Oil-fired Aga thermostat not operating properly

[Slonopotam]

Hi,

I have an oil-fired Aga with a BM oil control valve and a Flexitemp thermostatic control (see the image in this thread for an example).

When I bought the house I was told by the previous owner that the thermostat didn’t work and so he (and now I) have always run it by setting the flexitemp to the override setting which lifts the arm of the thermostat right up and allows the metering stem to rise - ie the aga runs on a high flame constantly. I assume that the high flow rate is lower than usual as the aga runs slightly cool and uses much less oil than the average oil aga. However it also that there is no way to increase the oil flow when there’s more demand on the aga.

As I was servicing it today, I wanted to get to the bottom of it. The float control itself seems to be working properly. I haven’t the equipment to measure the flow rate precisely, but the high and low flames both look to be about right.

However, when I turn the thermostat on at the socket and flip the switch from override to automatic, the aga will only ever run at a low flame - i.e. the arm on the flexitemp comes down onto the metering stem, restricting the oil flow, even if the aga is well below temperature.

I’m not an electrician so haven’t been able to test the circuit, but with the electricity switched on then the flexitemp box warms up, so it is clearly getting power.

All I can think of is that a) the arm itself needs adjusting, b) there is an electrical fault, c) there is a mechanical fault inside the flexitemp or d) the thermostat itself has failed somehow.

Does anyone have any experience with this sort of thing and could advise?

Thanks.

 

[Slonopotam]

Hi,

I have an oil-fired Aga with a BM oil control valve and a Flexitemp thermostatic control (see the image in this thread for an example).

When I bought the house I was told by the previous owner that the thermostat didn’t work and so he (and now I) have always run it by setting the flexitemp to the override setting which lifts the arm of the thermostat right up and allows the metering stem to rise - ie the aga runs on a high flame constantly. I assume that the high flow rate is lower than usual as the aga runs slightly cool and uses much less oil than the average oil aga. However it also that there is no way to increase the oil flow when there’s more demand on the aga.

As I was servicing it today, I wanted to get to the bottom of it. The float control itself seems to be working properly. I haven’t the equipment to measure the flow rate precisely, but the high and low flames both look to be about right.

However, when I turn the thermostat on at the socket and flip the switch from override to automatic, the aga will only ever run at a low flame - i.e. the arm on the flexitemp comes down onto the metering stem, restricting the oil flow, even if the aga is well below temperature.

I’m not an electrician so haven’t been able to test the circuit, but with the electricity switched on then the flexitemp box warms up, so it is clearly getting power.

All I can think of is that a) the arm itself needs adjusting, b) there is an electrical fault, c) there is a mechanical fault inside the flexitemp or d) the thermostat itself has failed somehow.

Does anyone have any experience with this sort of thing and could advise?

Thanks.

Just to add some additional information:

It now seems to me that the thermostat may be basically functioning, but the arm possibly isn’t lifting quite clear enough of the metering stem.

With the thermostat on and in automatic mode, the fire in the aga is not as small as a low fire, but is visibly smaller than the full high fire that you get when in low fire mode (thermostat switched off and set to automatic). To be clear: in low fire mode there is no corona of blue flame above the burner, in high fire mode there is a large corona, and in this case there is a small corona.

The net result is that the aga is held at a low temperature.

When in manual high fire mode, the aga runs at the correct temperature (130 Celsius in the middle of the roasting oven).

I’ve considered whether the thermostat setting could be off (ie the control deactivates at too low a temperature), but the thermostat should only have two settings (low fire or high fire) so why then would I be getting a medium fire? It must be that the arm just isn’t lifting enough.

I see that the Toby controllers have a screw to adjust the arm height. Does anyone know if there is a similar adjustment on the Flexitemp controller for a BM float?

 

[AgaDIY]

Just to add some additional information:

It now seems to me that the thermostat may be basically functioning, but the arm possibly isn’t lifting quite clear enough of the metering stem.

With the thermostat on and in automatic mode, the fire in the aga is not as small as a low fire, but is visibly smaller than the full high fire that you get when in low fire mode (thermostat switched off and set to automatic). To be clear: in low fire mode there is no corona of blue flame above the burner, in high fire mode there is a large corona, and in this case there is a small corona.

The net result is that the aga is held at a low temperature.

When in manual high fire mode, the aga runs at the correct temperature (130 Celsius in the middle of the roasting oven).

I’ve considered whether the thermostat setting could be off (ie the control deactivates at too low a temperature), but the thermostat should only have two settings (low fire or high fire) so why then would I be getting a medium fire? It must be that the arm just isn’t lifting enough.

I see that the Toby controllers have a screw to adjust the arm height. Does anyone know if there is a similar adjustment on the Flexitemp controller for a BM float?

Hi,

Delighted to help with all this, it is what I do for a living. You need to clean the Sliding Valve and hairline slit inside the OCV. Then test the flow rate (no special equipment needed). When the burner is working 100% on LOW and HIGH rates, you can then test to find which of the 3 components in the electric circuit have failed, and my money will be on the thermostat. All is very easy to do. Good luck - AgaDIYservice

 

[Lou]

That is fab that we have an AGA specialist on here now

Did it work @Slonopotam ?

 

[Andrew N Yorkshire]

Just to add some additional information:

It now seems to me that the thermostat may be basically functioning, but the arm possibly isn’t lifting quite clear enough of the metering stem.

With the thermostat on and in automatic mode, the fire in the aga is not as small as a low fire, but is visibly smaller than the full high fire that you get when in low fire mode (thermostat switched off and set to automatic). To be clear: in low fire mode there is no corona of blue flame above the burner, in high fire mode there is a large corona, and in this case there is a small corona.

The net result is that the aga is held at a low temperature.

When in manual high fire mode, the aga runs at the correct temperature (130 Celsius in the middle of the roasting oven).

I’ve considered whether the thermostat setting could be off (ie the control deactivates at too low a temperature), but the thermostat should only have two settings (low fire or high fire) so why then would I be getting a medium fire? It must be that the arm just isn’t lifting enough.

I see that the Toby controllers have a screw to adjust the arm height. Does anyone know if there is a similar adjustment on the Flexitemp controller for a BM float?

That is fab that we have an AGA specialist on here now

Did it work @Slonopotam ?