Boiler cycling, the ultimate evil or not?

[D_Hailsham]

Out of curiosity I trawled the Sedbuk database for the lowest output A rated wall mounted non-combi boiler; it was the Geminox 9.1kW. Went to their website and found that they make a 10kW boiler which modulates down to one kW, the THI 1-10C. There are also models which modulate from 2-17kW and 5-25kW as well as combis. The only problem is that they are large and weigh a lot - 63kg!! On the plus side (?) Geminox is part of the Bosch Group.

Now my question is: If they can make boilers with a 10:1 modulation ratio, why can't all the other manufacturers?

 

[D_Hailsham]

The wear in tear comes in when everything have to be started and stop 100 times a day, every day.

fans ,gas valve, APS, spark gens, relays... all opening and closing, starting and stopping.

Still negligible compared to the increased load on the engine bearings of a car when travelling at 100mph compared to 30mph.

In any case, apart from the fan which varies in speed, it's just on/off, open/close operations. Manufacturers will have tested these components so they know they will perform hundreds of thousand operations, if not millions, before there is a failure. So if the boiler manufacturer gives a five year guarantee, they are saying that they expect their components to perform more than 200,000 operations without failing.

 

[mehran]

'Hugely damaging' maybe over stating it... but have the boiler in a steady state could make the difference with a boiler lasting 10/15/20 years. the easer you make your boilers life the better its chances for the long haul.

I am sure it would be fine for 5 years but 20? some boiler makes(baxi) can even get their relays to last 2 and even in the car world there is a difference in motorway miles and city/town miles. I rather have a car that as been driven at a steady rate on the motorway for 100,000 mile then some town/city car that has had 100,000 mile of stop start driven...

anyway l agree about better modulation range.

 

[D_Hailsham]

On a "perfect" installation a modulating pump, linked to the burner, would ensure that the return was kept down and the burner level matched the demand with the modulating controls.

If a boiler is working to a set point fixed on the boiler, you will always get cycling, as the room temperatures rise, the trick of course is to let the flow temperature drop down as the room temperature rises... but not using a TRVS.

Are you saying that, on a "perfect system", there would be no need for TRVs on the rads as weather compensation would take care of everything. Have you forgotten about heat gain due to sunlight, people in a room, TV's, computers, cookers, refrigerators, washing machines etc?

Take my 20+ year old less than perfect system. The rads were probably about the right size when the house was built but, now it has been insulated, they are considerably oversized; I need 7kW but have 13kW of rads. OK, I can run the boiler at much lower temperatures, but the oversizing is not consistent. Most rads are oversized, by as much as 200%, others by as little 15%.

The only way, apart from changing all the rads, to allow for these variations is to have TRVs on all the rads.

 

[Agile]

If a boiler is running at maximum output the only difference, compared to minimum, is a faster running fan and a greater gas flow through the valve. The extra wear due to this is negligible.

I also think that most boilers are oversized, particularly now that Building Regulations specify a much greater level of insulation. I doubt if many new build houses need more than about 10kW for heating, which means that the boiler should be able to modulate down to 2-3kW.

If a boiler is running at or near minimum power then the heat exchanger is more oversized for the amount of heat and so mit can be more efficient from a gas point of view. The thermal stress is lower and that should extend the life. The wear of the fan bearings is lower.

Its the fault of many installers and some howm owners who insist on fitting over sized boilers. many installers dont agree with the 2 kW allowance in the calcs for water heating.

One lady who had bought a 32 kW heat only boiler for her small semi told me gleefully that she had bought the biggest boiler because she wanted the best. She then got a nupty east European installer! It never worked properly.

Tony

 

[ALEC1]

ow my question is: If they can make boilers with a 10:1 modulation ratio, why can't all the other manufacturers?

geminox and atag have downward firing burners avoiding "flame lift off"

they would have been tested for operation

this is a wild assumption, boilers are tested in germany holland and italy for the way they are designed to be used there, that is with modulating controls. I suspect WB test there junior series as this could not be sold in the rest or europe. I personally doubt that rigourous tests are run...ask any engineer who attends to break downs..we have to understand UK boiler manufacturers are actually import/export businesses with surprisingly little support from head office...

The only way, apart from changing all the rads, to allow for these variations is to have TRVs on all the rads.

well too a point.. what an efficient boiler needs is open passage to as many radiators as possible to ensure heat is dissipated. If it were a mild winters day the flow temperature is much lower. Experience suggest overheating as mentioned here isnt a problem, at least the boiler is not working to a high set point with a reduced flow causing all sorts of other problems...

 

[D_Hailsham]

they would have been tested for operation

this is a wild assumption, boilers are tested in germany holland and italy for the way they are designed to be used there, that is with modulating controls.

You misunderstood what I was saying. I was talking about component testing. Honeywell, Dungs etc etc will have tested their gas valves, diverters etc to destruction before they released them to the OEM market to incorporate in their boilers. The OEM assemblers wouldn't consider buying them unless they knew what the expected life was. There will be some which fail early, but that's inevitable when quality control is based on statistical criteria. It's the Friday Afternoon syndrome.

A true story, which I heard years ago.

A British company ordered some components from a Japanese supplier. The specification was very detailed and included the phrase "the batch must contain no more that 2% failures", meaning no more than 2% can be outside the tolerance set.

