This prompted by a comment by a builder coming to discuss our proposed
extension.
We said combi, he said that people are tending to go for Megflows now.
So is there any good reason to?

Rough very shortened recap of parts of the Wiki.

Q1. Do you need more hot+cold water flow than the maximum cold mains flow?
[e.g. two or more bathrooms and teenage daughters]

Yes - stored cold water in a tank, and pumped hot+cold to the baths and
showers.

No -

Q2. Do you want more continuous hot water than can be heated by your maximum
gas flow?

Yes - stored hot water, e.g. in Megaflow or see (1).

No -

Q3. *Well, here's the interesting part. If you got this far any of the
systems in (1) and (2) can meet your needs but so can a combi.*

So what is Q3 which decides between a Megaflow and a Combi?

Factors:

Combi uses less space, as there is no stored hot water, but has to be sized
for maximum immediate demand.
Conflict betweeen hot water demand and central heating demand.
Combis more complex (everything in one box) so probably lower MTBF.
Alleged shorter lifetime.
But they do free off the airing cupboard which is potentially a big plus.

Megaflow can be heated by a smaller boiler (possibly more efficiently?) and
once the stored water is up to temperature there should be no conflict
between CH and hot water.
Pressurised systems have higher maintenance - need testing and checking
every year.
Can run at the maximum mains flow if required by future changing demand.
Can be heated by backup power source e.g. immersion heater if there is a
problem with the boiler.
They do require more plumbing and take up significant storage space in
smaller properties.

So, leaving aside the space issue, it comes down to cost.

Which is cheaper to install.
Which is cheaper to run.
Which is cheaper to maintain over a 10 year period.

Answers on a postcard......


Oh, and I have deliberately ignored heat stores to try and limit the
options.
Also ignored additional heat sources linked to renewables such as PV and
back boilers on stoves.
Also ignored mixed solutions like having the combi heat a hot water store
for the showers but provide DHW to taps.

Cheers

Dave R

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")

On 06/04/2012 10:57, David WE Roberts wrote:

This prompted by a comment by a builder coming to discuss our proposed
extension.
We said combi, he said that people are tending to go for Megflows now.
So is there any good reason to?

Yup, there can be...

Rough very shortened recap of parts of the Wiki.

Q1. Do you need more hot+cold water flow than the maximum cold mains flow?
[e.g. two or more bathrooms and teenage daughters]

Yes - stored cold water in a tank, and pumped hot+cold to the baths and
showers.

No -

Q2. Do you want more continuous hot water than can be heated by your
maximum gas flow?

Yes - stored hot water, e.g. in Megaflow or see (1).

No -

Q3. *Well, here's the interesting part. If you got this far any of the
systems in (1) and (2) can meet your needs but so can a combi.*

So what is Q3 which decides between a Megaflow and a Combi?

Basically two things... flow rate, and "others". Where "others" are the
specific advantages/features that you list later like having immersion
backup etc.

Flow rate is the main driver for most I would guess. For either you need
a decent cold water supply obviously. Ignoring the very exotic and
elaborate combi solutions, its usually easier to get a decent flow rate
of "hotter than required, and hence can be mixed water" from a store of
water than from a combi, which will deliver its maximum rate at pretty
much final use temperature.

Factors:

Combi uses less space, as there is no stored hot water, but has to be
sized for maximum immediate demand.

Indeed, and there is the problem for many. Even using the full capacity
of your gas main may not meet your immediate demand.

Conflict betweeen hot water demand and central heating demand.

Yup, although it can be a problem with a store if its not large enough
to meet immediate needs without re-heating.

Combis more complex (everything in one box) so probably lower MTBF.

Compared with a system boiler, then not really. For that matter,
compared with a complete system of valves etc, you need the same basic
components somewhere - so the only difference is the location of the bit
that goes wrong!

Alleged shorter lifetime.

Not sure there is much in that. Buy a decent boiler, and the life is
likely to be similar in either case. However, getting the 30 years plus
the old cast iron lumps used to manage does seem harder.

But they do free off the airing cupboard which is potentially a big plus.

And leave it unheated, which may be a minus.

Megaflow can be heated by a smaller boiler (possibly more efficiently?)

With modern boilers running at 90%+ efficiency, low water content, and
pump overrun etc there is not going to be much in it, unless you have
very low hot water demands. Also note that hot water generation will
cost only a small fraction of your total heating bill.

and once the stored water is up to temperature there should be no
conflict between CH and hot water.

Yup.

Pressurised systems have higher maintenance - need testing and checking
every year.

Although I can't see any reason you could not do that yourself...

Alternatively there are heatbank solutions that will provide the same
benefits, but without the requirement for routing checking.

