Finding PROPER heating engineers in our area (Hants/Surrey)

[gas4you]

I think this topic is getting way too deep into detail that in the real world you would never notice.

Very few combi's condense when on for HW. If you go for a HW cylinder go for a fast recovery version, this will be smaller but as it heats the water up quicker is more efficient. The principle is you use less energy in storing less volume of hot water, then when you use the HW the cylinder re-heats a lot faster, so therefore a smaller cylinder supplies the same amount of HW as a standard larger one. (except when the boiler breaks down and you have to use the immersion)

 

[Agile]

I read on here comments suggesting that many combis don't condense on the water heating circuit, is that true, and if so, I assume that does affect their efficency somewhat?

Tony mentioned that the average cylinder loses 2.4kw per day, but if the boiler ran more efficently and so needed to output 2kw less energy per day to heat that water compared to a combi, then overall the losses are tiny. .

You have got that wrong! The boiler has to run a little every day to make up the losses from a cylinder. My figures for the cost of that were higher than the reality because I used 12p/unit for electricity whereas the energy would be provided by a gas boiler at perhaps 5p/unit for gas plus 2p for electricity.

A combi condenses as long as its providing water at point of use temperatures of say 44°C. That will be more efficient than some boilers heating a cylinder but all depending on the settings.

There is little difference between the efficiency of a combi and a cylinder in cost terms. 80% - 90% of the cost is space heating anyway. The difference is in the flow rates available.

Tony

 

[Locoblade]

You're probably right Dave, just having an engineering mind myself I always like to try and work these things out for myself and satisfy my curiosity.

You can see where Im coming from though I hope, Im not necessarily looking for outright figures to say that a combi is 0.x% more efficient than an unvented cylinder because no figures exist and it will be dependent on too many variables, its more to understand what has/hasnt been taken into consideration when reaching a conclusion that A is better than B.

 

[JohnD]

there is also bound to be a bit of heat loss from the primary circulation pipes to the cylinder. these are very often left uninsulated and can be short or long. I have recently put 20m of BS Climaflex on the primaries at my mother's house (gravity system) and this has made a substantial saving

 

[Locoblade]

You have got that wrong! The boiler has to run a little every day to make up the losses from a cylinder.

Tony

Im not sure you fully understood my point or perhaps I haven't understood yours . From what I understood, the figures you gave are the overheads for each system, where a system boiler runs to replenish that 2.4Kw lost, so thats the daily overhead, but this doesn't include anyone actually using water in the household. I accept that overhead but what I was proposing is perhaps overall, when you take hot water consumption into account (therefore more burn time on both boilers), the loss may not stay at 2.4kw overhead.

Say a house with a cylinder runs a couple of baths and drain the 150L tank, the system boiler then fires up and replenishes the cylinder. Lets say it needs 10Kw of energy (pure guess) to heat up that cylinder again. Add in the 2.4Kw per day loss and on this day its cost you 12.4Kw to supply the house with hot water.

The combi house runs baths and use that same 150L, as it's not condensing and therefore running less efficently, the boiler would surely use more energy (ie above 10Kw) to supply the same amount of water? If thats the case, the combi household may also need ~12Kw to heat it's daily requirement of water, therefore no net benefit?

 

[Agile]

A combi condenses as long as its providing water at point of use temperatures of say 44°C. That will be more efficient than some boilers heating a cylinder but all depending on the settings.

There is little difference between the efficiency of a combi and a cylinder in cost terms. 80% - 90% of the cost is space heating anyway. The difference is in the flow rates available.

Tony

The combi house runs baths and use that same 150L, as it's not condensing and therefore running less efficently, the boiler would surely use more energy (ie above 10Kw) to supply the same amount of water? If thats the case, the combi household may also need ~12Kw to heat it's daily requirement of water, therefore no net benefit?

You seem to be fixated into thinking that a combi is less efficient. The reality is that properly used a combi on hot water will be MORE efficient.

Tony

 

[Locoblade]

Hi Tony

Probably me misunderstanding then, I thought boilers were at their most efficient when condensing, and as mentioned above by gas4you, most combis don't condense on the HW circuit? In contrast, when heating a water cylinder, the system/heat only boiler would be condensing and therefore I assumed it would be running (slightly) more efficiently?

