I would go twin coil unvented hot water cylinder, as you say keep it simple. The only problem I see though is the limited height if the cylinder must go in the roof space. Could you not site the cylinder elsewhere?

You can't directly heat an unvented cylinder, you have to keep the water in the boiler separate from the water you use. So if you have two heat sources you need two coils - not strictly true, but close enough.

In a thermal store, the water in the store is heated by the boiler directly and a coil is used to heat the water that you use. This is where it gets expensive because that coil must heat up the water you use instantly and therefore needs a lot of surface area.

When you are comparing prices, are you comparing the installed price or just the tank? I don't know, but would have thought an unvented cylinder would be cheaper but with more complex controls and a thermal store would be more expensive but with simpler controls - thought it might balance out.

HTH, Bri.

squatter madras
You should be able to fit an UV cylinder in a 1500 mm loft. The expansion vessel for the potable water side can be at the same height as the potable side is now going to be un-vented (assuming you go for an un-vented cylinder). The boiler coil is un-sealed because you have said you have a vented CH system. Therefore, you already have unsealed CH and vented DHW water. I try to stick to those terms to avoid confusing myself (Not difficult). Sealed/un-sealed for primary side and vented/un-vented for DHW potable side.
Because your CH in un-sealed there must be a Feed & Expansion tank (smallish black plastic) that is connected to the CH circuit. This usually has to be at least 1m above the top of a radiator (i.e. primary flow), thus this would have to be 1m above the DHW cylinder coil (for an unsealed system). This would be true for both a vented or un-vented cylinder (as that term applies to the potable DHW water).
If you want to do away with the CH F&E tank then the CH primary side becomes sealed (that’s rads + hot water heating coil). A sealed hot water heating coil could heat either a vented cylinder (i.e. fed by big 50 gal black plastic tank) or an un-vented cylinder (fed directly from incoming mains).

First you need to confirm if you have sealed or unsealed CH and vented or un-vented DHW. If you say you have vented CH and are looking for an UV DHW cylinder I suspect you may have 1 small one and 1 big one.
So how many and how big are any black plastic tanks and where are they?

Cheers
Mike up North

The Gledhill boilermate has a coil for the boiler which means it can operate with the boiler either open vented or sealed. See the pictures halfway down: http://www.gledhill.net/building-products/bp-index.htm
Our TS (old Albion) only has a coil for the DHW, the boiler and rads are connected directly. This is more efficient (slightly), but means that you need gallons of inhibitor and flushing chemicals etc. as the whole store contents are circulated.

You would need to connect as per the left hand diagram as your system is vented.

Gledhill say that I need to add 400mm to the cylinder height, 100mm for the base and 300mm for the F&E tank, Ive only got 1500mm so snookered yet again I fear!

Any other ideas to fit in with the 1500mm height restriction (Victorian villa, one and a half storeys - combed ceilings)

I suppose that both at mains pressure is a tall order. I should have said two useable showers simultaneously.
You are absolutely right about pipe layout Cwatters, there are some 'plumbers' who are just 'tube solderers', the trick is trying to avoid them!

Would you use two seperate 22mm bosses for hot feed to each shower?

Bri, would you say that you'd prefer a coil rather than a direct boiler connection?

I'll measure the flow rates as you suggested.

The indirect thermal store is an easy swap from an existing system because either a cylinder (vented or UV) and the thermal store all have a primary coil through which the primary heat flow is passed. Therefore, from a primary perspective it’s a straight swap.
This would also be true for a sealed and unsealed system – all units would have of course to be below the height of the primary F& E tank.

Cheers, Mike up North

Hi,If this is an indirect thermal store then the primary heating is by within a coil on the radiator circuit. This is therefore served by the F&E tank on the radiator / CH heating circuit.
The thermal store contains a mass of water which basically doesnt go anywhere it just sits there, this needs a separate F&E tank which if a combo cylinder is sitting on the top of it. So this tank is higher wrt the store volume and the other F&E tank is higher wrt the rad/CH circuit. As the store water and heating coil circuits are separate the tanks can be at different places/heights – so long as they above their respective waters
Your potable DHW (mains fed) passes through another coil within the top of the store mass of water to heat it up, this will have an expansion vessel (and relief valve etc) on it just as would any UV cylinder. This takes up the expansion in the pipes as the DHW water is heated.
So for an indirect coil heated store there would be two separate F&E tanks
Thus for a direct non coil heated store where the volume of water in the store is the same as that within the primary circuit there would be only one F&E tank but its positioning might be harder.
Hence the suggestion that an indirect thermal store is almost a simple swap from a UV cyclinder wheer the primary connections are concerned - particualrly if a combo cyclinder is used as ther additional F&E tank is within the package.
Beware - non themrla store domestic cylinders (for use in flats etc) also come with a tank fixed on the top and are also called combo cylinders, in this case the water in the top is a feed tank of potable cold water from the mains to supply the cyclinder. If you are talking so someone who is not familiar with a thermal store they might be confused.
Cheers
Mike up North

Hi,
You'll find all sorts of recommendations about the height (head) difference, for example on the DPS heat banks with a separate F&E tank they quote only 100mm from the top water level surface to the rad. Basically its top to top and this difference in head should be as much as you can manage.
S plan just means that there is a route to the CH and a route to the DHW "coil" as per any normal heating system. There is a letter designation for every combination of valves and routes.
For a simple swap out the indirect is closest to any other cylinder, with direct the store is in the flow to the heating circuits, whilst with indirect its on a branch with a diverter valve.
Cheers
Mike up North

Hey,
Its only the direct version "they" are calling sludgebuckets over on talk SF, the indirect will be fine as a swap. The direct wont sludge up any way.
Cheers
Mike up North
(going south for the hols)