I own a Worcester 25si/28si series central heating boiler. In the user instructions it says to keep one radiator thermostat setting on max to prevent boiler from short cycling. It does not define "short cycling". Can someone please explain how I check whether the boiler is short cycling? What are the normal operation timings (e.g. is a 4 minute gap between boiler switching off then on short cycling)? I know nothing of this subject so please use layman's terms. Thanks.
What is the "normal" cycling frequency of a boiler
- Thread starter gammerz
- Start date
Sponsored Links
I can't believe nobody has replied to this. It is an excellent question, although obviously different people will define the terms differently.
Cycling is when a boiler turns on and off in order to maintain the desired temperature. This may occur under the control of a room thermostat as the room temperature varies, or under the control of the boiler itself as the water temperature in the radiators varies. A typical boiler, and especially a combi boiler, can heat up the water far faster than the radiators can cool it down. This is doubly true with TRVs when the boiler might only be feeding one or two radiators when the other rooms are up to temp.
All this is normal and not a huge cause for concern, although it does waste a bit of gas and cause some wear and tear on boiler components. The problem is limited in many cases because the heating isn't used continuously so only a few cycles will occur each day. A more serious problem is cycling when there is no demand for heating. This can be caused by a variety of control issues. It is usually prevented on modern installations by organising the controls in a way known as an interlock. You should consider any cycling at all on a new system without heating demand to be a fault, although don't be confused by the warm-start feature of many combi boilers which fire for a few seconds every few minutes to keep warm water available. This is sometimes called dry cycling because it isn't circulating hot water through your radiators.
Short cycling is a vague term that just means cycling more often than is desirable. By some definitions this would be any cycling on a boiler internal thermostat. Cycling on a room thermostat, for example, is perfectly normal and will usually occur no more than once or twice an hour and usually a lot less. Cycling on the boiler's internal thermostat can easily occur every minute or two if it cannot modulate its heating power low enough to balance the heat output from whichever radiators are open. Most people would see this as too often and many boilers now incorporate electronics to prevent it. A typical anti-cycling program would lock out a cycle for several minutes or perhaps a 10C water temperature drop. Even this is more often than ideal, but a great many people will have boilers doing this and never bat an eyelid. At 1-2kw per radiator, you need quite a few open radiators to balance the lowest power output of a combi boiler which may be 8-10kw on a modulating design and 20-30kw on a non-modulating design.
Specific to your question, if you have all the radiators turned off, manually or by TRVs, then the boiler has nowhere to send heat and nowhere to pump water. New boiler installations should always have a bypass to prevent damage, but even then it is just pumping hot water round a short pipe. Burner cycling would be inevitable. With one radiator permanently open you at least have somewhere for the heat and water to go although it will never be enough to prevent cycling. You can diagnose this by looking at a flame indicator light that many boilers have on the front, or watching the smoke plume outside. If it comes on and off frequently than you might have an issue. If it comes on and off every 5-10 minutes some of the time then it's not ideal and you might want to consider whether you can adjust your controls to provide more even firing.
Cycling is when a boiler turns on and off in order to maintain the desired temperature. This may occur under the control of a room thermostat as the room temperature varies, or under the control of the boiler itself as the water temperature in the radiators varies. A typical boiler, and especially a combi boiler, can heat up the water far faster than the radiators can cool it down. This is doubly true with TRVs when the boiler might only be feeding one or two radiators when the other rooms are up to temp.
All this is normal and not a huge cause for concern, although it does waste a bit of gas and cause some wear and tear on boiler components. The problem is limited in many cases because the heating isn't used continuously so only a few cycles will occur each day. A more serious problem is cycling when there is no demand for heating. This can be caused by a variety of control issues. It is usually prevented on modern installations by organising the controls in a way known as an interlock. You should consider any cycling at all on a new system without heating demand to be a fault, although don't be confused by the warm-start feature of many combi boilers which fire for a few seconds every few minutes to keep warm water available. This is sometimes called dry cycling because it isn't circulating hot water through your radiators.
Short cycling is a vague term that just means cycling more often than is desirable. By some definitions this would be any cycling on a boiler internal thermostat. Cycling on a room thermostat, for example, is perfectly normal and will usually occur no more than once or twice an hour and usually a lot less. Cycling on the boiler's internal thermostat can easily occur every minute or two if it cannot modulate its heating power low enough to balance the heat output from whichever radiators are open. Most people would see this as too often and many boilers now incorporate electronics to prevent it. A typical anti-cycling program would lock out a cycle for several minutes or perhaps a 10C water temperature drop. Even this is more often than ideal, but a great many people will have boilers doing this and never bat an eyelid. At 1-2kw per radiator, you need quite a few open radiators to balance the lowest power output of a combi boiler which may be 8-10kw on a modulating design and 20-30kw on a non-modulating design.
Specific to your question, if you have all the radiators turned off, manually or by TRVs, then the boiler has nowhere to send heat and nowhere to pump water. New boiler installations should always have a bypass to prevent damage, but even then it is just pumping hot water round a short pipe. Burner cycling would be inevitable. With one radiator permanently open you at least have somewhere for the heat and water to go although it will never be enough to prevent cycling. You can diagnose this by looking at a flame indicator light that many boilers have on the front, or watching the smoke plume outside. If it comes on and off frequently than you might have an issue. If it comes on and off every 5-10 minutes some of the time then it's not ideal and you might want to consider whether you can adjust your controls to provide more even firing.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
Similar threads
