Boiler constantly turning on and off

I posted about this some months ago so apologies if it's a bit of a repeat, but I've got updates.

Basically, I have a Potterton Suprima, small tank in the loft F&E, hot water tank upstairs in airing cupboard.

Bought the house 9 months ago and I turned the gas off straight off and the heating stayed off for 4 months. Some changes to pipe work, ie moving rads, and all new rads put on. When turned back on the boiler kept overheating and locking out. Engineer came out and changed the thermistor. No luck. Eventually found the the 'aerator' (small tin can looking thing with 3 pipes coming out of it) was blocked solid with sediment. When attaching the hose one end, nothing came out of the other two ends. Cleared it, back in. And the boiler worked. Sort of. Just kept firing up for a few minutes then stopping for a few minutes.

I've now had a new Grundfos pump fitted. Still the same problem. Have put a couple of bottles of X100 or whatever number it was, prior to the new pump to try and clean it. Still doing it.

Where do I go from here? Is it likely to be another partial blockage somewhere?

I've put the heating on tonight (trying to balance the rads after having bathroom towel rad fitted). I timed the boiler and below is the times of on and off;

On - 2m52s
Off - 2m12s
On - 1m22s
Off - 2m12s
On - 1m02s
Off - 2m12s
On - 50s
Off - 2m30
On - 40s
Off - 2m39s
On - 39s

Then I gave up but it's still doing it now.

I really don't get boilers and central heating but it seems as though the boiler is reaching the max temp in similar times at the start so shuts down? Then takes similar times to cool down enough to fire back up?

I will get an engineer back round but any heads up of possible causes would be great.

Thanks for reading and sorry for the ramble

Fordy

X100 is not a cleaner.

X400 is a cleaner.

Which did you use? This is important.

What colour is the water when you bleed the radiators?

Did you clean the mud out of the F&E tank?

When the boiler is short-cycling, feel the Return pipe on the boiler. Is it very hot, or warm, or cold?

How warm do the radiators get? (May vary between rads)

When you bleed the highest radiator in the house, does water squirt out forcefully, and without dying away?

JohnD said:

X100 is not a cleaner.

X400 is a cleaner.

Which did you use? This is important.

What colour is the water when you bleed the radiators?

Did you clean the mud out of the F&E tank?

When the boiler is short-cycling, feel the Return pipe on the boiler. Is it very hot, or warm, or cold?

How warm do the radiators get? (May vary between rads)

When you bleed the highest radiator in the house, does water squirt out forcefully, and without dying away?

Click to expand...

It was X400. Went through the receipts.

It's slightly brown. Leaves the toilet paper a faint yellowish colour. When I drained the system the very first time, it was black. But the second time it started to come through quite clear. This time it was grubbier than last time. So dirtier, after I'd added the X400

I didn't this time to be honest. Did last time. Did notice it was a bit mucky as it happens.

How do I work out which is the return pipe? Is it normally generic? One of them has a drain off valve.

Rads do get really hot to be fair. Or did. Living room wasn't getting hot so turned off all the others. That got hot quickly. Now the corner rad in the dining room, which is probably marginally closer to the pump, won't get hot. Was hot before I got the living room one hot. All other rads are running hot.

Yep, all rads have a good squirt for more than 5 seconds, then I burn my hands. lol

The boiler has two big pipes coming out of it. One is the flow and one the return. The flow pipe will (should) get hot as soon as the boiler fires, the return will (should) get warm after the hot water has flowed round the radiators, so are easy to recognise by touch.

Unless the pump has been fitted the wrong way round. Look up the installation instructions on the manufacturer's website to see if the feel of the pipes is as it should be. It does no harm to mark the F and R pipes with paint or marker pen for future reference.

Also, if you have turned off all tbe rads but one, there will be nowhere for the heat to go, which is another possible cause of short-cycling.

Feel the pipes and the rads and report back please.

JohnD said:

The boiler has two big pipes coming out of it. One is the flow and one the return. The flow pipe will (should) get hot as soon as the boiler fires, the return will (should) get warm after the hot water has flowed round the radiators, so are easy to recognise by touch.

Unless the pump has been fitted the wrong way round. Look up the installation instructions on the manufacturer's website to see if the feel of the pipes is as it should be. It does no harm to mark the F and R pipes with paint or marker pen for future reference.

Also, if you have turned off all tbe rads but one, there will be nowhere for the heat to go, which is another possible cause of short-cycling.

Feel the pipes and the rads and report back please.

Click to expand...

Both are roasting hot. One perhaps hotter than the other, but not by a lot. Can't touch them for more than a second each.

Pump is definitely the right way round.

All rads are now open. One still not getting hot though. Boiler still on and off for similar time periods.

If the Return pipe is very hot that would explain the short-cycling. Less heat is being used than the boiler is trying to supply.

Some possible causes:
The radiators are too small
Some of the radiators are not getting hot
The flow of heat is by-passing the radiators

I understand you to say that all the radiators are hot, except one, that can probably be fixed by balancing. Is that right?

