should heating pump stay on permanently?

[madammim]

Hi, we have a Greenstar Utility 18/25 oil boiler which we believe is a fully pumped heating system. We have an unvented cylinder upstairs (think this is hot water only?) There is a pump in the airing cupboard. The heating comes on and off ok but the boiler does not seem to stay on for more than about 5 minutes. Sometimes the rads get very hot, other times just warm.

The Honeywell CM927 is calling for heating and the green light on the relay box is on. The boiler comes on as does the pump and they both stay on for about 5 minutes. The boiler turns off after about 5 minutes despite the maximum temperature not having been achieved. When the boiler turns off so does the pump. The Honeywell controller is still calling for heat and the relay light is still on.

Should the pump still be running when the boiler switches itself off, so that the heat is still pushed round the system?

We have replaced the Honeywell controller and am wondering if the installer settings are incorrect. We have changed the cycle/hour and minimum on times as per the recommended installer settings for an oil boiler. However there is also a setting for pump exercise and heat/cool change which are both disabled but I don't know what they are or if they should be enabled?

So far we have had the pump replaced and the boiler thermostat to try and remedy this ongoing issue but it hasn't solved it.

Any advice greatly appreciated. Thanks for reading.

 

[oilhead]

Whilst your controller is demanding heat, the pump should be running all the time. It is not unusual for the boiler to cycle, as it is controlled by the boiler thermostat and if the boiler is hot it will stop burning until the pump has brought cooler water into the boiler. Get your installer/service technician to check the wiring.

 

[madammim]

Thanks for the reply. The wiring has not been changed. But I lost the wireless Honeywell controller and replaced it with the Honeywell CM927 which is also wireless. As it was supposed to be compatible with the model of relay box we have, we did not replace the relay box with the one that came with the CM927. I did look at the wiring and they looked the same but I think I had better get my husband to have a closer look! Do you have any idea if the Pump Exercise setting on the Honeywell CM927 should be enabled rather than disabled?

 

[D_Hailsham]

The heating comes on and off ok but the boiler does not seem to stay on for more than about 5 minutes. Sometimes the rads get very hot, other times just warm.

The boiler is controlled by two things: (1) the boiler thermostat, which sets the maximum water temperature; (2) the CM927, which turn it on/off according to its own algorithms.
If the room is very cold the rads will be hotter so it warms up faster. Once the room is up to temperature the rads do not need to be so hot as all they are doing is making up for the small heatloss while the boiler is off for a few minutes.

The boiler comes on as does the pump and they both stay on for about 5 minutes. The boiler turns off after about 5 minutes despite the maximum temperature not having been achieved. When the boiler turns off so does the pump. The Honeywell controller is still calling for heat and the relay light is still on.

Sounds as if the CM927 is working exactly as it should.

Should the pump still be running when the boiler switches itself off, so that the heat is still pushed round the system?

This is normal on many continental installations, but not in the UK.

We have replaced the Honeywell controller and am wondering if the installer settings are incorrect. We have changed the cycle/hour and minimum on times as per the recommended installer settings for an oil boiler.

I hope you didn't pay for the new controller as I suspect that you will have wasted your money.

However there is also a setting for pump exercise and heat/cool change which are both disabled but I don't know what they are or if they should be enabled?

Leave them as they are!

Pump exercise just runs the pump for less than a minute every day at noon. Presumably intended for systems which don't get used very often.

Heat/cool change is for air conditioning systems which can either heat or cool the room.

So far we have had the pump replaced and the boiler thermostat to try and remedy this ongoing issue but it hasn't solved it.

But there is nothing to 'solve'!

Read Honeywell's FAQ on TPI. You will get a better understanding of how the CM927 works.

 

[madammim]

D_Hailsham, thank you so much for your reply and taking the time to address each issue - I think I have a better understanding of it now. Really really appreciated.

I had to replace the Honeywell controller as during the summer the original which was free standing rather than fixed to the wall has got lost/hidden by the children!

I've been thinking there is an issue as the controller is set to a temperature of 20oc but the actual room temperature is for example only 18oc. The controller and relay all keep calling but the boiler and pump keep switching off and on and the radiators fluctuate between warm and very very hot!

In my head if the maximum temp has not been achieved then the radiators should permanently be very hot until the temperature has been achieved.

From the way you have explained it, it may be that I have an unrealistic expectations of a heating system and indeed, there is no issue at all.

Thanks again for your explanations - I think I'll try and be a little less hung up on my radiator temperatures!

Happy new year to you.

 

[D_Hailsham]

I've been thinking there is an issue as the controller is set to a temperature of 20oc but the actual room temperature is for example only 18oc. The controller and relay all keep calling but the boiler and pump keep switching off and on and the radiators fluctuate between warm and very very hot!

Is the controller kept in a fixed location, e.g. wall mounted? If so, it could be that it is in a warm area, so it turns the boiler off before the room has warmed up. Try moving the controller to the middle of the room and see what effect it has. If that solved the problem, then you can use the Temperature Offset parameter (12:t0) to make the thermostat think the temperature is lower than it really is.

Do you have Optimization switched on? If not, do so.

 

[madammim]

I've been thinking there is an issue as the controller is set to a temperature of 20oc but the actual room temperature is for example only 18oc. The controller and relay all keep calling but the boiler and pump keep switching off and on and the radiators fluctuate between warm and very very hot!

Is the controller kept in a fixed location, e.g. wall mounted? If so, it could be that it is in a warm area, so it turns the boiler off before the room has warmed up. Try moving the controller to the middle of the room and see what effect it has. If that solved the problem, then you can use the Temperature Offset parameter (12:t0) to make the thermostat think the temperature is lower than it really is.

Do you have Optimization switched on? If not, do so.

The controller is wall mounted in the hall - the coldest part of the house. Heating has been on for the past 4 hours and I have set it for a desired temperature of 25 degrees (which it will never achieve) So far we have achieved and stuck at 18 degrees. All the radiators are only warm, apart from the two smallest which are warmer. Sometimes they do get hotter. The boiler continues to cycle on and off and the pump turns on and off at the same time as the boiler. The CM927 is constantly calling for heat and the relay light is on. The heating will be left on all day and by the evening we may achieve a temperature of 20 degrees but never any hotter than that. I don't really want it to be hotter than that, it just seems like such a slog and takes such a long time to get there!