Boiler cycling on and off

[xMeta4x]

Hi,

I hope this is an easy question! Having fixed a few problems with my Potterton Profile 60e I'm wondering why it cycles on and off.

It will run for a minute or two, shut itself off, then fire up again in another couple of minutes. This goes on and on.

Do I have the boiler thermostat set too high?

Cheers,

Andy

(I should mention that hot water/heating are all OK)

 

[D_Hailsham]

Apart from the boiler thermostat, do you have a room thermostat?

 

[xMeta4x]

Yes I have both a room and water thermostat with a motorized 2 zone control valve. Until the room thermostat is up to temperature the boiler cycles like this, after that it shuts down as expected.

Not sure about the water, I don't really pay much attention to it as it's only on for half on hour twice a day.

 

[D_Hailsham]

Yes I have both a room and water thermostat with a motorized 2 zone control valve. Until the room thermostat is up to temperature the boiler cycles like this, after that it shuts down as expected.

No harm turning the boiler stat down slightly.

The boiler is producing heat faster than the radiators can get rid of the heat. This could mean that the boiler is oversized or the pump is set at too low a speed.

 

[xMeta4x]

I do have the pump set at the lowest setting. I found that otherwise the cavitation (spelling?) caused lots of air to enter the system!

The house was a 3 bed end of terrace, but has been extended with 2 extra rooms downstairs; would you say the boiler was overpowered?

 

[D_Hailsham]

I do have the pump set at the lowest setting. I found that otherwise the cavitation (spelling?) caused lots of air to enter the system!

Cavitation does not cause air to enter the system. There is something wrong with the layout of your system, in particular the cold feed from the F/E tank and the vent pipe.

Where do the feed and vent pipes enter the system?
How far apart are the entry points?
How high is the feed tank above the pump?

The house was a 3 bed end of terrace, but has been extended with 2 extra rooms downstairs; would you say the boiler was overpowered?

Can't say without knowing the heat loss of your house! However the size of your rads will give a good starting point - provided they are not way oversize.

Use the information in the Stelrad Elite Catalogue to work out the approximate total output of your radiators. Add on 2kW for hot water and you will get the size boiler you should have. The Profile 60E is 17.58kW (60,000BTU) but it can be down-rated to a minimum of 14.65kW (50,000BTU).

 

[xMeta4x]

Cavitation does not cause air to enter the system. There is something wrong with the layout of your system, in particular the cold feed from the F/E tank and the vent pipe.

Hmm, I solved a problem of constantly having to bleed air out of the radiators by turning down the pump - not sure how/why this worked. My system is *very* weird so bear with me.

Where do the feed and vent pipes enter the system?
How far apart are the entry points?
How high is the feed tank above the pump?

I have a tank in the loft which has the cold feed going into it, but I don't have a vent pipe, no idea why as I thought it was required (unless it vents elsewhere).
The pump is on the return of the boiler and is downstairs.

 

[tonybhoy]

Very rare not to have a vent pipe but some systems vent via the cold feed or a combined feed and expansion where the vent is just a loop into the F&E tank off the cold feed in the attic close to the F&E tank.

Sounds as though your cold feed is blocked.

Cavitation is air entrapped in the pump and is usually very noisy.

I would have a look for a vent pipe if I were you just to confirm you don't have one. There will be one for the cylinder which usually goes out through the roof (or it could vent into the cold water storgage tank) the heating vent normally vents into the F&E tank but it can also vent through the roof.

 

[xMeta4x]

Sounds as though your cold feed is blocked.

I suspected this a long time ago as the system didn't refill following a drain down and had to be filled from the mains. The plumber said that it wasn't blocked and was just a weirdly designed system. When filled from the mains the level in the hot water tank rose and overflowed which seems to confirm that the cold feed isn't blocked, and is also the vent..?

Cavitation is air entrapped in the pump and is usually very noisy.

It was *really* noisy until I turned the pump speed down. I bled it when it was fast and got no air or water..?

I would have a look for a vent pipe if I were you just to confirm you don't have one. There will be one for the cylinder which usually goes out through the roof (or it could vent into the cold water storgage tank) the heating vent normally vents into the F&E tank but it can also vent through the roof.

I do have a vent pipe over the cold water tank which is at a lower level to the hot water tank. I guessed that this was for the cold water system though as nothing came out when I filled via the mains.[/quote]

 

[xMeta4x]

Sounds as though your cold feed is blocked.

I suspected this a long time ago as the system didn't refill following a drain down and had to be filled from the mains. The plumber said that it wasn't blocked and was just a weirdly designed system. When filled from the mains the level in the hot water tank rose and overflowed which seems to confirm that the cold feed isn't blocked, and is also the vent..?

Cavitation is air entrapped in the pump and is usually very noisy.

It was *really* noisy until I turned the pump speed down. I bled it when it was fast and got no air or water..?

I would have a look for a vent pipe if I were you just to confirm you don't have one. There will be one for the cylinder which usually goes out through the roof (or it could vent into the cold water storgage tank) the heating vent normally vents into the F&E tank but it can also vent through the roof.

I do have a vent pipe over the cold water tank which is at a lower level to the hot water tank. I guessed that this was for the cold water system though as nothing came out when I filled via the mains.

 

[D_Hailsham]

You should have two tanks in the loft: a large one - which supplies cold water to the toilets, wash basins etc; and a small one - which is the Feed and Expansion Tank for the CH system.

There should be a vent pipe coming from the top of the Hot Water cylinder. This usually vents over the Cold Water tank.

You may have an other vent pipe from the CH system, which vents over the F/E tank. But sometimes this is not there as the system is using a "combined feed and vent". Apart from the obvious absence of a vent pipe, combined F and V systems use a 22mm feed pipe to the CH system.

I do have a vent pipe over the cold water tank which is at a lower level to the hot water tank

Do you mean that the smaller tank in the loft is higher than the larger and the vent pipe hangs over the larger tank? If so, this should be the vent pipe for the Hot Water cylinder.

 

[xMeta4x]

I have two tanks, the F&E is in the loft and the cold water tank is in a cupboard in the upper floor (don't ask me why); this is the one with the vent pipe over it.

The vent over tne CW tank is 22mm but the feed for teh F&E tank is smaller.

Will this have an effect on the boiler cycling? The manual says that it will cycles when it gets to temperature so I'm wondering if it's just normal...?