Boiler thermostat query

[Reshweb]

I have a Worcester Bosch Greenstar CDi30 conventional boiler and have a query about the boiler thermostat performance.

When the room stats are calling for heat and with the boiler temperature control at maximum, the temperature display goes up to 77 deg then the burner is turned off. All good so far .. except that the burner doesn't fire up again until the display is down to around 25 deg although the room stats are still calling for heat.

I have had an engineer look at it today and he can't work out what is going on.

My question is how does this thermostat work. Does it solely look at the flow temperature and should trigger when the flow temperature drops by xx deg or is there some other control that can influence the differential ?


As it is at the moment I have radiators that start to get warm then get cooler water circulated which cools them and then they get hot water again. It doesn't sound right to me.


Any thoughts much appreciated.

Graham

 

[Chuck]

I suspect that what you are seeing is:

The boiler thermostat will be set at, say 70°C and the burner modulates to try to maintain that flow temperature. If the emitters (radiators, etc,) are dissipating less power than the minimum the boiler can provide, normally about 8kW, the flow temperature will gradually rise above 70. When it exceeds 70°C by 5°C the burners shut down. The residual heat in the HX can cause the flow temperature to overshoot by a couple of degrees, i.e 77°C before it stops rising. The boiler continues to circulate until the flow temperature drops to 5°C below the setpoint, i.e. 65°C when the burners come back on and the next cycle starts.

If the cycle takes longer that the boilers minimum cycle time, which is normally 10 minutes IIRC, this cycling between 65 and 75 will continue indefinitely. If the second cycle completes more quickly than this, the boiler decides that it is 'short cycling' and then shuts the burners down for 10 minutes. The flow temperature then keeps dropping past 65°C down to, in your case, 25°C before the short-cycle prevention timer allows the burners to come on again and the whole show starts from the beginning again.

At this time of year, this is normal behaviour for systems with TRVs and single boilers with typical modulation ratio specs. If you have some pipe thermometers and a lot of patience you can precision-balance your system so that the rooms are at the correct temperature with the TRVs not quite closing when the house thermostat stops calling for heat. It's not really economic to pay someone to do this, however, as it takes a long time to get right.

 

[Reshweb]

Thanks for the detailed response Chuck.
It is starting to make a bit more sense now however I have a couple of follow on questions if I may.

I don't have TRV's as I have zoned the rooms with individual room stats and two port valves. There is a bypass radiator to maintain a path should all valves close and the boiler overrun keeps the pump running for a while.

So if I understand you correctly if it takes longer than 10 minutes to cycle between the 65 deg and 70 deg cycle all works ok. Or if it never reaches the 70 deg mark the burner stays on until all room are up to their desired temps. However if I only have a couple of rooms calling for heat the cycle may be less than the short circuit prevention timer hence what I am seeing.

What balancing could I try given that I don't have TRV's.
Would the pump speed have any impact on things.
Is there anything else I can do to reduce the impact.

Many thanks
Graham

 

[Chuck]

I don't have TRV's as I have zoned the rooms with individual room stats and two port valves. There is a bypass radiator to maintain a path should all valves close and the boiler overrun keeps the pump running for a while.

Having a zone valve closed can have the same effect as TRVs being closed, i.e. the system can't transfer enough power because the zone(s) is off.

From your description (no TRVs, always-on bypass radiator) the system hasn't been updated since the 80s. You might be able to improve matters by getting a relatively sophisticated controller with 'multizone synchronisation' so that the zone valves all open (or close) at the same point in the cycle.

Zoning in a small house or flat can cause more problems with boiler-cycling than it's worth so binding all the zones to a single thermostat might also be worth trying.

You really need a good local heating engineer to take a look at your system, check the sizing and balance, see how it's operating and make some recommendations for improving its performance.

I fear that the guy who couldn't explain what's happening is problably not the man for the job.

 

[Reshweb]

Thanks Chuck
It's not a small house its a large 5 bedroom bungalow that I zoned as I didn't want to overly heat unused rooms.

At least I now understand what's happening so can try a few things to alleviate the situation.

graham