Viessmann boiler

[bengasman]

... but the radiator temperatures will be lower - flow 72°C return 52°C.

But say you only need a 20kW boiler and have 38kW rads. Then the temperatures would be 61°C and 42°C.

That looks good in the theory book, but have you actually ever achieved this in real life?

This of course assumes that it is:
A, a conventional boiler,

And there have been several topics about OPs with problems when the heating output of a combi boiler has not been range rated.

None of which where solely, or even predominantly due to not being range rated, but caused by poor installation/maintenance.

B, the smaller boiler will have a lower minimum rate than the bigger model,
C, that the house will actually need less than the minimum rate of the boiler,

The second (C) is easy to determine as the heat required is proportional to the outside temperature. If you need 22kw to raise the temperature from -1°C to 21°C then you will need 11kw to raise the temp from 10°C to 21°C and 5kW to raise the temp from 16°C to 21°C.
Item B can be resolved by checking the manufacturers specifications and selecting the most appropriate boiler - though installer's preferences may distort this.

That looks good in the theorybooks, but have you ever achieved these figures in real life?

D, it will never get colder than the design temperature outside,

Then you discuss this with the client and get him to agree what temperatures to use.

The client will have no clue about what that all actually means, nor what is realistic; it is the designer's job to do that for him.

E, the occupant is not, and never will be, Jamaican and likes the house warmer than the standard design temperature,

You install a system to meet the current residents requirements, not some hypothetical future requirement.

That sounds fine on paper, but in real life, it is not that easy as very few people are clairvoyant. Discussing future needs is one of the first things I do, and fewer than 1 in 5 is sure there will not be any extensions or alterations.

F, at not time in the future will there be an extension built, a loft converted, or any other need for more output,

Another case where you discuss this with the client before making your recommendation.

And another case where 4 out of 5 are not sure.

G, that it is actually a demonstrable disadvantage to have the boiler cycling during some periods, rather than a theoretical issue,

Loss of unburnt gas prior to ignition every time the boiler comes on?

There might be a minute loss in theory.
Even at a cycling speed of only 5 minutes, you would be looking at something like 0.00001 cubic metre per day, IF the boiler had such a poor performance that gas would be out of the flue before ignition; less than one single pound over the lifespan of the boiler.

H, the owner believes that the demonstrable comfort of over capacity does not outweigh the theoretical downside of cycling.

This reminds me of the early days of central heating when the sales pitch was "guaranteed temperatures". This was achieved by oversizing everything. Fine when gas was cheap and the problem of global warming had not appeared over the horizon.

It seems you have reached the heart of the problem: your ideas are based on common believes held some 30 years ago.

Oversized rads actually make modern boilers more efficient.

Contrary to 30 years ago, boilers modulate, reducing the cycle rate by a factor 10, if not more.

The issues using more gas and global warming are totally irrelevant for a number of reasons:

A slightly oversized modulating premix boiler does not use noticeably more gas than one that is "exactly" sized.

There is no evidence whatsoever that global warming is still happening; a sharp dip started in the second halve of the nineties.
Even if global warming were still happening, there are only vague theories that it is anthropogenic, and 3 very strong indications that it is not.

Even if there was a measurable increase in gasuse of an oversized boiler, it if absolutely negligible in terms of total national/global use.

 

[D_Hailsham]

... but the radiator temperatures will be lower - flow 72°C return 52°C.

But say you only need a 20kW boiler and have 38kW rads. Then the temperatures would be 61°C and 42°C.

That looks good in the theory book, but have you actually ever achieved this in real life?

The flow temp of my rads never get above 65°C. That's because they were installed when the house had no insulation or double glazing, which it now has. The rads and the boiler are now oversized for the heat loss.

This of course assumes that it is:
A, a conventional boiler,

And there have been several topics about OPs with problems when the heating output of a combi boiler has not been range rated.

None of which where solely, or even predominantly due to not being range rated, but caused by poor installation/maintenance.

I think you will find that most of them were due to not being range rated. The OPs had 30kW combi boilers, fine for HW, installed in houses which only needed 15kW for heating. The boilers were cycling like fury.

D, it will never get colder than the design temperature outside,

Then you discuss this with the client and get him to agree what temperatures to use.

The client will have no clue about what that all actually means, nor what is realistic; it is the designer's job to do that for him.

