Just though I'd call in here to run this one past the more experienced...
I currently have a gravity fed hot water system. I'm about to build a small
extension and will replace the current boiler while doing so. I have
pondered for a while the idea of a mains pressure hot water system but am
really concerned about the lack of pressure at the upstairs shower. I
currently have a pressure pump installed and the shower is brilliant. I also
like the idea of a combi simply because of the fact that I'll never "run
out" of hot water.
Anyway, after reading loads of stuff on the web I have concluded (maybe
wrongly!!) that I should have a combi to feed everything but the showers and
retain the current hot water cylinder and pump just for the showers. I'd
obviously have to somehow plumb in the system to heat the water cylinder and
that will no doubt necessitate the use of motorised valves etc. I realise
that I'll still need to keep the loft tanks but don't mind that. I'll also
have two hot water supplies in case one fails and stored water in the loft
is always handy should there be a mains supply problem.
So, am I thinking right? and is there any where on the web where I can
source a plumbing & wiring diagram for this type of set up? I can plumb and
wire but haven't a clue where to start on this. I'll obviously need the
boiler to be on call for the central heating and the hot water and these
need to be independent.
Any other idea's appreciated too.
Assuming that the mains pressure and flow is good, I'd do it the other way
round. Have the shower (and possibly kitchen tap) off the combi, and run the
bath from the cylinder, possibly using the pump.
Of course, you shouldn't rule out a mains pressure storage system which
gives the best of both worlds. I've just collected such a system this
morning, a 180l DPS Pandora. I'll let everyone know if it works once it is
finally connected up.
You wire and plumb it just like you would a conventional heating system. You
will have to discard any boiler built in programmer, as it will only have
one channel. Either lose it, or set it to 24/7.
Decide on either Honeywell 'Y' or 'S' plan. Wire up in the conventional
fashion and lash up the final call for heat to the room thermostat
connection on the boiler. You'll probably need a volt free contact at the
end, so choose a programmer that has volt free contacts, as otherwise,
you'll need a relay, unless the boiler can accept a 230V call for heat.
I've got one. Shoehorned the HW tank into the loft. Saves a lot of
space. Plumbing is more intricate - lots of safety pipes to dump
pressure outside etc, but overall performance is better than anything
else. Combis have pressure, but no flow rate IMHO, unless you go for a
really BIG one. Combis need to have heating capacity for peak hot water
flow. Mains pressure tank? You have three or four baths worth on tap all
teh time. Boiler needn't be as big.
I'd only use a combi on a strict budget, or in a small house with no
room for a HW tank.
I'd never use a gravity fed header tank unless teh local water supply
was known to be highly unreliable.
Here, water is 100% reliable, but electricity is not. No sparks means no
boiler. But HW tank is good for several baths, aga needs no psarks to
cook, and open fires take cxare of heating, and candles just about sort
out lighting. And battery radio allows local news coverage of 'national
AND I can recharge its batteris from the car.
The DPS Pandora is different from your system, though. Lower maximum flow
rates and won't work at all without electricity. However, it doesn't require
lots of safety pipes to dump pressure, which is the main reason I bought it.
Yes. You can add anything to a combi that could be added to a conventional
boiler. A combi is just a conventional boiler with an additional
instantaneous hot water heat exchanger and associated controls.
You'll need a pile of extra controls, programmers and valves, as would be
required for a heating system using a conventional boiler. These are
frequently packaged into convenient kits with substantial discounts.
There are two main contenders. Both are similar in purchase cost. Both
vastly superior to a combi boiler in performance, assuming your mains can
supply the water.
a) An unvented hot water cylinder. This provides the ultimate flow rate,
assuming your mains can supply it. It requires extra safety controls, which
mean that you need a specific safety inspection for building regulations or
a specifically qualified installer. Example: Heatrae Sadia Megaflo.
b) A heat bank (or thermal store, which is similar). This provides good flow
rates (again assuming good mains supply), but not as good as the unvented
cylinder. It is inherently safer as the hot water storage itself is at
atmospheric pressure, so is not subject to potentially dangerous pressure
rises when heated. This makes is much easier to install. Example: DPS
Pandora (also available with complete CH+DHW 'Y' or 'S' plan system controls
and 2 channel programmer ready mounted to the cylinder).
I've just bought a DPS Pandora, but it won't be connected for a week or two.
It looks the part, though. We've managed to hoist it into the loft, where
there is no convenient overflow or pressure relief outlet due to a planned
Showers need pressure as much as they need flow. This means that an
excellent shower can be obtained by a combi, which uses mains pressure but
is limited in flow rate. It will also be less noisy, as no pump is needed.
The cylinder is capable of supplying much higher flow rates. However,
gravity feed might not be able to get sufficient flow rate at the taps, for
which a pump can be beneficial.
That would depend on the pipe diameter and routing. Most gravity fed baths
I've used have had substandard flow rates, sometimes not much more than a
combi boiler. A pump is often quite an improvement. The problems occur when
the hot water cylinder is tucked away somewhere quite different from the
location of the bath and the cold water tank, leading to convoluted pipe
runs, sometimes even run in 15mm pipe. It is these situations that a pump is
often quite an improvement as far as baths are concerned. With showers, you
usually need a pump to improve the pressure so that the water can squeeze
through the holes.
Thats right. One of the thngs they teach you at a snotty UNI is that
flow rate is a function of both pressure and bore diameter, and indeed
I guess where IMM went they didn't even explain what 'function of' means
in the mathematical sense...its a bit beyond the Boys Bumper Book of How
The website isn't quite up to date. AFAICT, the "2002" specification
heatbanks have been replaced with "5670" specification types referred to as
Pandoras (which are mentioned). However, the general principles and
specifications are similar and (I'm told) prices reduced. The prices aren't
listed on the website, but are very similar to equivalent Megaflos as listed
by discountedheating.com if you want some idea.
The company is of the a small friendly variety. This means that lead times
aren't that great (it isn't off the shelf at a plumber's merchant like a
Megaflo), but they are able to build the things to a precise specification,
if a standard model isn't quite right.
No blending valve. The indirect coil comes unadorned. There is a
thermostatic mixing valve on the DHW outlet, though.
I'm certainly interested in anything that can reduce the return temperature,
though. I wouldn't mind reducing the radiator flow temperature, too, for
safety reasons. I have a five year old, whose bed is adjacent to a radiator.
Can blending valves achieve this?
On Wed, 1 Oct 2003 09:18:20 +0100, "Christian McArdle"
Are you running the radiators from the store as well, Christian?
Otherwise, if it's a condensing boiler, does it have separate
temperatures of operation when in DHW mode vs. CH?
On mine, there are zone valves on the feed from the boiler - there
being one quite close to it for a new radiator. There is then the
original 22mm feed to the airing cupboard where there is one zone
valve for CH and one for the cylinder.
When the CH demands, the valves for that open and the flow is limited
to 70 degrees. When the cylinder demands, these valves close and
the DHW one opens. Then the boiler will run at 82 or even 85 degrees
If your boiler doesn't do this but modulates based on the return
temperature, then adding a blending valve to the radiator circuit to
feed some water back to the return, the effect would be to reduce the
heat output because the boiler will see it as reduced load.
Then you have to think about whether the radiator sizes are adequate
at the reduced temperature to give the heat output you need.
To email, substitute .nospam with .gl
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.