Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.

On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.

It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.

It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Dave wrote:
Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.
It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.


Maybe there are losses associated with the fact that the incoming water
for DHW is always cold, but the CH will normally have a hot value in the
return.

On 2006-09-25 10:32:04 +0100, "Dave" said:


Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.

It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Typically the burners are modulating so will back of the power anyway
as the CH circuit heats up.

There are larger combi boilers around - e.g.

http://www.mhg.de/ProCon_GWB_15_-.89.0.html?&L=2

The 45 model goes up to 47kW on heating and will provide a 20 lpm flow
of hot water at 40 degrees.

I have one of this company's previous models and it's an excellent
product. Not cheap, however.

On 25 Sep 2006 02:32:04 -0700 someone who may be "Dave"
wrote this:-

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:
On 25 Sep 2006 02:32:04 -0700 someone who may be "Dave"
wrote this:-

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.


True but I plan to use the combi section for a single shower room and
kitchen tap. The rest of the house will be served from a traditional
hot water cylinder heated by the CH part of the boiler.

On 2006-09-25 13:23:34 +0100, "Dave" said:

David Hansen wrote:
On 25 Sep 2006 02:32:04 -0700 someone who may be "Dave"
wrote this:-

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.


True but I plan to use the combi section for a single shower room and
kitchen tap. The rest of the house will be served from a traditional
hot water cylinder heated by the CH part of the boiler.

Then you are looking for something that is the opposite way to most
requirements.

All you can really do is to go for the CH requirement and be pleased
that you will get a respectabl combi solution into the bargain.

On 25 Sep 2006 05:23:34 -0700 someone who may be "Dave"
wrote this:-

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.

True but I plan to use the combi section for a single shower room and
kitchen tap.

I trust that the combination boiler will be close to both.

The rest of the house will be served from a traditional
hot water cylinder heated by the CH part of the boiler.

Then you are trading recovery time for input power. I doubt that 5kW
will make much difference, unless people are used to bathing in
relays. Perhaps what you need is a thermal store rather than a
cylinder.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Normally, it is better to fix the insulation than increase the boiler size.
Using reasonable insulation, 35kW is enough to heat a small hotel. No house
should require this, unless it is a listed property and fundamentally
uninsulatable.

Christian.

On Mon, 25 Sep 2006 10:39:33 +0100, The Natural Philosopher wrote:

Dave wrote:
Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.
It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.


Maybe there are losses associated with the fact that the incoming water
for DHW is always cold, but the CH will normally have a hot value in the
return.

There are a number of factors, one of which would be that circulating more
than 30kW of heating is starting to need 28mm pipes and larger than the
'normal' "6m" circulators.

Another is that by limiting the CH power the likelihood of short cycling
is reduced and/or the extent of it is softened.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

On Mon, 25 Sep 2006 05:23:34 -0700, Dave wrote:

David Hansen wrote:
On 25 Sep 2006 02:32:04 -0700 someone who may be "Dave"
wrote this:-

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.


True but I plan to use the combi section for a single shower room and
kitchen tap. The rest of the house will be served from a traditional
hot water cylinder heated by the CH part of the boiler.

Have you, formally, calculated/guestimated the heating needs of your
mansion yet?


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

Andy Hall wrote:
On 2006-09-25 10:32:04 +0100, "Dave" said:


Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.

It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Typically the burners are modulating so will back of the power anyway
as the CH circuit heats up.

There are larger combi boilers around - e.g.

http://www.mhg.de/ProCon_GWB_15_-.89.0.html?&L=2

The 45 model goes up to 47kW on heating and will provide a 20 lpm flow
of hot water at 40 degrees.

I have one of this company's previous models and it's an excellent
product. Not cheap, however.

Nice but slightly over budget!

David Hansen wrote:
On 25 Sep 2006 05:23:34 -0700 someone who may be "Dave"
wrote this:-

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.

True but I plan to use the combi section for a single shower room and
kitchen tap.

I trust that the combination boiler will be close to both.

Yes, very.


The rest of the house will be served from a traditional
hot water cylinder heated by the CH part of the boiler.

Then you are trading recovery time for input power. I doubt that 5kW
will make much difference, unless people are used to bathing in
relays. Perhaps what you need is a thermal store rather than a
cylinder.

