Hot water system

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Hello all,
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.
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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.
Christian.
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Christian McArdle wrote:

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 electrical crisis'
AND I can recharge its batteris from the car.
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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.
Christian.
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That can be sorted by installing another plate heat exchangers in parallel. That's if you need very high flow rates of course.

Or an overflow pipe either, so can be fitted in the centre of a house, like under the stairs , right out of the way. Large metals pipes penetrating walls can be a large heat sink.

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At the risk of starting WW3.5, is it possible to add a mains storage system to an existing (Bosch Worcester Greenstar) combi?
And if so any recomendations?
TIA
Bax

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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 dormer extension.
Christian.
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Eh?!
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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.
Christian.
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You obviously mean just for a shower as a tank fed bath can supply very high flow rates, but poor pressure.
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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.
Christian.
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Christian McArdle wrote:

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 bore length..
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 Things Work.
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You really don't know, do you?
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Most gravity systems are fitted with 22mm pipe and lately the cold feed to the cylinder in 28mm. The really do flow.
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Christian, do the manufacturers of the Pandora have a web presence?
TIA
Bax

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www.heatweb.com
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.
Christian.
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What is your spec? Does it have a blending valve on the flow and return?
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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?
Christian.
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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 flow.
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.
.andy
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