Cycling query

[MickyP10]

Read a few on this and trying to understand. Say if a boiler is over-specced (a bit), shuts off (and pump continues to run) while radiators lose enough heat to trip the boiler to fire up again, why is this uneconomical? I would have thought the pump only transporting the heat - ie no oil or gas being used - was a good thing.

 

[MJN]

I will let the pro's give the full run down but a few reasons why cycling is inefficient are:

1. By definition the maximum efficiency of any energy transfer system is obtained when the input matches the output. If a boiler can modulate the burner to match the load then this can be achieved to a certain extent, however if it has to turn off/on due to either overshooting or not being able to modulate down low enough then this goal cannot be met.

2. Steady-state conditions will always produce better efficiency e.g. your car engine running at its full operating temperature compared against startup in cold conditions. Boilers are just the same and cannot be as efficient whilst starting and stopping. Furthermore, initial combustion will never be complete due to the need to guarantee ignition and the explosive nature of the burn - some unburnt fuel will always be ejected out of the flue when it fires up.

3. In order to prevent uncontrolled explosions the combustion chamber must be evacuated of any accumulated unburnt gas prior to introducing it for ignition so not only will this gas be wasted fuel but it also inevitably reduces the burn environment temperature due to the throughput of (cool) air not used directly for combustion. Heat is a fundamental requirement of combustion hence loss here will again reduce efficiency.

4. The internal boiler control feedback loop is not instantaneous and the hysteresis delay will mean the boiler remains fired up for longer than is necessary i.e. it still produces heat for a period until it realises it needs to turn off. The inefficiences of such an overshoot will occur each and every on/off cycle.

I'm sure others will chime in with even more reasons.

You are right that the continuing circulation from the pump is making use of the latent heat of the water, however you need to consider the efficiency of the system as a whole - the boiler component of which is the most significant energy consumer hence inefficiencies here similarly carry the most signficance.

Mathew

 

[MJN]

[Oops - double posted]

 

[johnnojob]

arrange room stat to shut down system when that area is heated.

had several vokeras many years ago and they would cycle on and off a lot with noise from those massive relays and gas valve cutting in.

i think that the fan never stopped running although on half voltage much heat was lost from heating body via the flue.

nice people at above company fitted a delay timer so the heating stays off for much longer periods up to 20 minutes if required.adjustble.

these must still be available as a kit ,its just a small stand alone printed board with a few wires attached.

 

[MickyP10]

Hi Mathew

Thanks for the reply. As an untechie, I agree with a lot of what you're saying, but still find it hard to understand why if the boiler's off for five mins and then on for, dunno, eight minutes, it's less efficient than it being on for a solid 13 minutes. Granted if the thing fired up and turned off every minute it would be a bit silly. Say you're in summer now - same boiler etc, but you're only heating the water circuit. That'd shut off pretty quick, surely. What do you do about that, apart from knocking the temp back a bit?

If you cruised down a hill in neutral in a car, you'd use less fuel than driving down or on overrun!

Not trying to be flippant, but there must be a balance when time off v time on. No idea what it is, mind. But there must be one!

Edit to say: I'm making the times up but they're not far off what I get with my (oil) burner. ** When I say 'efficient' I mean uses less fuel.

 

[Tibbot]

Efficiency has nothing to do with using less fuel. Leave it off and it'll use no fuel at all, but that won't make it efficient.

The only way of measuring efficiency for a boiler is how much input is turned into output, whilst burning, and how much energy is wasted through the flue.

How much heat wasted out the house is another matter but doesn't alter boiler efficiency.

In your example the reason it goes off is that it's reached temp at the control stat before the pump can disperse the heat. A 13 min continuous burn means heat is able to be dissipated more evenly. System will warm and boiler cut without enless stop start wear & tear and poor early combustion. You won't use more fuel.

 

[MJN]

As an untechie, I agree with a lot of what you're saying, but still find it hard to understand why if the boiler's off for five mins and then on for, dunno, eight minutes, it's less efficient than it being on for a solid 13 minutes.

If we assume that our desired output during those 13 minutes is constant - 20kW for arguments sake, in a steady-state condition our boiler could provide a nice smooth 20kW for the duration such that the input matched the output and, by definition, efficiency is achieved.

If, however, the boiler is going to be off for 5 minutes and on for 8 then the latter part of the cycle will need to compensate for the 5 minutes at 0kW by burning at 32.5kW for the 8 minutes (from (20x13)/8). In this hypothetical scenario this 32.5kW might be outside of its capabilities, or if a condensing boiler then its ability to condense, and we have increased our likelihood of overshoot. All of these are inefficiencies compared with the steady-state non-cycling operation. Furthermore, this is just one cycle - it's the repeated cycling inefficiencies that eventually add up to a potentially significant waste of energy, particularly given some of the problems with ignition I mentioned earlier.

Say you're in summer now - same boiler etc, but you're only heating the water circuit. That'd shut off pretty quick, surely. What do you do about that, apart from knocking the temp back a bit?

