apologies for multiple post last time - I was getting the message to
try again and again ... from google

3) heating on demand is more economical from storing water
Is it though? If you have to run a big ****-off boiler flat out to heat
the water, is it as economical as a modulating boiler running at tickover?
ok - to heat up X litres of water from temp A to temp B you need
certain amount of calories from gas. As far as I understand it, it
doesn't matter if you heat it
fast (combi) or slow (storage tank) - you still have to burn the same
amount of gas. Big ****-off boiler will run flat out for say 15mins
while you take shower. At the same time modulating boiler will
tickover for 1 hour in reality burning the same amount of gas. Now,
when you store the water in the tank heat will escape and you have to
burn some more gas to keep water at temp B all the time. This is where
combi wins I think (at least in terms of energy savings)
Additionally if you work irregular hours you don't have the trouble of
programming when exactly you need to have your hot water.

4) I can free some loft space for future conversion
do I miss some important point here ? feel free to comment

The size of a smaller boiler and small (150L tank is not much greater
than a 20KW boiler.
Neither is the cost a great deal different in my case. YMMV.
Yep - I agree. But you also have to have run additional pump (press
tank) which presumably is noisy and cost you electricity.

it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.
You won't get that from a combi.
hmm... - no good, will have to discuss that with my girlfriend then =(

I stayed in a house once..with someone with a teenage daughter. The HOWLS of anguish when I turned on a hot tap
to wash my hands from the teenage daughter showering..
how about using thermostatic mixer showers ? Flow would drop but you
won't get temp fluctuations ... Just like you have with your tank
setup.

Seriously look closely at what peak hot water rates you CAN get with a combi.
15.2 ltr/min for big ****-off Vaillant Ecotec 837. Initially I thought
this would be enough for shower + kitchen tap (i.e. dishes).

Mmm. The pressurized tank will go anywhere..doesn't have to be in the
loft. Could be in the garage..
As could the boiler.. Mines in a corner of the loft as it happens..

Yes I am sure I could find some corner space even with a loft
converted. Thanks for your input I will definitely consider this
option.

Mike

wrote:
apologies for multiple post last time - I was getting the message to
try again and again ... from google

3) heating on demand is more economical from storing water
Is it though? If you have to run a big ****-off boiler flat out to heat
the water, is it as economical as a modulating boiler running at tickover?
ok - to heat up X litres of water from temp A to temp B you need
certain amount of calories from gas. As far as I understand it, it
doesn't matter if you heat it
fast (combi) or slow (storage tank) - you still have to burn the same
amount of gas.

not necessarily. Boiler efficiency varies with output power. Also duty
cycle (unburnt gas up chimney before it lights sometimes)Lots of
conflicting evidence on the net on this. No axe to grind, jutrs be aware
that boiler thermal efficiency is not 100% and isd not constant with
outut power.



Big ****-off boiler will run flat out for say 15mins
while you take shower. At the same time modulating boiler will
tickover for 1 hour in reality burning the same amount of gas. Now,
when you store the water in the tank heat will escape and you have to
burn some more gas to keep water at temp B all the time. This is where
combi wins I think (at least in terms of energy savings)
Additionally if you work irregular hours you don't have the trouble of
programming when exactly you need to have your hot water.


A lot depends on tank insulation. Usually this is pretty good, and a
pressurized tank has about as high a volume to surface area ratio as you
can get. So it doesn't lose heat very fast at all. In any case you cvan
swich it off overnight after the shower goes cold and have it come on
half an hour before you get up.



4) I can free some loft space for future conversion
do I miss some important point here ? feel free to comment
The size of a smaller boiler and small (150L tank is not much greater
than a 20KW boiler.
Neither is the cost a great deal different in my case. YMMV.
Yep - I agree. But you also have to have run additional pump (press
tank) which presumably is noisy and cost you electricity.

? what pump? No pump at all. It runs off mains pressure. The only pump
is the circulation pump for the CH and HW primary, and that only runs
when the boiler runs in any case.. and boilers make a fair amount of
noise. A combi doesn't use a pump when heating hot water, but it still
has one for rads etc. So theres just as many pumps.



it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.
You won't get that from a combi.
hmm... - no good, will have to discuss that with my girlfriend then =(

I stayed in a house once..with someone with a teenage daughter. The HOWLS of anguish when I turned on a hot tap
to wash my hands from the teenage daughter showering..
how about using thermostatic mixer showers ? Flow would drop but you
won't get temp fluctuations ... Just like you have with your tank
setup.

Well lets put it this way, when I went to wash my hands, I didn't get
enough hot water to fill a basin in under 5 minutes, and that was tepid.
OK she had a small wall mounted combi, nit a big ****off one.


Seriously look closely at what peak hot water rates you CAN get with a combi.
15.2 ltr/min for big ****-off Vaillant Ecotec 837. Initially I thought
this would be enough for shower + kitchen tap (i.e. dishes).

Mmm. The pressurized tank will go anywhere..doesn't have to be in the
loft. Could be in the garage..
As could the boiler.. Mines in a corner of the loft as it happens..

Yes I am sure I could find some corner space even with a loft
converted. Thanks for your input I will definitely consider this
option.


I have been into this in depth twice now..and each time the combi didn't
work out cheaper or better in any sense.

Hot water is a pretty small element of my heating budgets, and our tank
stays hot for a couple of days so its pretty well insulated.

I think where I would install a combi is a bachelor flat of small
dimensions, with a single occupant and a single bathroom there I can see
the virtue of the very small foot print. And overall heating
requirements are not huge.

Once you get to a family situation, I think the approach loses all merit.



Mike

The Natural Philosopher wrote:
I think where I would install a combi is a bachelor flat of small
dimensions, with a single occupant and a single bathroom there I can see
the virtue of the very small foot print. And overall heating
requirements are not huge.

We stuck one in a one bed flat, two people.

The space saved by not having a cylinder meant we could move the washing
machine out of the kitchen, which made space for a dishwasher. Which
solved the dishes problem. We vented the boiler through the roof, which
meant we could put it in a big cupboard.

Still couldn't shave while my wife was showering, but other than that it
was great. Including the shower. It had a 5 litre heated store for
proper instant hot water, which worked well.

Once you get to a family situation, I think the approach loses all merit.

Absolutely.

Ben Blaukopf wrote:
The Natural Philosopher wrote:
I think where I would install a combi is a bachelor flat of small
dimensions, with a single occupant and a single bathroom there I can
see the virtue of the very small foot print. And overall heating
requirements are not huge.

We stuck one in a one bed flat, two people.

The space saved by not having a cylinder meant we could move the washing
machine out of the kitchen, which made space for a dishwasher. Which
solved the dishes problem. We vented the boiler through the roof, which
meant we could put it in a big cupboard.

Still couldn't shave while my wife was showering, but other than that it
was great.

I think you have made the most useful point here for the OP to consider.

A small cheap combi will not do more than one taps worth of hot water at
a time.

If that is a non starter, then a small cheap combi is not the way to go,
and a large not so cheap combi, becomes almost the same footprint and
cost as an alternative approach.



Including the shower. It had a 5 litre heated store for
proper instant hot water, which worked well.

and of course, since the combi was inside the insulation layer, any loss
from that is heating to the house.

Once you get to a family situation, I think the approach loses all merit.

Absolutely.