Can run at the maximum mains flow if required by future changing demand.
Can be heated by backup power source e.g. immersion heater if there is a
problem with the boiler.
They do require more plumbing and take up significant storage space in
smaller properties.

So, leaving aside the space issue, it comes down to cost.

Which is cheaper to install.

Combi, by a large margin.

Which is cheaper to run.

Not enough in it to matter.

Which is cheaper to maintain over a 10 year period.

Probably not much in it.

Answers on a postcard......


Oh, and I have deliberately ignored heat stores to try and limit the
options.

Also ignored additional heat sources linked to renewables such as PV and

PV etc seems to only make significant contribution to your hot water
heating, and hence will not offset enough value to ever be economic in
most cases.

back boilers on stoves.

That may make more sense if you have access to cheap fuel.

Also ignored mixed solutions like having the combi heat a hot water
store for the showers but provide DHW to taps.

Probably only worth doing if the layout of the house is such that it
solves particular problems.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"John Rumm" wrote in message
o.uk...
On 06/04/2012 10:57, David WE Roberts wrote:

This prompted by a comment by a builder coming to discuss our proposed
extension.
We said combi, he said that people are tending to go for Megflows now.
So is there any good reason to?

Yup, there can be...

Rough very shortened recap of parts of the Wiki.

Q1. Do you need more hot+cold water flow than the maximum cold mains
flow?
[e.g. two or more bathrooms and teenage daughters]

Yes - stored cold water in a tank, and pumped hot+cold to the baths and
showers.

No -

Q2. Do you want more continuous hot water than can be heated by your
maximum gas flow?

Yes - stored hot water, e.g. in Megaflow or see (1).

No -

Q3. *Well, here's the interesting part. If you got this far any of the
systems in (1) and (2) can meet your needs but so can a combi.*

So what is Q3 which decides between a Megaflow and a Combi?

Basically two things... flow rate, and "others". Where "others" are the
specific advantages/features that you list later like having immersion
backup etc.

Flow rate is the main driver for most I would guess. For either you need a
decent cold water supply obviously. Ignoring the very exotic and elaborate
combi solutions, its usually easier to get a decent flow rate of "hotter
than required, and hence can be mixed water" from a store of water than
from a combi, which will deliver its maximum rate at pretty much final use
temperature.

Factors:

Combi uses less space, as there is no stored hot water, but has to be
sized for maximum immediate demand.

Indeed, and there is the problem for many. Even using the full capacity of
your gas main may not meet your immediate demand.

Conflict betweeen hot water demand and central heating demand.

Yup, although it can be a problem with a store if its not large enough to
meet immediate needs without re-heating.

Combis more complex (everything in one box) so probably lower MTBF.

Compared with a system boiler, then not really. For that matter, compared
with a complete system of valves etc, you need the same basic components
somewhere - so the only difference is the location of the bit that goes
wrong!

Alleged shorter lifetime.

Not sure there is much in that. Buy a decent boiler, and the life is
likely to be similar in either case. However, getting the 30 years plus
the old cast iron lumps used to manage does seem harder.

But they do free off the airing cupboard which is potentially a big plus.

And leave it unheated, which may be a minus.

Megaflow can be heated by a smaller boiler (possibly more efficiently?)

With modern boilers running at 90%+ efficiency, low water content, and
pump overrun etc there is not going to be much in it, unless you have very
low hot water demands. Also note that hot water generation will cost only
a small fraction of your total heating bill.

and once the stored water is up to temperature there should be no
conflict between CH and hot water.

Yup.

Pressurised systems have higher maintenance - need testing and checking
every year.

Although I can't see any reason you could not do that yourself...

Alternatively there are heatbank solutions that will provide the same
benefits, but without the requirement for routing checking.

Can run at the maximum mains flow if required by future changing demand.
Can be heated by backup power source e.g. immersion heater if there is a
problem with the boiler.
They do require more plumbing and take up significant storage space in
smaller properties.

So, leaving aside the space issue, it comes down to cost.

Which is cheaper to install.

Combi, by a large margin.

Which is cheaper to run.

Not enough in it to matter.

Which is cheaper to maintain over a 10 year period.

Probably not much in it.

Answers on a postcard......


Oh, and I have deliberately ignored heat stores to try and limit the
options.

Also ignored additional heat sources linked to renewables such as PV and

PV etc seems to only make significant contribution to your hot water
heating, and hence will not offset enough value to ever be economic in
most cases.

back boilers on stoves.

That may make more sense if you have access to cheap fuel.

Also ignored mixed solutions like having the combi heat a hot water
store for the showers but provide DHW to taps.

