View Full Version : something strange beyond the aquastat.

12-21-2009, 05:26 AM
this is my first post so i would like to introduce myself and say thank you for this enlightening and valuable forum!
i am the co-owner of a residential construction company in princeton. i have limited knowledge of boiler systems but have learned a bit. probably still closer to a homeowner than a professional with regards to heat systems..
i have an old farmhouse that was built in 1780. the heating system is steam radiators on an oil boiler. the domestic hot water is heated on the same boiler, controlled by an aquastat.
i keep my thermostat quite low, 59 degrees. i keep my DHW set at around 120 and my aquastat is set at 135. the probe for the aquastat is rather short, doesn't actually enter the boiler but instead terminates about 4" outside the furnace.
problem: the aquastat is calling for heat so the boiler comes on and stays on. it goes all the way to steam temperature (212, well past the 135 that it is calling for) and starts heating the house. it usually happens early in the morning, i wake up because it's 75 degrees in the house and my thermostat is set at 59. i know the problem is coming down the aquastat line because when i disconnect it the problem is solved and the heat functions normally. as a test i have disconnected it for days and had no problems (and had hot water because the boiler is hot from heating the house). as soon as i hook it back up, the boiler kicks on the the furnace rages. the circulation pump is off on the water tank, the water has reached the desired temperature, yet the aquastat calls for heat.
history: this has happened a few times in the last two years. to my furnace techs' credit, they have replaced nearly every part on the furnace (switches etc.) and the aquastat and circulation pump on the DWH side. it won't happen for ages, i don't think it has ever happened in the summer. often a 'fix' will work for a period of time, then the problem will happen again. replacing the aquastat worked for 24 hours. replacing the wire from the aquastat to the junction of the thermostat and the furnace worked for a few months (i think the wire was bare in places). replacing the DHW circulation pump worked for a few months, then presto! problem returned.
i am really baffled and so are all the technicians from my oil company.

12-21-2009, 06:10 AM
From what you are saying it sounds like the aqua-stat is not sensing the water temperature. Where is the aqua-stat on the boiler, it should be in a tapping below the water level? A picture of the system would be helpful. You also say the probe for the aqua-stat is 4" outside the boiler. Are you talking about a strap on?


Peter Griffin
12-21-2009, 07:30 AM
Is the aquastat well below the water level in the boiler?

12-21-2009, 07:53 AM
thanks for the quick response!
here is a picture of the aquastat tap. the probe goes perhaps 4" past the backside of the aquastat, leaving it out fairly far from the boiler. don't know if that is a problem, one technician told me i should keep it set low because that probe is set so far out. otherwise for the probe to read 135 the boiler will be much higher. he told me often there are two taps, one for the aquastat and one for the circulation pump. because they are both on one tap it is awkward. not sure if that is valid.

thanks again!

Peter Griffin
12-21-2009, 11:05 AM
You need to move the aquastat well so that if is emmersed in the boiler water

12-21-2009, 12:41 PM
thank you for your responses
so, if i need to move the aquastat lower, how would i do that? it is impossible to create an after-market tap, is it not?
regarding the temperature probe on the aquastat, i have attached a picture of what it looks like (not sure what a strap on is in this context). the picture is of the one i replaced, the probe in the photo is longer than the probe on the new one.
since the probe is in a shared line with the circulation pump, shouldn't it be reading that the water is hot as it is drawn through to the DHW tank?
isn't the intermittency of the problem puzzling?

12-21-2009, 01:53 PM
Is there another taping below the water level? There may be one that is hidden behind the jacket and the knockout hasn't been removed. If not I would tie the circulation pump into the copper return line below the water level and use the existing tapping for the aqua-stat alone.


12-21-2009, 06:42 PM
thank you john and peter.
it's good advice to check if there is another knockout for an additional tap. john, you said the aquastat might not be tapping uner the water level at it's existing location. then you said i should tie the circulation pump into the return line below the water level and leave the aquastat where it is if i move the circulator. does that mean the picture satisfied your question of whether the aquastat tapping is below the water level?
peter, it looks like from your advice you think the tapping for the aquastat is not below the water level. is this correct?
thanks fellas.

12-21-2009, 07:16 PM
The tapping for the aquastat is below the water level. The problem is the probe is not reaching the water in the boiler.


12-21-2009, 10:59 PM

One possible way that the well which holds the aquastat probe is not sensing the boiler water in the loop to the water heater is because it becoming air locked.

I think that the configuration of the nipple and coupler from the T is to keep the probe well out of the direct flow and prevent a loss of flow in the loop. It should normally be wetted, but if it becomes surrounded by air it will not sense the water temperature.

I don't know how air can be getting into the region around the well, but it may be related a low pressure zone at the pump inlet. You say the problem doesn't happen in the summer. Is there less make up water added when the boiler water does not see steam temperatures but only goes to 135 on the aquastat. Perhaps the make up water is a source of additional dissolved air.

I would explore removing the nipple and coupler and seeing what happens with the probe well mounted in the flow path. If it blocks flow, you may be able to go to a larger T. You might also be able to change the orientation and plumbing from the T (or go to a 4 way) so that the aquastat well is mounted below the flow path and could not become an air trap.

The tech who talked about the common configuration of two taps below the water line for indirect dhw applications (instead of the long armed T you have) may have been indicating that there are known problems with that geometry.

The intermittent nature of your problem is indeed puzzling. Good Luck