Over the last winter our (rather old) boiler has been trouble keeping
the house warm. The boiler itself is running continuously but the
radiators do no better than get warm.

We've been living in this house for about 6 years, and not noticed this
as a problem before - but it is possible that in previous years the TRVs
on the rads in the bedrooms may have been turned down lower.

Can anyone think of any likely reason why the boiler should be producing
less than 100% output?

The boiler is an old Potterton Netaheat 16-22 - not the most
sophisticated unit. I am intending to replace it shortly and will use
it as a guide to the correct size of the replacement (which will be
bigger) so need to check that the problem is the boiler being
underspecified for the house rather than just not putting out its rated
output.

Any suggestions gratefully received...

On Sun, 06 Mar 2005 21:17:44 GMT, Bedouin
wrote:

Over the last winter our (rather old) boiler has been trouble keeping
the house warm. The boiler itself is running continuously but the
radiators do no better than get warm.

We've been living in this house for about 6 years, and not noticed this
as a problem before - but it is possible that in previous years the TRVs
on the rads in the bedrooms may have been turned down lower.

Can anyone think of any likely reason why the boiler should be producing
less than 100% output?

A reasonable possibility is that the heat exchanger is part filled
with sludge. clues here would be noises from it, or if it is
cycling on and off before the radiators are getting hot.

It is possible that some or all of the TRVs are past their sell-by
date and contributing to poor system balance.

Either way, if you are changing the boiler, it will make sense to give
the system a very thorough cleaning and also perhaps replace TRVs if
they are more than about 7 years old.

It may not be worth spending anything on desludging the boiler,
though.




The boiler is an old Potterton Netaheat 16-22 - not the most
sophisticated unit. I am intending to replace it shortly and will use
it as a guide to the correct size of the replacement (which will be
bigger) so need to check that the problem is the boiler being
underspecified for the house rather than just not putting out its rated
output.

Any suggestions gratefully received...

If you want to be rigorous about this, you'd need to do heat loss
calculations. However, if the house has been comfortable with 22kW
before, going for a 28kW boiler should give a comfortable margin.







--

..andy

To email, substitute .nospam with .gl

"Andy Hall" wrote in message
...
On Sun, 06 Mar 2005 21:17:44 GMT, Bedouin
wrote:

Over the last winter our (rather old) boiler has been trouble keeping
the house warm. The boiler itself is running continuously but the
radiators do no better than get warm.

We've been living in this house for about 6 years, and not noticed this
as a problem before - but it is possible that in previous years the TRVs
on the rads in the bedrooms may have been turned down lower.

Can anyone think of any likely reason why the boiler should be producing
less than 100% output?

A reasonable possibility is that the heat exchanger is part filled
with sludge. clues here would be noises from it, or if it is
cycling on and off before the radiators are getting hot.

It is possible that some or all of the TRVs are past their sell-by
date and contributing to poor system balance.

Either way, if you are changing the boiler, it will make sense to give
the system a very thorough cleaning and also perhaps replace TRVs if
they are more than about 7 years old.

It may not be worth spending anything on desludging the boiler,
though.




The boiler is an old Potterton Netaheat 16-22 - not the most
sophisticated unit. I am intending to replace it shortly and will use
it as a guide to the correct size of the replacement (which will be
bigger) so need to check that the problem is the boiler being
underspecified for the house rather than just not putting out its rated
output.

Any suggestions gratefully received...

If you want to be rigorous about this, you'd need to do heat loss
calculations. However, if the house has been comfortable with 22kW
before, going for a 28kW boiler should give a comfortable margin.

The pump could be running slow.
Not on full.


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In article ,
Doctor Evil wrote:
The pump could be running slow.
Not on full.

Please explain how this could be the fault if the boiler is running
continuously, as stated?

--
*Arkansas State Motto: Don't Ask, Don't Tell, Don't Laugh.

Dave Plowman London SW
To e-mail, change noise into sound.

I am intending to replace it shortly and will use it as a guide to the
correct size of the replacement (which will be bigger)

Modern boilers are modulating. In the past, you guessed the heat output and
bought a boiler that matched (i.e. 12kW or 14kW or 8kW etc). Now you just
buy a 28kW model (which is halfway to heating a palace) and it just turns
itself down when less is needed. It means that careful consideration of
boiler sizing is no longer required. The only issue is that instantaneous
combis need as much power as you can find for reasonable performance when
heating water. For heating, any 24kW or 28kW system boiler will heat almost
any house. If it doesn't, then you need more insulation, not a bigger
boiler.

For smaller houses, the ability to modulate really low is really useful to
avoid cycling. Indeed, it is useful on larger houses, too, but not quite so
important. Some only go down to 8kW, whilst others go as low as 4kW or
better.

