I've got a low hot water pressure problem at the kitchen tap due to a
low head and SWMBO's request for a mixer tap on the end of a hose (I
recommend these by the way but be aware of the high insertion loss).
The question of a pump has been discussed in a recent thread "Hot Water
Pump" and one of the options is a basically a CH pump from Grundfos
which is designed to address this problem. I have my concerns about
this pump as it's flow switch and head requirements will only just be
met by my set up, plus physically plumbing it in will not be easy due
to lack of space under the kitchen sink.

By chance I stumbled on this website - think someone had posted it in
this group :-

http://www.mcdonald-engineers.com/pr.../thermflow.htm

This is a mains pressure thermal store system where the mains is heated
by a heat exchanger inside the tank.
In terms of plumbing effort and cost this is possibly times 10 in both
but what other advantages would I get - it would allow me to get rid of
the Triton electric shower and fit a decent pressure one as well as
sort out the kitchen hot water flow rate I know, but what are the other
advantages? - and disadvantages !

Rob

On 24 Jan 2006 04:13:50 -0800, "
wrote:


This is a mains pressure thermal store system where the mains is heated
by a heat exchanger inside the tank.
In terms of plumbing effort and cost this is possibly times 10 in both
but what other advantages would I get - it would allow me to get rid of
the Triton electric shower and fit a decent pressure one as well as
sort out the kitchen hot water flow rate I know, but what are the other
advantages? - and disadvantages !

Rob
Advantages-bags of hot water at mains pressure whenever you need it.
No more struggling with a combi

Disadvantages, it takes up space,needs to be fitted by certified
tradesman.



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Make free phone calls

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Tried,tested,approved !

On Tue, 24 Jan 2006 12:29:36 +0000, tarquinlinbin
wrote:

needs to be fitted by certified
tradesman.
Since when?

Mr F.

This is a mains pressure thermal store system where the mains is heated
by a heat exchanger inside the tank.

I'd go for a model that uses an external plate heat exchanger. Some are more
sophisticated than others.

You can only get the advantage of these systems if you have good mains flow
and pressure.

I have a DPS Pandora. It is really good and works entirely as advertised
(unlike their clunky website).

http://www.heatweb.com/

Christian.

What's the advantage of an external heat exchanger ?

This is not a pressurised tank - it has a heat exchanger inside the
tank it therefore doesn't need a 'certified' fitter.

Space is gained as it's not that much larger than my dhw tank and I get
rid of the cold water tank.

I didn't realise that combis were a problem - I'm running off a log
burning stove and an oil burner.

Rob

What's the advantage of an external heat exchanger ?

Faster heat exchange and lower pressure drop leads to higher potential flow
rates compared with an internal coil exchanger. External nature of heat
exchanger allows descaling and replacement.

The disadvantage is the requirement for a pump, and, hence, an electrical
supply. They should also be more expensive.

Christian.

I have an Albion Mainsflow installed in my loft. This type has the coil
inside the cylinder (the 22mm pipe splits into four 10mm tubes to
increase the surface area). I have a proper ion-exchange water
softener, so scaling isn't a concern for me, but Christian is quite
right that having an external plate heat exchanger allows for easier
descaling when necessary. The external models are much more expensive
(several hundred) but marginally more efficient.

Showering is fantastic (Mira Excel), and having potable hot/cold mains
pressure water at all taps is great (with aerators fitted in the
bathroom). You can remove the 50 gallon water tank from your loft (you
may need a small feed/expansion tank depending on design). My unit
(140litre direct cased) cost £600 from www.theheatingshop.co.uk. The
roughly equivalent model from DPS was about twice the price.

-Antony

My unit (140litre direct cased) cost £600 from www.theheatingshop.co.uk.
The roughly equivalent model from DPS was about twice the price.

Yes, it does seem that DPS have put their prices up. I paid 750 for my
Pandora, which was a 180L indirect.

Christian.

Interesting - both you and Anthony have got smaller tanks than I was
advised; it was suggested that I should have a 210 litre with 180l as
an option but 140l is a long way smaller. The 210 was so that we could
have 2 baths of it but that is a very unlikely scenario with
effectively only 2 of us in the house now. Is 140l big enough ?

I'm still not getting good vibes on why I should go the mains pressure
way rather than solving my low hot water pressure problem with a pump.
I fancy the mains pressure route but the effort and cost do mitigate
against it.

Rob

Sad person replying to his own mails but a further thought came
overnight - if the heat exchanger is external why do you need a
specific tank; will the standard dhw tank not act as the store if the
feed plumbing is appropriately modified ?