The installer should still explain to the installer the reason for his recommendation. If the client says: "Whatever you recommend is fine with me", then you are home and dry. But what happens when you have a client who does understand what you are talking about and starts asking in depth pertinent questions about your recommendations? It's always safer to err on the side of caution and not assume too much about your client.

E, the occupant is not, and never will be, Jamaican and likes the house warmer than the standard design temperature,

You install a system to meet the current residents requirements, not some hypothetical future requirement.

That sounds fine on paper, but in real life, it is not that easy as very few people are clairvoyant. Discussing future needs is one of the first things I do, and fewer than 1 in 5 is sure there will not be any extensions or alterations.

Sorry, I mis-read what you wrote. I thought you were talking about a future owner being Jamaican and want the house hot. If the client wants his rooms at 25°C, then it's up to the installer to design a system which meets his requirements.

F, at not time in the future will there be an extension built, a loft converted, or any other need for more output,

Another case where you discuss this with the client before making your recommendation.

And another case where 4 out of 5 are not sure.

Then you recommend for the situation as it is; not as it might be in ten years time. (The boiler will probably have failed by then!)

G, that it is actually a demonstrable disadvantage to have the boiler cycling during some periods, rather than a theoretical issue,

Loss of unburnt gas prior to ignition every time the boiler comes on?

There might be a minute loss in theory.
Even at a cycling speed of only 5 minutes, you would be looking at something like 0.00001 cubic metre per day, IF the boiler had such a poor performance that gas would be out of the flue before ignition; less than one single pound over the lifespan of the boiler.

I agree it is only small, but it's a real loss. There can of course be cycling at both high and low output, particularly if the boiler is oversized.

H, the owner believes that the demonstrable comfort of over capacity does not outweigh the theoretical downside of cycling.

This reminds me of the early days of central heating when the sales pitch was "guaranteed temperatures". This was achieved by oversizing everything. Fine when gas was cheap and the problem of global warming had not appeared over the horizon.

It seems you have reached the heart of the problem: your ideas are based on common beliefs held some 30 years ago.

Not sure what you are saying. Are you suggesting that companies, mainly the Gas Board in those days did not deliberately oversize boilers and radiators to ensure that the "guaranteed temperature" was always achieved?

Oversized rads actually make modern boilers more efficient.

Agreed. Though I have never said otherwise.

A slightly oversized modulating premix boiler does not use noticeably more gas than one that is "exactly" sized.

"Slightly", yes. But when a boiler is 50% or 75% oversized it is relevant.

There is no evidence whatsoever that global warming is still happening; a sharp dip started in the second halve of the nineties.
Even if global warming were still happening, there are only vague theories that it is anthropogenic, and 3 very strong indications that it is not.

That's another debate.

 

[bengasman]

... but the radiator temperatures will be lower - flow 72°C return 52°C.

But say you only need a 20kW boiler and have 38kW rads. Then the temperatures would be 61°C and 42°C.

That looks good in the theory book, but have you actually ever achieved this in real life?

The flow temp of my rads never get above 65°C. That's because they were installed when the house had no insulation or double glazing, which it now has. The rads and the boiler are now oversized for the heat loss.

But did you ever actually achieve the results of your theory above?

I think you will find that most of them were due to not being range rated.

You think that, but I know from experience that this is not the case. I have not come across a single boiler that broke down, where cycling was even a possibility this year or last year. How many have you seen over the last couple of years?

F, at not time in the future will there be an extension built, a loft converted, or any other need for more output,

Another case where you discuss this with the client before making your recommendation.

And another case where 4 out of 5 are not sure.

Then you recommend for the situation as it is; not as it might be in ten years time. (The boiler will probably have failed by then!)

I doubt very much that boilers I install will perish in under 10 years, I don't do budget work. Recommending a boiler that is only just big enough today, for the sake of less cycling and totally ignoring the fact that in the next couple of years that boiler may have to be replaced by a larger one, is commercial suicide.

G, that it is actually a demonstrable disadvantage to have the boiler cycling during some periods, rather than a theoretical issue,

Loss of unburnt gas prior to ignition every time the boiler comes on?

There might be a minute loss in theory.
Even at a cycling speed of only 5 minutes, you would be looking at something like 0.00001 cubic metre per day, IF the boiler had such a poor performance that gas would be out of the flue before ignition; less than one single pound over the lifespan of the boiler.

I agree it is only small, but it's a real loss.

You are not seriously considering an amount of less than 0.1% of the gasbill to be of any consequence? And that is only IF the gas is unburnt which I doubt.