The consignment arrived from Japan and inside the container was a separate box. Inside that was a number of the component with a small note which said: "These are the 2% failures you requested".

The only way, apart from changing all the rads, to allow for these variations is to have TRVs on all the rads.

well too a point.. what an efficient boiler needs is open passage to as many radiators as possible to ensure heat is dissipated. If it were a mild winters day the flow temperature is much lower.

But you are assuming that all rads are equally oversized; that's not correct in my case. The smallest bedroom has a 1200W radiator, but only needs 300W - four times the size required; the dining room has a 1200W but needs 1000. You cannot reduce the flow temperature so all rads get the correct amount; some will still be dishing out more heat than is necessary. The only way to resolve this is either change the rad or install a TRV.

I agree about the open passage. In general, I can't see any point in having a zone valve on a heating circuit if you have any sort of compensation. OK if you have two heating zones you might need them and also if you have stored HW you would need a zone valve on the HW circuit. A number of manufacturers now suggest using a diverter (not mid position) valve the "wrong way round". So the normally open (B port) is used for the heating circuit and the normally closed (A port) is used for the HW side. When HW is called for the valve closes the CH side and opens the HW side and the boiler automatically ramps up to full output. Come to think of it, I can't see why this arrangement could not be used on a non-modulating boiler if you have a thermostat with TPI control, e.g Honeywell CM series.

Experience suggest overheating as mentioned here isnt a problem, at least the boiler is not working to a high set point with a reduced flow causing all sorts of other problems...

Are you talking about heat gains form TVs etc?

 

[ALEC1]

The smallest bedroom has a 1200W radiator, but only needs 300W

agreed no technology will compensate for an inadequately designed system!

TRV's may have a place on these systems, but in general care must be taken to ensure there is a big enough open path for an almost continuously running pump.
My experience really suggests that overheating is not really an issue with weather compensated or modulating controls...I suspect the reason is that radiators become less efficient at lower temperatures so a localised heat source of small proportions automatically compensates....

 

[D_Hailsham]

If a boiler is running at or near minimum power then the heat exchanger is more oversized for the amount of heat and so might can be more efficient from a gas point of view.

That's an interesting observation. The Energy Systems Research Unit at Strathclyde University recently published a report on research they had carried out for the Building Research Establishment on Domestic Heating Control Systems. It contains graphs showing how the efficiency of boilers increases as the return temperature decreases; and this is true of non-condensing as well as condensing boilers.

 

[ALEC1]

its the way condensing boilers were designed to operate many years ago in holland and germany but with the proviso that modulating load controls were used as the most cost effective way of doing this..

alos of course the components are tested by manufacturers, but not the whole boiler...

its obvious that the less operations a valve goes through over a given time period the longer it will last...

modulating controls achieve this, by keeping everything ticking over... the use of on-off controls causes alot of issues...that are unique to this country because we are not encouraged to install boilers as the designers intended!

 

[D_Hailsham]

agreed no technology will compensate for an inadequately designed system!

Hear! Hear!

I suspect that the rad sizes in my house (they are the originals) were based purely on the length of window above, not on the heat loss of the room.

TRV's may have a place on these systems, but in general care must be taken to ensure there is a big enough open path for an almost continuously running pump.

Continuous Circulation eh! I first came across this on a USA Forum - see This Article. It starts about halfway down. Search for "constant circulation". According to them, it's all the rage in Europe.

 

[ALEC1]

given that heat loss is variable but constant if you want to keep a space at a certain temperature or temperatures you need to carry a quantity of heat somehow

The ideal is a modulating pump which needs as the heat loss lowers so does the speed of the pump, forcing the boiler efficiency up!

contrary to popular opinion of our many organisiations pumps dont consume much energy, a 15/50 about 100 watts and a modern alpha pump around 30w.

Indeed I have seen a house with a 28kw heat loss and an alpha pump run on less than 25w...

 

[transam]

I have followed this thread with interest !! are some saying ?? or is it being ststed that the fitment of these control's will extend the life of a boiler to 15/ 20 year's , what is meant by this all component's , some ???
component's , ?????

 

[ALEC1]

clearly a boiler that is off will last for ever, so one that hardly works will last longer than one that is lurching between all and nothing which on-off controls make them do. Installing the right controls that make boilers work as little as possible seems to be the way to go...

several boiler manufacturers do talk quietly about a 15 year boiler life, but not on the cheapest models..and only on ones that come from parts of europe that have high regard for engineering....

Assuming the movable high stress parts are changed say every 5-7 years, I dont see any problem on the better boilers lasting 15 years...get out clauses for system water quality apply, etc, etc

 

[transam]

I can see the logic in what you are saying ! ultimately prety much anything mechanical can last for as long as one want's it to , they can be repaired , part's can be made ect ect , providing you can find someone to do it !! is it economic to do so ??? large boiler manu's make alot of money out of spare's , it's a good source of revenue , would they want there boiler's lasting 20 year's , unless of course we are talking about preventive maintenance e.g changing part's prior to any percieved failure ? Has any one got any expierence of domestic boiler's that are never turned off ??? I know of several 3 different make's , that have Never switched off for up to 5 to 7 year's & when I say never switched off , I mean 24/7 they turn off onboiler stat only , constantly short cycling , in some case's around a by-pass only , I even have record's some where, on any part's used , during this use or abuse ?