If a system boiler is less efficient at doing this though, what magic does a combi utilise to make it more efficient at heating water up?

 

[Agile]

Hi Tony

Probably me misunderstanding then, I thought boilers were at their most efficient when condensing, and as mentioned above by gas4you, most combis don't condense on the HW circuit? ?

Oh dear!

I am telling you ( for the third time! ) that a condensing boiler in DHW mode supplying water at the point of use temperature of 43°C, or lower, will be in mostly condensing mode and operating very efficiently.

More efficiently than most boilers heating a cylinder where the flow temperature is set to about 70-75°C.

I am not advocating use of a combi rather than a cylinder, only correcting you on your misunderstanding of boiler operation.

Tony

 

[tonybhoy]

Hi Tony

Probably me misunderstanding then, I thought boilers were at their most efficient when condensing, and as mentioned above by gas4you, most combis don't condense on the HW circuit? In contrast, when heating a water cylinder, the system/heat only boiler would be condensing and therefore I assumed it would be running (slightly) more efficiently?

It won't be condensing

WB greenstar combis boilers condense on hot water,one of the few combis that do

Anyway you really are nit picking here. The cost savings on hot water vis both systems will be very little but I'd stake my mortgage on the combi being cheaper to run on hot water.

Obviously you want stored hot water but you are obviously having difficulty deciding because you know a combi is the proper solution for your requirements.

Do you really want to spend a grand more than you have to?

Only you can decide.


Cheers.

Tony.

 

[Locoblade]

Agile, if that is the case then I completely understand why it might be as efficient if not more so, but it seems to contradict what was said by gas4you and what Ive read elsewhere hence why I was questioning it. Are the others wrong?

 

[doitall]

I would sooner spend a grand more and get a far superior and more reliable system.

=Tony More efficiently than most boilers heating a cylinder where the flow temperature is set to about 70-75°C.

That's a lot less than a combi on full blast for 43c, in addition you forgot to mention the heat only boiler modulate right back to a simmer once it's up to temperature.

 

[Deleted member 105431]

locoblade why don't you write a 10 000 word thesis on the subject? You are nit picking the most minor of details. By the time you decide (if you ever do) fossil fuels will have been exhausted and we will be using hydrogen powered heat systems.

 

[Locoblade]

Hi Tony

Probably me misunderstanding then, I thought boilers were at their most efficient when condensing, and as mentioned above by gas4you, most combis don't condense on the HW circuit? In contrast, when heating a water cylinder, the system/heat only boiler would be condensing and therefore I assumed it would be running (slightly) more efficiently?

It won't be condensing

WB greenstar combis boilers condense on hot water,one of the few combis that do

Anyway you really are nit picking here. The cost savings on hot water vis both systems will be very little but I'd stake my mortgage on the combi being cheaper to run on hot water.

Obviously you want stored hot water but you are obviously having difficulty deciding because you know a combi is the proper solution for your requirements.

Do you really want to spend a grand more than you have to?

Only you can decide.


Cheers.

Tony.

Hi Tony

I agree the differences are likely to be the thin end of bugger-all difference and maybe perceived as nit-picking. The discussion originated from the claim that hot water cylinders were significantly more expensive to run than a combi though, and I wanted to find out how those conclusions were reached as I couldnt find any definitive info elsewhere on the net on the comparable efficiencies of the various solutions. The fact that it's been debated as much as it has and there's still some unknowns and assumptions in there suggests to me that its not such a clear-cut thing as many make out, which in itself answers the question I guess.

Im not trying to convince myself of anything though, if it was up to me I'd actually most likely get a good combi, but the missus really doesn't want one and i know if I persuaded her and then the same old combi downfalls cropped up, Id never hear the end of it If the alternatives genuinely are significantly more expensive to run then that might be a good bargaining tool, but if not then there's only really the upfront cost difference to weigh up as the downside of a cylinder install.

Cheers chaps

 

[gas4you]

Apart from WB I do not know of another manufacturer that states their boilers condense on dhw. Certainly none of the manufacturers that I have attended for training.

 

[doitall]

Apart from WB I do not know of another manufacturer that states their boilers condense on dhw. Certainly none of the manufacturers that I have attended for training.

Atmos do