O/T ancient boiler costing us a lot

[charlie_pank]

Just got gas bill for last 3 months - it's up by about 100% over last year! Went and had a look at the boiler and discovered that the CH Danfoss valve wasn't closing properly. As a result the CH was on all the time - I had been thinking the house had been unusually warm - don't know how long it has been like that, but fixed it now...

 

I had a further rummage around and started looking at its gas consumption generally. It's an old boiler which has 2 circuits each controlled by a Danfoss valve - one for CH and one for the HW cylinder. There's a bypass valve controlled by a tap for allowing some flow around a short-circuit when the boiler is cooling down.

 

For the last 6 months.I've had the HW off most of the time because we don't use much of it (electric shower). I noticed last night that the boiler was switching on/off every couple of minutes to keep the CH at the right temp - which really doesn't seem like an efficient way to behave. I think that it might actually be more efficient to remove the motorized part of the Danfoss valve from the HW circuit and leave it in the loop all the time. That way when the CH comes on, it heats the water in the cylinder too, which will act as a heat store, so the boiler shouldn't switch on/off so much thus using less gas. It also means I can close the short-circuit tap which I think was allowing the hot water to recirculate back to the boiler very quickly causing it to shut off again before it should.

 

I've implemented this plan and I'll be watching the gas consumption over the next few days. Any advice on whether I've done a good thing/bad thing. Anything else I should look out for?

 

 

[TonyR]

I can’t see how the cylinder would act as a heat store. What will happen is the temperature of the domestic hot water will now only be controlled by the boiler thermostat.

The cylinder thermostat on these systems are normally set around 60 degrees C, the boiler thermostat around 80C.

As there is now no cylinder stat the domestic hot water will be excessive, and if the heating circuit is off the boiler will cycle keeping the water in the flow & return pipes and cylinder to the boiler stat temperature.

 

What should happen is that if the heating circuit is off and the cylinder reaches temperature the power to the boiler and pump is switched off to prevent boiler cycling.

 

The stuck motorised valve may have been the cause of the cycling

 

TonyR

 

[charlie_pank]

The cycling was happening AFTER I fixed the stuck valve.

 

Agreed, the cylinder will be at the temp of the CH system, that's fine with me. The cylinder stat is still in use, it just can't close the HW valve - it should have the same effect as having the CH and HW always come on and off together on the timer.

 

Having the cylinder always in the circuit means that the circulating water will take longer to heat up and longer to cool down, thus reducing the on/off behaviour of the boiler.

[charlie_pank]

What is the normal behaviour for a boiler, how often should it cut in and out?

 

 

[TonyR]

Boilers will normally cycle when all the radiators and pipe work reach the temperature of the boiler thermostat.

If the boiler starts cycling well before the hot water and radiators are up to temperature indicates a potential problem.

You normally aim for a differential of 11C across the boiler flow and return pipes when the system gets up to temperature.

Excessive cycling can be caused by poor system design i.e. oversized boilers,old gravity systems, pipe work not sized correctly, etc.

Sludging/blockages, and failing pumps can cause cycling on older heating systems

 

TonyR

[charlie_pank]

Could be an over-specced boiler as we don't use all the radiators most of the time. I'll turn down the boiler thermostat...

[The caterham6]

This is my advice as 42 yrs BG I have this system....You need Vaillant Boiler/Cyl this is Unvented System this is the way forward everything is stat controlled its not cheap but your bills will drop loads....here

[pugwash]

Charlie,

 

I have the opposite problem to you in that my radiators only come on when there is a call for hot water. Not normally a problem as water tank cools down over night and in the morning radiators come on as water tank starts to heat. Family hits showers which cools tank and keeps radiators on. All leave house and system switches off on timer. End of day all come home, tank still not up to temp from morning showers and all comes on on the timer. Radiators get hot as water tank heats up. Only a problem if tank hits stat temp before end of evening as radiators all then go off. Wall stat has no effect. Sooner or later I need to get someone to look at it. Perhaps we need to cross my system with yours to get one that works! I am sure that POBC will advise what is wrong here - plumbing not my strong point.

 

 

[Martin J]

I think you will find your hot water will get too hot.

What kind of boiler do you have? Some are range rated so the heat output can be adjusted, although this is done by a service engineer or the installer.

Lots of my customers are worried about fuel bills & I have to offer advice to get the best out of a boiler.

I went to an average sized house recently in an exposed rural location & despite saying they ran it economically & with only a condensing vaillant boiler fitted they had burned their way through £500 of LPG in 6 weeks. They assumed the boiler had just broken down but the bulk tank was empty. They had only been in the house for a couple of months & were truly shocked. ☹️