Is my boiler short cycling?

[thomsob]

How does this effect gas consumption also?

 

[D_Hailsham]

I have worked the outputs from the link you supplied (thanks very much).

It comes in at 40,646 BTU.

If we then add on about 9,400 BTU for hot water we get a boiler size of 50,000 BTU, which is much less than the 80,000 BTU your boiler supplies.

It might be worth using the Sedbuk Whole House Boiler Calculator to find out what size boiler you really should have.

As you have found out, an oversized boiler will lead to short cycling as the boiler is heating up much faster than the radiators can give out the heat.

The gas consumption will increase as the boiler will be going on and off more often and there is a slight loss of un-burnt gas every time the boiler lights.This loss will obviously be greater.

Unfortunately the Suprima 80L is a fixed output boiler, so you can't reduce the output. However it might be worth reducing the boiler temperature "one hour" at a time and seeing what the effect is.

 

[thomsob]

thanks very much for this. I will turn down the stat a little at a time and let you know how i get on.

 

[BigBurner]

Was this boiler previously working OK with no cycling before the lockouts?

 

[thomsob]

As i said i have just moved into the house 2 months ago, it has always been liek this i moved in so can't comment im afraid.

Would getting a quicker pump help get the heat away from the boiler quicker?

 

[JohnD]

Have you got thermostatic radiator valves?

If you bleed all your radiators, how long is it before any of them have air/gas in then again?

What colour is the water that comes out?

You say the rads are not very hot. If you feel the two pipes that supply each rad, is there much difference between the flow (hot) side and the return (less hot) side?

How does the temperature of these radiator pipes compare to the pipes next to your boiler?

Have you by any chance taken any of the rads off for decorating, and did sludge come out when you emptied it?

How hot is the HW cylinder and the tap water? Can you see the Cylinder Stat setting?

I am just a householder, but I am thinking the hot boiler/not hot rads suggests poor CH circulation, possibly sludge obstruction or near-closed valves or a bypass is open or too much flow is going through the cylinder. The system probably contains 100 lites of water, so if it is circulating, it should take more than a minute to bring this back to 80C or whatever, even without much heat transfer from the rads.

 

[D_Hailsham]

The system probably contains 100 lites of water, so if it is circulating, it should take more than a minute to bring this back to 80C or whatever, even without much heat transfer from the rads.

The boiler only seems to come on for 1 minute bursts after it has been on for an hour or so.

How long between the one minute on periods? 4 minutes.

So the boiler is taking about an hour to get up to temp and then it is cycling - one minute ON, four minutes OFF.

As I have shown, the boiler is about 60% oversized. Even if there was no sludge etc in the system, the boiler will cycle.

Don't forget this is not a modulating boiler, so it whacks out its full output every time it fires up. There is nowhere for the excess heat to go, so the return temperature will be higher than the designed value and the boiler quickly reaches max temperature. The pump then continues to run for about four minutes, controlled by the pump overrun, until the temp is low enough for the boiler to relight. I suspect that the over-run is time controlled and not temperature, so the water has not really cooled down enough before the boiler relights.

 

[BigBurner]

As I have shown, the boiler is about 60% oversized. Even if there was no sludge etc in the system, the boiler will cycle.

Best install a CH buffer cylinder. A small 40 to 50 litre direct cylinder. Have the boiler only heat this with a flow pipe to the top or the shower connection on the side, and a return pipe back from the cylinder cold feed to the boiler . You may need a Surrey flange on this port. Then take a flow from the cylinder to the rads. the return from the rads to the bottom of the cylinder.

Best have two cylinder stats on the cylinder to eliminate cycling (need to use a relay to do this). The boiler only heats the cylinder and the stats switches it off when up to temperature. When the cylinder cools substantially, the rads are taken off the other side. You can have a Willo Smart pump on the rad circuit and TRVs on all rads and no wall room stat.
Then boiler cycling will be eliminated as the cylinder acts as a buffer.

 

[JohnD]

for clarity, why will a 50 litre cylinder work when a 100 litre CH system doesn't?

 

[BigBurner]

for clarity, why will a 50 litre cylinder work when a 100 litre CH system doesn't?

Read what I wrote. Draw it out and think it through.

 

[JohnD]

Please don't be offensive. Just provide your explanation.

You may wish to include the two cyl stats you added to your original post after your snotty reply.

 

[snb]

you could get an RGI to check the burner pressure. these boilers can be adjusted. they have 2 setting suggested. normal being 80000 btu output.

This can be reduced to about 70000 btu output.All this info is in the manufacturer instructions for this boiler.

Your biggest problem and the one that needs to be sorted is the flow around the rads.

Check what the pump speed is. Should be number 2. Check that the pump valves are fully open. The bypass valve is open only 1 turn if manual. if automatic then put it on number 2 or 3.

Check that the rad lockshields are open enough. Down stairs about 1.5 open. upstairs about 1 turn open. Failing this open the lockshields fully and see what happens and then turn them down so that the rads get hot but with the lockshields as closed as much as possible.

I would put in a new pump after that if the above fails.

 

[BigBurner]

Please don't be offensive. Just provide your explanation.

It was provided. It was simple.

You may wish to include the two cyl stats you added to your original post after your snotty reply.

It was in the original post, I re-wrote so the uninitiated can understand a little easier.

 

[JohnD]

Then boiler cycling will be eliminated as the cylinder acts as a buffer.

Please explain how and why

 

[thomsob]

Hi Guys, thanks for all the advice. I have been out of the country for the festivities and have an update.

I put some de-sludging chems in the system as the rads are only getting hot at the top, the bottom middle is luke warm at best.

There is a towel rail in the bathroom which as far as i can tell is plumbed straight in just above the boiler. This towel rail needs bled every week pretty much, but is the only rad in the system that is burning correctly, i.e. all over. I have turned the towel rad off at the TRV and the lock shield, and have noticed that my rads are now burning much better.

For some reason the boiler is still short cycling however, which means it takes longer for the house to reach temperature.

Any further suggestions? Also can anyone tell me why turning the towel rail off has helped the rest of the system, was it acting as a by pass or something?