Boiler cycling, the ultimate evil or not?

[ALEC1]

Its a bit cynical to ask that!

install a boiler as the designers intended on a system sympathetic to that design, care for it and all boilers will last.

slap it on a poorly designed and maintained system, ignore the boiler designers criteria and it will fail...

of course while we ignore engineers, deride installers and ride rough shod over a "critical" industry with programmes like rogue traders we get the installations we deserve, and the industry as it happens...

 

[ianniann]

It sounds like Alec still hasn't grasped that virtually every boiler is going to cycle because it can't modulate low enough, and that this is doubly true when the flow temperature is lowered to reduce the heat flow into rooms near their setpoint. I'm just going to take that as a given.

Here's another observation for the people that think switching a boiler on and off will kill it by next weekend. Virtually every combi boiler in the country burns for a few seconds every ten minutes or so, as a type of warm standby for providing hot water. This can usually be turned off but most people don't know or don't bother. My Vaillant Sine managed it for maybe 20 years until I inherited it and switched off that feature. My biggest fear would be for the igniters, something the Vaillant doesn't even have, but plenty of combis are switching every 10 minutes 24/7 without dying. Cycling the heating maybe every half hour when there is heat demand seems trivial in comparison.

 

[ALEC1]

With respect I agree that short cycling is inevitable in smaller systems given that systems can't dissipate enough heat.

In the average semi a typical uk house a correctly sized system boiler wont cycle..but boilers are repeatedly over sized that leads to this inevitable issue..

It all depends on how the boilers are installed...

cycling is hugely wasteful of gas and puts stress on components that thoughtful installers will avoid..although it must be pointed out that we are not encouraged to be thoughtful or install the boilers in such a way that they are effiecient!

 

[transam]

The boiler's which never switch off that I am refering to , is because the user's do not pay the gas bill's , the institution in question show very little interest in sophiisticated controls , despite being offered them , on numerous occasion's , as for being cynical , yes very cynical , I have very little time for any boiler manu' , pick the best of a bad bunch , & hope u get it right ?? inbuilding some control system into a boiler installation that would keep the boiler in the condensing mode all the time , might well have the opposite effect , & cause the boiler to fail , reduce it's life span , not increase it ???


I wonder if the sale of the baker dunsley neutralizer is on the increase ??? as more people go back to basic's ??

 

[ALEC1]

agreed gas should be much more expensive to focus peoples mind...

 

[ianniann]

cycling is hugely wasteful of gas

Could you quantify that, please? I'll give an upper limit to start you off. The 100 times a day or so that a combi boiler cycles to keep warm water on standby uses perhaps two or three kwh worth of gas (maybe even less on a new boiler?), and most of that is legitimate heat going to keep the exchanger hot (and then warming up your airing cupboard or garage!). Try it, you can measure this at home, folks. Compare that to the 100 kwh or so that you'll use for heating and the maybe 10-20 times it might conceivably cycle doing that heating and I'm not seeing huge waste.

 

[ALEC1]

cycling is the burning of gas to keep a boiler hot when there is no demand. as opposed to a warm start facility which isn't really cycling in the trues form

What makes cycling inefficient is that it is indicative of a call for heat with no where for it to go.

If you are talking about a combi preheat facility this isn't cycling. In fact research has been done suggesting that keeping plate heat exchangers warm actually saves gas and water in the typical domestic set up. Like everything to do with fuel efficiency it depends on usage...

what people should be doing is turning the warm start off and open a hot tap slowly increasing the flow as the temperature rises...but most people just turn the tap on and let it run.. thats actually the wasteful part!

 

[ianniann]

Now you're just changing your definitions. If you want to say that cycling is only when the boiler is turning on and off for no reason then of course it's wasteful. Not "end of the planet" wasteful, but since there's no need for it ... no brainer. In fact I'd call it a boiler fault if it happens.

I'm talking about cycling on and off when there is a demand for heat (as I thought you were). This is the cycling that is inevitably caused by a boiler output that can't be modulated low enough for the heat demand. I just don't see any significant waste involved. The pump stays running when the burners are off (or it should), hot water circulates through the radiators just as if the burners were on, and after a few minutes delay or when the flow temperature drops a bit then the burners will kick in again. Your (I think your, but certainly some people's on here) master plan of pumping mildly warm water through the radiators as the room temperature approaches the setpoint virtually guarantees cycling which is why I keep trying to get to the bottom of why you think it's so wasteful.

 

[ChrisR]

The benefit is obvious. At lower temperatures the boilers dont work so hard, and given that every mechanical component has a given life span the less it works the longer it will last.

It isn't obvious, and the statement is tripe.

cycling is the burning of gas to keep a boiler hot when there is no demand.

A lie. Read the question, don't make up what your opposition is saying so that you can rubbish it. This is the tactic of a fool.

cycling is hugely wasteful of gas

More unsubstantiated drivel from Alec.

Alec is a major blight on anyone who reads his diatribes, prancing about on his hobby-horse, plucking fact and fiction at random to support his religion.

We all have a right to an opinion. Mine is that Alec is one of the worst people you could listen to.

To the original poster, asking about short-cycling, you might consider old, old boilers, say 20 years old, working on sludgy system with poor circulation. Or combi boilers which didn't modulate, or didn't do it much, with outputs of 24kW or more working in flats with 3 rads. Sometimes they only come on for 20 seconds at a time. From the hundreds I've seen like that, they don't seem to suffer particularly in the components which get exercised every time the boiler comes on.
The cycling is something you notice while you're there, changing a hot water diaphragm say. Some of them come on many times more frequently than they would if the system was working properly, but the circuit boards, for example, don't fail any more often. I'd think they would, so I'd readily accept that as an explanation IF it happened.

It could be, I don't know, that short "on" periods are less wearing on components, because they don't attain the same temperature that they would if they were on for longer periods. It certainly isn't "obvious". To the really ignorant, everything is obvious.

 

[ALEC1]

if your talking about cycling as not particularly long bursts of burner activity then as I have said in smaller systems its inevitable..

but it would be hardly a good idea to size a flat or house around the minimum loading of a heating system

what is certain is that a lot of the sophisticated systems that the public are now demanding have control logic in them to minimise cycling and enhance system efficiency....