Rob

Sad person replying to his own mails but a further thought came
overnight - if the heat exchanger is external why do you need a
specific tank; will the standard dhw tank not act as the store if the
feed plumbing is appropriately modified ?

A heatbank is little more than a standard HWC modified with the addition of
a few flanges, a pump, a flow switch, a heat exchanger and a thermostatic
mixing valve. They are certainly DIYable with skill.

The Pandora adds a proprietary expansion system that prevents water
evaporation and so does not need a header tank, float valve and overflow.
These items are not essential. Some systems use a variable speed pump
instead of a TMV. This provides better stratification performance.

Christian.

Is 140l big enough ?

If you want to run 2 baths at the same time, you need a bigger
cylinder. My unit recovers very quickly (with an 18kW boiler), so
there's no problem running one bath after another. 140litres is enough
for 1 bath + 1 shower in parallel. If your requirements are greater
than this, use a bigger unit. Also, if you have mutliple heat sources
(e.g. solar, Aga) you would want to go for a larger store.

I'm still not getting good vibes on why I should go the mains pressure way rather than solving my low hot water pressure problem with a pump.

I was replacing my entire DHW/CH system, so it made sense to install a
thermal store. If you're happy with all the other aspects of your
system, then just adding a pump may be more sensible as going mains
pressure may involve major changes to pipework (going from 15 to 22mm),
replacing taps (some can't handle high pressure), etc. Bear in mind
that quality high-pressure shower pumps aren't cheap.

-Antony

The 210 was so that we could have 2 baths of it but that is a
very unlikely scenario with effectively only 2 of us in the
house now. Is 140l big enough ?

To give you some idea of the performance of my 180L attached to a 28kW
boiler, it managed to fill a 750 litre birthing pool (approximately 6 full
bathloads) to 40C in 41 minutes.

I'm still not getting good vibes on why I should go the mains pressure
way rather than solving my low hot water pressure problem with a pump.

The advantages of a heatbank over pumped gravity feed:

1. More reliable (pumps break down).

2. Cylinder lasts longer (is filled with corrosion inhibitor and doesn't get
new supplies of fresh hard water every day).

3. Silent operation.

4. Doesn't require large cold tank in loft.

5. Can supply outlets above cold tank level without fiddly pull switches or
expensive and unreliable pressure switches. (Very useful in loft conversions
and flats).

6. All taps, including the hot taps, are potable drinking water.

7. Can have multiple primary heating sources, so you can add solid fuel back
boilers and solar heating panels, as well as your gas boiler and electrical
immersion.

8. Can run the radiators off it, too, particularly when (7) above is
implemented, allowing your rads to be heated from many alternative sources
depending on economy and availability of energy.

Disadvantages:

1. Needs good pressure and flow from the mains.

2. More expensive to install.

3. No short term backup in the event of mains water failure.

Christian.

On 26 Jan 2006 01:46:36 -0800, "
wrote:

Sad person replying to his own mails but a further thought came
overnight - if the heat exchanger is external why do you need a
specific tank; will the standard dhw tank not act as the store if the
feed plumbing is appropriately modified ?

Rob


You could do this. The water in the cylinder must be fed from an
open vented header tank, not pressurised. However, said tank could be
a small CH type. Some thermal store integrated products have this
built in.

The store needs to be maintained at 80 degrees plus if you want to
have the benefit of being able to use a smaller cylinder than would be
implied for the volume of usable water at the tap.
A side implication is that although plate heat exchangers have a very
good transfer rate, (e.g. 100-200kW), if the water in the store is
only at DHW temperature (60 degrees), you would not be able to reach
this at the taps.
An implication of storing water in the cylinder at 80 degrees is that
heat losses increase by a third as compared with a conventional
arrangement. Although these are not large, it may be prudent to
order a cylinder with more insulation.

It would be a good idea to use a cylinder with fast recovery heating
coil to the boiler. This means that the boiler can start to deliver
all of its output to the cylinder soon after heat starts to be removed
from it. The result is of a longer run time before the store is
depleted and one is down to the rate that the boiler can deliver. Of
course this only matters if the volume and rate of use exceeds the
capacity of the cylinder.


--

..andy

will the standard dhw tank not act as the store if the feed plumbing is appropriately modified ?

It depends on your setup. My Albion Mainsflow has a built in
feed/expansion tank (the whole store is in fact a feed/expansion
tank...). As the store is direct, it also acts the F&E for the primary
boiler/CH circuits. Other stores work differently, where the boiler
heats the store indirectly, you might need two F&E tanks, or the
boiler/CH might be sealed. There's a lot of variation in design.

-Antony