Boiler cycles on and off

[Gert_Lush]

I have a vented CH system and conventional boiler (standard set up)
The boiler was recently given its annual service. The bloke said to have the boiler thermostat up to the max position, the temp sensor up to about 22 degrees, and then balance each room temperature as required on the radiator TRV. (I've been on a couple of energy saving websites and they seem to concur with all this.)

Now, the boiler cycles on and off. On for about 1 minute and off for about 3 mins, all the time.

The circ pump is set to position 2 and all the rads have been bled. There doesn't appear to be any over-pump into the F&E tank or cavitation in the pump.

Is this cycling normal? (i haven't a clue having never had a properly working CH system!)

Cheers.

 

[ChrisR]

Only if most of the trv's are shut. Has the rad in the room with the room stat been turned down too far?

 

[Gert_Lush]

All the TRV's are either half way or up to max. The room stat isn't getting cold enough to operate the 'call for heat' contact inside it.

With the boiler stat being up to max, is it a case of the boiler getting the CH loop water up to temp and then cutting off? (If i turn the circ pump speed up, i get loads of over-pump)

 

[Gert_Lush]

Some more info, when the system starts up from cold, it runs as expected. When the house starts to get warm (but not cutting out on the room stat) the boiler then starts to cycle on and off as described.

Any ideas?

 

[meldrew's_mate]

It seems your boiler may be too large an output for the load. You may also have some loads "short circuiting" the system. From your description of pump speed I'd assume a boiler of size 40,000 to 60,000 btu/hr. Try the following.

1. Contact the fitter who serviced your boiler, ask if it's range rated. If it is ask him/her if it has been set to maximum output, and can it be turned down to minimum?

2. Add up the approximate btu/hr outputs of all your radiators, a bit laborious but necessary. This is your maximum heating load for the purposes of this test.

3. Look on your DHW cylinder (airing cupboard?), is there a gate valve or similar restricting the flow to or from the cylinder heating coil? Make sure you have located this correctly, then set it to 1.5 to 2 turns open (start with it shut)

4. Do you have a fixed bypass valve? This is normally only a 15mm gate valve just downstream of the pump, usually set 1 turn from shut.

5. Now balance the system, search for this on this site's "how to.." area; my preference is to start the system from cold with all TRVs at maximum and both HW and CH on. Starting at the smallest rads and upstairs, check the lockshield valve and adjust these down to reduce the flows until you have between 10 and 15 C difference on the pipe flow to return.

6. Rush around checking each radiator is warming up at the same rate, including downstairs, and that the hot water heating coil has a similar temperature drop, though this is more difficult as it can be confused by the contents temperature at this stage!

7. When all the rads are hot (and with all the running around you'll be hot too ) once again check each rad has 10 to 15 C across it, usually a large rad downstairs will need its lockshield fully open, the rest between 1/4 turn and 2 turns from closed dependant upon their size/output and position.

8. Now check the boiler flow and return pipework, if all else is working this too should have 10 to 15 C across it (abracadabra!!). Do your timing routine over 30 minutes and if the boiler is firing for less than 50% of your timing period it has too large an output. The solution is to have your heating fitter turn down the output (CORGI only) or consider building an extension.

9. Finally set your TRVs to the desired room temperature (your comfort level)


One final point, energy efficiency QWANGOs correctly quote that standard efficiency (not condensing) heating systems run most efficiently at boiler maximum temperature. I don't dispute that, but it must be tempered by considering the safety of the elderly and the young as 75 C + surface temperatures can be dangerous for them.

 

[Gert_Lush]

That's brilliant, certainly alot to get through but it all sounds good stuff. I i will let you know how i get on.

Thanks again.

 

[ACOperson]

Just a thought on your problem:

Have you called the service engineer about this? It is possible that he/she may have misplaced the thermostat phial, so it is overheating and turning the boiler off prematurely.

I know a service engineer who did that on a back boiler once to similar results

 

[Agile]

The boiler will actually be more efficient with a lower flow temperature.

The greatest efficiency will be obtained by reducing the flow temperature as the rooms heat up and reducing the heat input to match the heat loss.

The boiler is also more efficient if it is running all the time but there is then no spare capacity for colder weather. I would set a non modulating boiler to be on for 80% of the time with the outside temperature in London now at 3*C. That would then give a 20*C spare capacity.

Tony

 

[Gert_Lush]

Just a thought on your problem:

Have you called the service engineer about this? It is possible that he/she may have misplaced the thermostat phial, so it is overheating and turning the boiler off prematurely.

I know a service engineer who did that on a back boiler once to similar results

Is this something i can check and possibly rectify or not?

 

[Gert_Lush]

Further to this, came home today, boiler is cycling on and off and the house was freezing cold. No heat from rads. Circ pump was making no noise at all, even when i changed the speed.
Would the boiler cycle on and off if the circ pump was knackered?