Pumped unvented cylinder vs combi cylinder?

[Niall Smart]

Hi all...

I'm replacing the boiler and DHW system in my 3 bed semi-d that I'm renovating.. so far so good - thanks to all the regular posters! I've putting the boiler and DHW cylinder in the loft (attic) space to save room downstairs. Hoisting the boiler up was fun!

Anyway, I've run into one issue which I haven't seen covered here much. I live in Dublin, where the mains water supply / pressure can be quite erratic, especially during summer. That pretty much rules out a mains pressure system. The local water bye-laws also mandate cold water storage for cisterns, showering, etc. I guess this is to stop our mains distribution system getting overloaded at peak times.

So I can't get rid of tanks in the loft, and if I want a pressurised (power) shower I'm going to have to use a pump. The two options seem to be either (a) regular vented / unpressurised system where the pump is on the appliance side of the DHW cylinder or (b) use a pump to pressurise an unvented cylinder from a cold water storage tank. These diagrams illustrate what I mean:

a -

- the kit will be in the attic, so if I go with (a) I'll need to use a combination cylinder to get a positive head to feed the cylinder.

Has anyone installed (b) the pumped unvented system?

As far as I can see the only pro is that I can have a larger DHW cylinder as I don't need the cold water directly overhead, but with the drawbacks that I will have pump noise on any hot water draw off, no hot water if the pump fails, and more complicated pipework. Advantage combi cylinder?

Thanks for any input...

Regards, Niall

[fred]

In article , Niall Smart writes

I suppose the advantage with b) is that the pump is simpler (single) and so may cost less, but the twin pump is a standard retrofit for those wishing higher pressure so economies of scale may have eliminates the price difference.

Bear in mind that with a) you can use the cheapest (pressure) grade of cylinder available whereas with b) you may require a more expensive one, plus grade 3 (lowest) cylinders are on the shelf in every plumbers' merchants.

Remember also that in a) there is no requirement for the hot and cold tanks/cylinders to be co-located, the cold could be a coffin tank high up in the apex of the roof or elsewhere.

[Doctor Drivel]

In the attic it is best to have a combination cylinder with a large cold water tank attached, then a pump on the oulet side for ashower. There rest will gove and not much pressure wjich is fine.

Or use a cold water accumulator and a heat bank or combi

[Niall Smart]

Good point, although the difference in costs between the two types pumps is unlikely to sway the decision much either way as I'm doing all the labour myself.

Now that is likely to be more of a cost differential. I do have to get the combi made to order though as I need a short squat one.

Yeah, its probably easier to go with a combi cylinder though in my particular scenario

Interestingly I rang Stuart Turner technical dept and they said that even with option (b) you need some head over the pump (he said at least 1m) to prevent cavitation, etc. So given that, I'm probably almost definately going with option a. Until I change my mind again. That has to be the worst bit about DIY, too many options!

Niall

[fred]

I was thinking life is just a bit easier with a higher head, it's a bit of a pain to have the pump operating for all water flows, it might be nice to have an unpumped feed on say the toilet sink to avoid middle of the night pump operation for a toilet visit.

[James Salisbury]

Why not conventional storage tank, shower pump and a thermal store?

[Niall Smart]

I did consider this, by using a submersible pump in the storage tank I would even get away with having the storage tank at negative head with respect to the cylinder. However any hot water draw-off would then activate two pumps - the pump from storage, and the plate exchanger pump. Even if I raised the cold water storage tank over the thermal store I don't think I'd get enough positive head to overcome the flow resistance of the plate heat exchanger.

I probably would have went for a thermal store if I had a sufficient quality mains supply.

Regards, Niall

[Niall Smart]

Yeah I intend to use a separate tap off the cylinder to a secondary circulation loop feeding the kitchen and bathroom taps. Similar for the cold.

Regards, Niall

[Dave Plowman (News)]

There's a lot of rubbish written about showers. You don't need high pressure for a decent one - it's the flow that matters. And you don't need high pressure for that - consider tipping a bucket of water over yourself where there's no pressure and little head. ;-)

If you're rejigging the system, consider moving the header tank as high as possible in the roof space and take special care with the pipe runs avoiding elbows etc - bend the pipe instead. Run separate 22mm feeds from the tanks to a proper shower designed for low pressure use - Aqualisa make them. Whilst the extra cost of the pipework will negate the savings of the pump cost you'll end up with a better and quieter shower.

[Doctor Drivel]

This man should eff off as he is a total idiot.

A shower needs "pressure". The high pressure on the skin gives the sensation. A high pressure low flow shower is generally acceptable to most people.

[Dave Plowman (News)]

Yes - stick with your electric one. You know it makes sense.

[Doctor Drivel]

Please eff off as you are an idiot.