There's not that much you can do, other than relying on the cylinder stat to indicate it is satisfied prior to the boiler running itself continuously. If the cylinder stat is set to 65C and the boiler is set to 55C then, yes, it'll have no option but to continuously cycle. Going back to matching input to output if the cylinder cannot extract the heat at a rate that the boiler is producing then we'll get cycling - we've simply created the ultimate mismatch in this scenario.

If you cruised down a hill in neutral in a car, you'd use less fuel than driving down or on overrun!

Be careful with analogies as they all too often don't compare like with like, particularly when the important bits are the nuances which get conveniently left out! In this case I'd disagree anyway with your premise. You'd use less fuel on overrun because the injectors would be turned off for the duration hence zero fuel usage. When in neutral your engine would have to at least idle, to avoid stalling, hence you'd still be burning fuel to keep it ticking over.

Not trying to be flippant, but there must be a balance when time off v time on. No idea what it is, mind. But there must be one!

If you were to graph a chart of instantaneous energy consumption vs time for a boiler that cycles on/off/on/off/... and compared it with a continuously running modulating boiler you would see that the former would be stepped, perhaps saw-toothed, whereas the latter would be smooth. Going back to our definition of ultimate efficiency being where input equals output the smooth curve would necessarily win every time given the inherent inefficiencies of stopping and starting.

Edit to say: I'm making the times up but they're not far off what I get with my (oil) burner. ** When I say 'efficient' I mean uses less fuel.

Moving from the theory to practice others might be able to comment on what is acceptable for an oil burner, and what measures you can take to increase its efficiency.

Mathew

 

[MickyP10]

Thanks again Mathew, and I'm not playing devil's advocate, or owt. (run down the hill with the engine off will use no fuel at all, and a carbed engine on overrun would still use fuel anyway. Besides, you mentioned cars first!).

I added the edit that when I mentioned 'efficiency', I meant oil usage (note Tibbot). Clearly there are many, many things which make the boiler more efficient in technical ways - self-cleaning temps, output-input temps, etc, and I concede all the points you have made regarding 'Boiler' efficiency and the science of matching it to the number of radiators and calculating outputs. BUT if we are talking about amount of oil used per given heat output: - boiler reaches stat temp, turns off, pump continues to run for five mins, boiler fires up again, are you seriously saying that it would burn more oil with this scenario than being on continusously? I'm probably wrongly assuming that an oil-burning boiler's output is linear - it fires up, gives full flame until it reaches the stat temperature, then shuts down. Are you saying it's throws out more oomph when it starts up. If so, I can see your logic better.

Again, this isn't a wind up and thank you everyone for chipping in.

 

[MJN]

Thanks again Mathew, and I'm not playing devil's advocate, or owt.

Of course - it's good to discuss these things and I think it is good not to accept everything as read.

run down the hill with the engine off will use no fuel at all, and a carbed engine on overrun would still use fuel anyway.

This is where our analogy is breaking down. It is too simplistic a comparison. Perhaps it might be more realistic if we were to change it to: You have to travel from A to B (both at the same height) and can go via one of two routes: 1. A flat road all the way, or 2. A hilly route with continuous ups and downs. Both routes are the same distance. Which would likely use more fuel? Perhaps to make it even more realistic we could blindfold you such that you don't know when you're coming up to reaching the top of the hills - you only know when you're already over the top, still with your throttle open despite you now coming down the other side. Similarly at the bottom of the dips you're going to be slow to get back on the throttle and will lose some of the advantage of any additional momentum gained from the initial overshoot.

In general terms maintaining steady always requires less energy, per unit time, than getting there in the first place. (I hesitate to mention that in case we get into a debate about whether to turn the heating off at night or not! There are additional complexities there which cloud this point)

Besides, you mentioned cars first!

Fair enough! Let's drop the car bit then..

boiler reaches stat temp,

That's the thing - it doesn't do this. It will always overshoot as the feedback loop inevitably has hysteresis . This overshoot is wasteful and given it happens prior to every time the boiler turns off it is thereby inefficient and good to avoid.

Are you saying it's throws out more oomph when it starts up. If so, I can see your logic better.

It pretty much has to to ensure ignition. It can tick along nicely once up-and-running.

Inefficiency is not down to any one single factor - merely minor ones which add up to something approaching significance.

Mathew

 

[MickyP10]

Ta. Think I asked an unanswerable question and you patiently picked away with the science, which I thank you for.

I'm stuck witht the boiler and system I have, so, tbh, don't give a monkey's how 'efficient' it is. If I threw a couple of grand at a new boiler I'd never get the investment back before I moved, so my only consideration was oil usage; ie whether five mins, for example, of running hot water on the pump only was really that bad and less 'efficient' (in my terms!) than not. As I said, it was an inconsidered question, but I'm sensing you know where I was coming from.

So... should I leave it on all the time then, or just fire it up a couple of times a day when needed?