Christian.

Christian McArdle wrote:
I am intending to replace it shortly and will use it as a guide to the
correct size of the replacement (which will be bigger)


Modern boilers are modulating. In the past, you guessed the heat output and
bought a boiler that matched (i.e. 12kW or 14kW or 8kW etc). Now you just
buy a 28kW model (which is halfway to heating a palace) and it just turns
itself down when less is needed. It means that careful consideration of
boiler sizing is no longer required. The only issue is that instantaneous
combis need as much power as you can find for reasonable performance when
heating water. For heating, any 24kW or 28kW system boiler will heat almost
any house. If it doesn't, then you need more insulation, not a bigger
boiler.

This is certainly a comparatively expensive house to heat - every room
has 2 outside walls and the rooms we use most all have 3! Yes in theory
more insulation would be a good idea but that is not easy (solid walls
and all that); and I also have what is probably the best ventilated
under-floor void you've ever seen.

The problem with an over-specified boiler is cost and size so I'd rather
get the size right.

Going up to a 28-30kW model will give me modest increase over my current
22kW - but will that be enough?

On Mon, 07 Mar 2005 19:57:26 GMT, Bedouin
wrote:

Christian McArdle wrote:
I am intending to replace it shortly and will use it as a guide to the
correct size of the replacement (which will be bigger)


Modern boilers are modulating. In the past, you guessed the heat output and
bought a boiler that matched (i.e. 12kW or 14kW or 8kW etc). Now you just
buy a 28kW model (which is halfway to heating a palace) and it just turns
itself down when less is needed. It means that careful consideration of
boiler sizing is no longer required. The only issue is that instantaneous
combis need as much power as you can find for reasonable performance when
heating water. For heating, any 24kW or 28kW system boiler will heat almost
any house. If it doesn't, then you need more insulation, not a bigger
boiler.

This is certainly a comparatively expensive house to heat - every room
has 2 outside walls and the rooms we use most all have 3! Yes in theory
more insulation would be a good idea but that is not easy (solid walls
and all that); and I also have what is probably the best ventilated
under-floor void you've ever seen.

The problem with an over-specified boiler is cost and size so I'd rather
get the size right.

Going up to a 28-30kW model will give me modest increase over my current
22kW - but will that be enough?



You'd have to do the sums to know.

There are two major elements.

- Heatloss through surfaces. You can get a reasonable number for
that if you are methodical about materials used etc.

- Heatloss through air changes. This assumes that you either use
standard numbers suggested by the Buikding Research Establishment or
make a reasoned estimate. You can't really know if your dining room
has 1.5 or 2.5 air changes an hour.


You can arrive at reasonable numbers for radiator sizing this way.
i.e. you can out in enough capacity to be sure of good results. it's
hard to do that fo the whole house and sizing a boiler though.

If the house was OK consistently on 22kW (meaning burner on the
Netaheat at max) then you whould be comfortable at 28kW and have a lot
of margin at 35kW.

A house needing 22kW suggests to me that it's either large or has poor
insulation because it's older and you can't put insulation in enough
places to make a big difference. If that's the situation, then it's
more important to look at heat loss because you will have little to
address the variations during a day. The temperature will change
quite quickly with the outside temperature. If you have a situation
where the insulation is better, there will be less intra-day
temperature change and so you don't have as much sensitivity to sudden
weather changes.

There's no magic way, unfortunately. If you are concerned, and the
house is not well insulated, then I would go for higher power. This
will cost more, but not as much as getting it wrong and having to
replace the boiler with a larger one......






--

..andy

To email, substitute .nospam with .gl

Andy Hall wrote:
On Mon, 07 Mar 2005 19:57:26 GMT, Bedouin
wrote:


Christian McArdle wrote:

A house needing 22kW suggests to me that it's either large or has poor
insulation because it's older and you can't put insulation in enough
places to make a big difference. If that's the situation, then it's
more important to look at heat loss because you will have little to
address the variations during a day. The temperature will change
quite quickly with the outside temperature. If you have a situation
where the insulation is better, there will be less intra-day
temperature change and so you don't have as much sensitivity to sudden
weather changes.

That's certainly true - it has comparatively large surface area - being
L shaped, and as a solid walled 1920 construction there is no easy way
to improve the insulation further without major work. The low power of
the boiler, and the comparatively high thermal mass of the house, means
that it is very slow to warm up in cold weather - hence the desire for
considerable more power.

There's no magic way, unfortunately. If you are concerned, and the
house is not well insulated, then I would go for higher power. This
will cost more, but not as much as getting it wrong and having to
replace the boiler with a larger one......

That's my feeling too - but the range of boilers in the 35-40kW size is
obviously lower than in the more common 28kW area.