Gas Ch Boiler Problem

[ladyalex]

Hello. Any gas boiler experts out there? Or anyone who has had a similar problem and solved it ?
We have a gas boiler. A conventional one, not a condenser type. We have a hot water tank and central heating. The boiler is more than 20 years old but has been regularly 'serviced' and recently had both the hot water pump and the central heating pump replaced.
The problem is that the boiler fires up and gets the rads nice and hot, then everything cools down. The heat gets too low for comfort. Then the boiler fires up again but only for 5 minutes at the maximum, so the rads never get properly hot again.
We have had a procession of British Gas men, often 2 at a time in the house over the last 10 days, all coming up with different 'soutions' none of which work.
They have 'rebalanced' our rads, checked that there is no significant 'sludging' changed the schedule so that the Hot Water heating is not on at the same time as the Central heating, made sure that the rads that we don't need to be on are set as low as possible, given us a new room thermostat....all to no effect. And yes, you have guessed it, they say that our boiler is too small for the system, we need a new boiler and a powerwash. That boiler has kept us warm and toasty for years.

We think that there is a problem with the boiler timing mechanism.

Any ideas anyone ?
Thanks a lot.
:-))

[ladyalex]

Hello again Karl. Thanks for getting back to me.
Our observations today are:
Boiler started up at 6:45.
Rads hot at 8:00.
Rads gradually cooled down so we started trying to monitor the boiler
Boiler off at 10:53 back on at 10:59
Boiler off at 11:00 back on at 11:06
Boiler off at 11:30 back on at 11:35
Boiler off at 11:39
Then we stopped monitoring.
Most of today the rads have been cool but now they are hot...but no doubt they will cool down again.

[bhg481]

The rads are not supposed to be hot all the time - they only need to come on if the temperature drops in the room with the thermostat. Your boiler pattern matches mine - it just comes on for a few minutes every now and again to maintain the temperature of the water in it as determined by the boiler thermostat. What you really need to monitor is the temperature of the room with the thermostat; that is what the system is trying to retain.

[KARL]

It is quite clear that there is nothing wrong with either the boiler or the pump. If you are not getting the heat that the system is designed to deliver then the problem is one of control.

bhg is correct, assuming there is a thermostat installed somewhere to monitor/govern the temperature in that location/room then you need to monitor how close to its setting the actual temperature sits. For this you need a fairly good thermometer - position it pretty much in free air but not very far from the thermostat at between 1.0 and 1.2 metres above floor level. If there is a significant variation from (i.e. below) the setting on the thermostat then raise the thermostat setting by two degrees and see if the deviation changes.

If the deviation after initial warm-up is steady for hours then you can choose how to set the thermostat accordingly. If the deviation varies significantly over time then further experimentation/observation is necessary. Note that areas/rooms away from the thermostat location can behave quite differently as to temperature, all depending on the characteristics of the house and the heating system.

[Etch]

Ladyalex says "that boiler has kept us warm and toasty for years" so I find it difficult to believe that the problem is simply down to not understanding how a thermostat controlled system works! The boiler is fitted with an 'NTC sensor', which controls the temperature of the water flowing through the boiler, but also an 'overheat thermostat' which cuts in if the boiler temperature exceeds a pre-set value. I still reckon that a misbehaviour of one of these devices might cause the symptoms described. There could even be some strange fault (blown capacitor, dry joint, etc.) on the circuit board that handles the output from the two temperature sensors, so I don't agree that the boiler has been eliminated from suspicion.

As an example of the latter possibility: I have an electrically operated gate which suddenly stopped working properly. It would still open, on demand from the remote or in test mode, but wouldn't close. I tracked the fault to a dry joint on the circuit board, re-soldered it, and it now works perfectly again. These things do happen!

[KARL]

One question occurrs to me: Are you getting hot tap water as and when you want it ? I have assumed throughout that you are and that your concerns are limited to only the space heating via the radiators.

[ladyalex]

Thanks, guys. I really appreciate you getting back to me....which is more than the British Gas Men have. 1 text and 2 phone calls today to get them. They claim to be coming back tomorrow with a new gas valve. I am afraid that I have no confidence in them at all now and seriously doubt that they have any clue as to what they are doing.
In response to Karl re the temperature near the room thermostat...
our room thermostat is in the hall. We have a thermometer in the hall. During the course of all this bother we have had the Stat set at maximum (30c) 25C and 22C the last of these is our normal setting for cold days. When I have checked the thermometer it has barely reached 20c and is usually hovering between 16 and 18. The Gas Men had it at 30C for ages when they were here but the temperature on the thermometer as, I said barely reached 20.
I have to add that the thermometer is very old and part of a barometer.
Thanks again. :-))

[Togo]

Is the diverter valve sticking, or is the electric housing that holds the little motor and sits on the valve not connecting to the valve spindle properly? Most have a little retention clip that must click to show that it will engage with the spindle or sometimes the plastic lug that engages the brass spindle becomes rounded because of long use and does not turn the spindle fully.