As a rule of thumb, a single radiator 600mm high and one metre long can emit about 800W to 1kW of heat into a cold room. How does the capacity of the rads compare to the power of your boiler and to the needs of your house? If the boiler is oversized the engineer can probably reduce its output. But make sure that the radiators are not too small to heat the house on a cold winter's night.

The boiler's thermostat should have enough hysteresis to prevent it turning off, then on again until the water temperature has dropped quite a bit. If all the radiators (and the cylinder) are fully hot, that should take longer than a couple of minutes. You say you recently had a new thermistor fitted. That is outside the scope of my DIY plumbing. I am not a heating engineer but have been through the usual DIY problems including circulation, blockages, cleaning, balancing, airlocks and even incorrect pump fitting by "professionals"

I would be looking for a blockage in pipework
take a magnet and see if you have a magnetic attraction on all the pipework around the area of the cold feed, vent and pump the stronger the magnetic attraction the more the pipes will be blocked
If there is an attraction then you will need to replace pipework on flow in area of cold feed/vent and system will need a good flush

hope this helps

Steve

One other thing

Turn down the boiler's temperature control knob. Does this cause the Flow pipe to get less hot? I am wondering if the control has failed and it is firing until the safety overheat control cuts it off.

UPDATE:

Sorry all, went for a bath, thought I'd leave the hot water and heating on anyway. Turned the room thermostat down. Rads were still really hot.

Just turned the hot water back on to the timer on the programmer. Left the heating on. However, room stat is saying 25degress (yes Im sweating), but I've set it to maintain room temp at 20. But the boiler is cycling on and off still.

Edit: I should say the little flame icon has gone out on the thermostat, but the boiler is still firing up

JohnD said:

If the Return pipe is very hot that would explain the short-cycling. Less heat is being used than the boiler is trying to supply.

Some possible causes:
The radiators are too small
Some of the radiators are not getting hot
The flow of heat is by-passing the radiators

I understand you to say that all the radiators are hot, except one, that can probably be fixed by balancing. Is that right?

As a rule of thumb, a single radiator 600mm high and one metre long can emit about 800W to 1kW of heat into a cold room. How does the capacity of the rads compare to the power of your boiler and to the needs of your house? If the boiler is oversized the engineer can probably reduce its output. But make sure that the radiators are not too small to heat the house on a cold winter's night.

The boiler's thermostat should have enough hysteresis to prevent it turning off, then on again until the water temperature has dropped quite a bit. If all the radiators (and the cylinder) are fully hot, that should take longer than a couple of minutes. You say you recently had a new thermistor fitted. That is outside the scope of my DIY plumbing. I am not a heating engineer but have been through the usual DIY problems including circulation, blockages, cleaning, balancing, airlocks and even incorrect pump fitting by "professionals"

Click to expand...

I've got one less radiator then I did have when I bought the house, but all the replacements are bigger.

Yes balancing should remedy the one cold rad. It has been hot before as have they all, at the same time.

That's something that is probably a bit beyond me but I'll try and figure it out tomorrow with a fresher head.

Thanks for your help.

Fordy81 said:

UPDATE:

Sorry all, went for a bath, thought I'd leave the hot water and heating on anyway. Turned the room thermostat down. Rads were still really hot.

Just turned the hot water back on to the timer on the programmer. Left the heating on. However, room stat is saying 25degress (yes Im sweating), but I've set it to maintain room temp at 20. But the boiler is cycling on and off still.

Edit: I should say the little flame icon has gone out on the thermostat, but the boiler is still firing up

Click to expand...

Bump

I had a very similar issue when I moved into our current house, the manual bypass valve was open too wide which caused most of the flow to go straight back to the boiler causing it to cycle.

Assuming all the radiators have been sized correctly a check to confirm this is when the boiler is cycling check the temperature of the return pipe if it is a similar temperature to the flow you either need to adjust the bypass valve or recheck the balancing of your radiators. If the return pipe is fairly cool you have a flow issue where the heat is unable to get taken away from the boiler.

hossacd said:

I had a very similar issue when I moved into our current house, the manual bypass valve was open too wide which caused most of the flow to go straight back to the boiler causing it to cycle.

A check to confirm this is when the boiler is cycling check the temperature of the return pipe if it is a similar temperature to the flow you either need to adjust the bypass valve or recheck the balancing of your radiators. If the return pipe is fairly cool you have a flow issue where the heat is unable to get taken away from the boiler.

Click to expand...

Thanks for that.

What exaclty is the manual bypass valve? At the bottom of my hot water tank, I have a pipe coming out that feeds to the pump I think. I've got a simple isolation valve. Is it that? Or is that where it should be?

Also, my 3 port valve (Honeywell), the outer case was quite warm, and taking that off, the round motor cover was roasting hot, even though the heating and hot water have been off for about 2 hours. Is this normal?

no. It sounds like it is jammed. You should be able to turn the spindle with your fingers. Does it need pliers?