The combi is just an extra to replace an electric shower. I thought I'd
add this because I a 35kW combi is almost as cheap as a 35kW heating
boiler. I might still do this if I think I can get away with 30kW of
heating.

The existing HW cylinder is adequate and HW use is low so I plan to
keep the existing cylinder rather than use a thermal store. If the
kitchen also uses the combi the HW cylinder will be left unused much of
the time. I appreciate that this is not a normal situation. The house I
plan to heat is big, old and very draughty.

Ed Sirett wrote:
On Mon, 25 Sep 2006 05:23:34 -0700, Dave wrote:

David Hansen wrote:
On 25 Sep 2006 02:32:04 -0700 someone who may be "Dave"
wrote this:-

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Sounds good to me. A house that is large enough to need that much
heating is likely to be too large for one combination boiler to
serve adequately.


True but I plan to use the combi section for a single shower room and
kitchen tap. The rest of the house will be served from a traditional
hot water cylinder heated by the CH part of the boiler.

Have you, formally, calculated/guestimated the heating needs of your
mansion yet?

Not exactly! Heat calculation is done based on the existing boilers.
Each floor has a 60kBTU Potterton Kingfisher. These run almost flat out
when it's cold and windy. I would say the house is adequately warm
rather than hot!

Christian McArdle wrote:
I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Normally, it is better to fix the insulation than increase the boiler size.
Using reasonable insulation, 35kW is enough to heat a small hotel. No house
should require this, unless it is a listed property and fundamentally
uninsulatable.

Christian.

The house is 300 years old and listed! The draughty windows are the
problem. They could be replaced but at huge cost.

Dave wrote:
Christian McArdle wrote:
I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.

Normally, it is better to fix the insulation than increase the boiler size.
Using reasonable insulation, 35kW is enough to heat a small hotel. No house
should require this, unless it is a listed property and fundamentally
uninsulatable.

Christian.

The house is 300 years old and listed! The draughty windows are the
problem. They could be replaced but at huge cost.

Sounds like you need some tight-fitting secondary glazing panels that
can be
fitted in the winter.
Simon.

Ed Sirett wrote:
On Mon, 25 Sep 2006 10:39:33 +0100, The Natural Philosopher wrote:

Dave wrote:
Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.
It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.


Maybe there are losses associated with the fact that the incoming water
for DHW is always cold, but the CH will normally have a hot value in the
return.

There are a number of factors, one of which would be that circulating more
than 30kW of heating is starting to need 28mm pipes and larger than the
'normal' "6m" circulators.

Sounds plausible and presents the opportunity for a DIY upgrade!


Another is that by limiting the CH power the likelihood of short cycling
is reduced and/or the extent of it is softened.

Ed Sirett wrote:
On Mon, 25 Sep 2006 10:39:33 +0100, The Natural Philosopher wrote:

Dave wrote:
Andy Hall wrote:
On 2006-09-25 10:07:35 +0100, "Dave" said:

Why do most (big) combi boilers have reduced power to the CH? For
example the Worcester Greenstar 35CDI is rated 35KW but the CH is
limited to 30KW.
It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would
be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as
possible and even 35kW doesn't give exemplary flow of hot water in the
winter.

This makes sense for small boilers but if the burner and pump can
deliver 35KW to the DHW then I can't see any reason to limit the power
to the CH. The heating would heat up quickly then the thermostat(s)
would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and
it looks like I'm stuck with a non combi version.


Maybe there are losses associated with the fact that the incoming water
for DHW is always cold, but the CH will normally have a hot value in the
return.

There are a number of factors, one of which would be that circulating more
than 30kW of heating is starting to need 28mm pipes and larger than the
'normal' "6m" circulators.

Sounds plausible and presents the opportunity for a DIY upgrade!


Another is that by limiting the CH power the likelihood of short cycling
is reduced and/or the extent of it is softened.

On 26 Sep 2006 02:16:10 -0700 someone who may be "Dave"
wrote this:-

The house I
plan to heat is big, old and very draughty.

Far better to insulate it as much as possible than continue trying
to heat the garden. How are the draughts caused and what have you
done to reduce them. Loft insulation? Presumably cavity wall
insulation is not possible.

When you do a heat loss calculation what are the steady state
losses?


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54