset high limit should be set to 20 degrees above the highest operating temperature level which is controlled by the auto-reset limit. If the manual limit is resetting itself or responding to a temperature lower than it's set point, then it is worn out or defective and needs to be replaced. There are 2 types of expansion tanks used in homes; 1 has a bladder and 1 does not. The pressure in the bladder tank is set with no pressure in the system and compressed air is introduced into the tank and pressurized to the set point of the cold water fill pressure. The standard tank (non-bladder) is drained completely empty and then the system pressure fills the tank somewhat compressing the air in the tank to the system starting pressure. Either tank should be accessible and not be covered with a building material in case they need attention. The heating system is controlled by a thermostat and the limit and high limit are just for operating safety so the boiler temperature does not exceed the normal limits. This explains the use of the expansion tank and the temperature limits. Hope this helps
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