We have a gas combi boiler, not condensing, for central heating and hot water.

We also have a contract with British Gas for servicing.
Periodically we have asked them to stop it short cycling, they have sent an engineer to look who has found no problem and told us to turn up the boiler stat when this happens. We can fix it by turning the stat way up or way down, guess which we do!

Yes, heating is on.

Anyone have experience or expertise to share.

Many thanks,

Peter

How about range rating the boiler down a bit to a lower kW load ? ( would need gas engineer to do this )

We have TRVs on every radiator, but no house thermostat.

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Does it only short cycle once the house is warm? If so this is what I think is happening...

When the house is upto temperature most of the TRV switch off and the return temperature starts to rise. Worse case all the TRV switch off and the pump has nowhere to send the hot water from the boiler except through an automatic bypass fitted for the purpose. The bypass sends the hot water coming out if the boiler straight back round to the return. Either way the boiler detects that the flow temperature (return water + heat) is getting too hot and shuts off quickly. I think I would get a room stat fitted in the living room and leave the TRV in that room wide open (or remove it). If the temperature on that stat is set slightly higher than the TRV in other rooms it should switch off the boiler rather than short cycle.

If it short cycles even when the house is warmng up from cold then it could be one of a number of problems including boiler much too powerful for the rads, faulty bypass, pump too slow or faulty etc.

Most pumps have a knob with three positions on it for controlling the speed. Typically marked I, II, III.

Yep, get rid of one TRV and replace it with a thermostat that controls the boiler. Out of interest, what are you using to control the boiler currently? A programmable thermostat would be an excellent investment if you haven't got one.

We had a short cycling problem for years on a condensing boiler. Turned up pump, upgraded pump, system flushed. It was only when we had the boiler replaced that someone checked the gas pressure and found it was too low. Cue upgraded pipe into house.

Worth checking if nothing else solves it.

You need a room stat in the main room and remove the TRV in that room. Without a room stat all boilers will short cycle most of the time once the house is up to temp. The britsh gas guys should have spotted the lack of stat when you first raised the issue with them. In the interests of energy conservation, I'm pretty sure building regs require a room stat.

you can have a basic stat that will control the temp at the same temperature all day every day for £15 ish plus install. If you want to do a proper joB you can have a programmable stat for around £50 that will control the heating at different temperatures at diffent times of the day and allow different programs for each day of the week so you can set the heating to match your lifestyle. You'll recoup your money in no time with one of these and your short cycling should be sorted.

Posted By: Peter Clark
Can this be wireless?
Peter

Absolutely, CWatters mentioned that above.

Posted By: CWattersNo idea if they are any good (20m range?) but Amazon has a programable wireless stat for just £27 +p&p

http://www.amazon.co.uk/Programmable-Wireless-Room-Thermostat-Volt/dp/B0076YWXIU/ref=sr_1_5?ie=UTF8&qid=1351006608&sr=8-5" rel="nofollow" >http://www.amazon.co.uk/Programmable-Wireless-Room-Thermostat-Volt/dp/B0076YWXIU/ref=sr_1_5?ie=UTF8&qid=1351006608&sr=8-5

Well known brands go from £50 to over £100.

Thanks for helping, I will have a look.

Peter

Posted By: Peter ClarkWe have TRVs on every radiator, but no house thermostat. So, what you said seems to imply that the boiler was supplying more than enough heat and the house was at the correct temp?

Peter

you've got what?

that's madness IMO, and fairly obviously the reason for the boiler cycling as it's relying solely on the return stat in the boiler to control it.

it should have a main thermostat in a space heated by a rad without any trv fitted to provide main control for the entire system, otherwise you get this happening where the boiler will cycle even when there's no actual requirement for heating on any of the rads, just because the boiler stat has cooled to it's switch on temp.

Whoever fitted this was a complete muppet.

Posted By: Gavin_AWhoever fitted this was a complete muppet.

That's interesting. Peter said it was a combi boiler, i.e. it also provides hot water direct to the taps.

People turn on hot taps for anything from 10 seconds for a quick hand rinse, to 5 minutes running a bath. If short cycling is so inefficient, doesn't that mean that combi boilers will be very inefficient producers of domestic hot water?

If there are TRVs in all rooms, as we've seen, there is a risk of short cycling.

If you have one radiator without a TRV, as Seret suggested, then either you must set the room stat so high that that radiator is basically uncontrolled, or you run the risk of a satisfied room stat starving other radiators of heat.

Really you want to have room thermostats in every room, so the boiler always comes on when a room needs heat, and never otherwise.

The wired type only cost a tenner each, but the whole-house wiring could be expensive and disruptive.

Alternatively you could do it with pricey wireless stats in each room.

Either way, you would still need TRVs to ensure that when the boiler does come on, the heat goes only to the radiator(s) that need(s) it. Or (the expense is mounting - but I guess some people do this) you could have a motorised zone valve for each room.

The building regs already want us to have at least two zones in an average house. I suspect that as the cost of things like wireless stats comes down, a zone-per-room arrangement will become normal. Does that sound plausible to those of you who know more about heating than I do?

Interesting that it comes down to the 'balancing' of the system, which, if I remember correctly, is how they did it in the days before central heating had any thermostatic controls at all.

Posted By: rhamduInteresting that it comes down to the 'balancing' of the system, which, if I remember correctly, is how they did it in the days before central heating had any thermostatic controls at all.

all systems will operate better if they're reasonably well balanced, so that the heat being sent to each radiator roughly matches the heat requirements of that room in proportion to all the others.

it does seem to be a bit of a lost art, with some like the heating engineer in this case obviously thinking that TRV's are some sort of magic bullet that means they don't need to bother with anything more complex than just banging a trv on each radiator.

Balancing is simply about ensuring the correct mass flow rate of heated water at the correct temperature reaches the radiator to ensure it can perform at the design day condition. You can measure it (or deduce it from pressure drop) or on a simplified basis you can use a heat balance method - the pumbers hand being the most basic method.

From there, operation of the TRV's simply gives you the reduced mass flow you need to meet the part load conditions

regards

Barney

It's always annoyed me that plumbing control systems are fundamentally crappy. You have an electronic master control (timer, maybe plus thermostat), but then add a lot of flow-based temp controls. The two systems are entirely independent so then boilers get a load of fancy heuristics in them to try and do 'the right thing' by looking at flow and return temps. It's fundamentally rubbish engineering. A proper system would take a temp reading from all rooms and timer settings (also preferably per-room/zone), (and outside, and prob DHW too) then decide how/when to run boiler.

None of this is hard but it used to be too expensive. It should be (farily) cheap and easy now but unless you DIY something with 1-wire and HR20 motorised valves it generally still isn't.