Probably only worth doing if the layout of the house is such that it
solves particular problems.


Thanks John - I think cost of installation will rule here :-)

On the PV front there is a minor additional gain if you already have your
snout well entrenched in the greenbribe trough.
With proper circuitry (as suggested in a recent post) you can turn on the
immersion heater whilst you are generating and make even more money.
Not having suitable roof space we just look on in envy as others take our
hard earned money.

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")

On 06/04/2012 18:23, David WE Roberts wrote:

"John Rumm" wrote in message
o.uk...
On 06/04/2012 10:57, David WE Roberts wrote:

This prompted by a comment by a builder coming to discuss our proposed
extension.
We said combi, he said that people are tending to go for Megflows now.
So is there any good reason to?

Yup, there can be...

Rough very shortened recap of parts of the Wiki.

Q1. Do you need more hot+cold water flow than the maximum cold mains
flow?
[e.g. two or more bathrooms and teenage daughters]

Yes - stored cold water in a tank, and pumped hot+cold to the baths and
showers.

No -

Q2. Do you want more continuous hot water than can be heated by your
maximum gas flow?

Yes - stored hot water, e.g. in Megaflow or see (1).

No -

Q3. *Well, here's the interesting part. If you got this far any of the
systems in (1) and (2) can meet your needs but so can a combi.*

So what is Q3 which decides between a Megaflow and a Combi?

Basically two things... flow rate, and "others". Where "others" are
the specific advantages/features that you list later like having
immersion backup etc.

Flow rate is the main driver for most I would guess. For either you
need a decent cold water supply obviously. Ignoring the very exotic
and elaborate combi solutions, its usually easier to get a decent flow
rate of "hotter than required, and hence can be mixed water" from a
store of water than from a combi, which will deliver its maximum rate
at pretty much final use temperature.

Factors:

Combi uses less space, as there is no stored hot water, but has to be
sized for maximum immediate demand.

Indeed, and there is the problem for many. Even using the full
capacity of your gas main may not meet your immediate demand.

Conflict betweeen hot water demand and central heating demand.

Yup, although it can be a problem with a store if its not large enough
to meet immediate needs without re-heating.

Combis more complex (everything in one box) so probably lower MTBF.

Compared with a system boiler, then not really. For that matter,
compared with a complete system of valves etc, you need the same basic
components somewhere - so the only difference is the location of the
bit that goes wrong!

Alleged shorter lifetime.

Not sure there is much in that. Buy a decent boiler, and the life is
likely to be similar in either case. However, getting the 30 years
plus the old cast iron lumps used to manage does seem harder.

But they do free off the airing cupboard which is potentially a big
plus.

And leave it unheated, which may be a minus.

Megaflow can be heated by a smaller boiler (possibly more efficiently?)

With modern boilers running at 90%+ efficiency, low water content, and
pump overrun etc there is not going to be much in it, unless you have
very low hot water demands. Also note that hot water generation will
cost only a small fraction of your total heating bill.

and once the stored water is up to temperature there should be no
conflict between CH and hot water.

Yup.

Pressurised systems have higher maintenance - need testing and checking
every year.

Although I can't see any reason you could not do that yourself...

Alternatively there are heatbank solutions that will provide the same
benefits, but without the requirement for routing checking.

Can run at the maximum mains flow if required by future changing demand.
Can be heated by backup power source e.g. immersion heater if there is a
problem with the boiler.
They do require more plumbing and take up significant storage space in
smaller properties.

So, leaving aside the space issue, it comes down to cost.

Which is cheaper to install.

Combi, by a large margin.

Which is cheaper to run.

Not enough in it to matter.

Which is cheaper to maintain over a 10 year period.

Probably not much in it.

Answers on a postcard......


Oh, and I have deliberately ignored heat stores to try and limit the
options.

Also ignored additional heat sources linked to renewables such as PV and

PV etc seems to only make significant contribution to your hot water
heating, and hence will not offset enough value to ever be economic in
most cases.

back boilers on stoves.

That may make more sense if you have access to cheap fuel.

Also ignored mixed solutions like having the combi heat a hot water
store for the showers but provide DHW to taps.

Probably only worth doing if the layout of the house is such that it
solves particular problems.


Thanks John - I think cost of installation will rule here :-)

In which case, the combi rules (which is why builders love them!).
Boiler will be no more than £100 more than a system boiler, and you save
yourself £600+ on the unvented cylinder.

On the PV front there is a minor additional gain if you already have
your snout well entrenched in the greenbribe trough.

Yup possibly, but the lions share of the gain is the bribe. The actual
energy you need for hot water production over a year is not usually that
significant.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/