[ladyalex]

We are getting hot tap water, but not as hot as it used to be . The gas Men have also been fiddling with the hot water thermostat as we complained to them about this , they raised it a bit, but not yet enough . another issue we have to take up with them when they return tomorrow....if they come. They are bored with us now, I think. But not as bored as we are with them and with being cold.

[ladyalex]

Thanks togo.
I think I know what you mean. There are 2 valves one for the CH , one for HW. Both of htem have been replaced within the last 2 years because they were sticking and as far as I can see both are working fine now. I will double check, though.

[Etch]

KARL 12:48 - see ladyalex's post at 11:47 Sun 13th Dec 2020: "our hot water temperature is lower than it was"

[teacake44]

^^ Yep, I've had that problem in the past, the valve itself becomes clogged up with scale, therefore makes it very difficult for the plastic/ nylon key way to turn it, and sooner or later cracks, not turning it at all, but sometimes lucky and partly turns it. It may be that first thing in a morning the plastic key has set hard over night and just manages to turn it, but once warm, the crack is larger and softer and will stretch.

[teacake44]

Togo^^^

[bhg481]

Your main thermostat is in the hall. You don't have a TRV on the radiator in the hall, do you?

[ladyalex]

I do have a valve on the rad in the hall...don't know what TRF means, so cannot answer your question.
Can I just repeat, this system has worked very well for a number of years. There have been occasional issues but nothing like this or nothing taking the Men as long to fix.
I am sure that the boiler is adequate for our needs. The room stat has been replaced, the pump (s) have been replaced the valves that Togo was taking about have been replaced.
The boiler fires up , is on long enough to get all the rads up to temperature, then it cuts out. It then comes on after some time , but only for a short time. This coming on and off repeats as the house gradually cools down.

[bhg481]

TRV is thermostatic radiator valve - the sort where you dial-in a temperature as opposed to a manually adjustable tap on the radiator. They should only be fitted in rooms which don't have a room thermostat

[ladyalex]

I have a manually adjustable valve on the rad in the hall which I can set from 1 to 5. We have always had a valve like this on the rad in the hall.

[bhg481]

If it has numbers on it it's almost certainly a TRV; they're not usually calibrated in degrees, just numbers, so that you can turn them up or down a number. Strangely, that could make your house overheat; if that TRV is set to a low temperature the hall will remain cool and the room thermostat in there will never kick in to cut off the heating to the rest of the house. If it's set to Max the room thermostat will control the whole house.

[ladyalex]

It is set to max as it usually is. We have a big house with a big hall and stairs.

[KARL]

From your descriptions I will assume that your controls include a common type of two port (in and out) zone valve.

When the thermostat demands heat it does so by providing power to the zone valve. Inside the zone valve (almost certainly all of them) is a small motor which turns a spring loaded cam which in turn is fixed to the valve spindle. As the cam/spindle turn so the gate/barrier within the valve turns and opens the waterway. Once the cam reaches the very end of its travel it trips a micro switch.

The micro switch energises both the boiler and the pump, the boiler comes on and the pump drives the heated water toward the radiators. Once the thermostat is satisfied that the set temperature has been reached, it cuts the power to the zone valve motor which allows the cam to return, cut the power to boiler and pump and close the valve - until next time.

The motor is a synchronous motor which is something rather special. It exerts a certain amount of torque on the cam/spindle even when the cam/spindle has stopped rotating, in order to keep the valve open and the micro switch closed. However, with age the motor can lose torque - this is not wear as we think of it but more a form of deterioration. It is possible that the micro switch is closing initially but then the cam slips, perhaps back and forth. Ultimately the motor may not even make it to throwing the micro switch. The motor is easily replaceable, readily available and not at all expensive - replacing it saves replacing the entire valve head.

It is only possible to systematically go through the checks which should reveal the problem within two hours at most by being on site. I still think the boiler is not the prime suspect and I would go straight for the controls - a crucial tool would be a multimeter to follow the electrical circuit's behaviour. I would first eliminate zone valve problems because it governs everything. From/with/at it one can establish whether all the other elements are being switched on and when energised whether they respond correctly.

bhg correctly pointed toward the zone valve and the lever on it. It is possible that the lever moves but still does not complete its normal travel. It is just possible that moving it manually and locking it in position at "on" the boiler and pump are not being reliably/consistently energised, only an electrical check will tell for certain.

It might be the synchronous motor, it could even be the micro switch, not to mention simple electrical faults (contacts/continuity). It is possible that some obstruction in the waterway is preventing the valve from fully opening and therefore preventing the cam tripping the micro switch. This can be a case of a hunt for a needle, but the haystack is not so big.

[KARL]

Please allow us to follow developments in the tradesmen's visit(s).