Something like this is 4 foot high.
http://www.heatandplumb.com/acatalog/Telford_Direct_Combination_Cylinder_1200_x_450.html
How large do you need your DHW cylinder?

Large thermal losses from these sorts of cylinders though.

Thanks @djh! I'm going to ask the manufacturers. I think it's possible to remove the legs to make it slightly smaller, but probably not quite enough.

The header tank only has to be big enough to take the expansion of all the water connected to it. (TS + boiler + rads etc. or what you have) It does not have to be the fill point as well so no need for a float valve. (mine is filled / topped up manually from a feed at the base of the TS) The header tank has to be the highest point on the system and whilst best practice has a separate pipe to the header tank (and I'm not sure of the UK regs) over here the header tank is often taken off at a convenient point in the system and providing that there is no means of closing off the pipe work then it is considered safe here to tee off at some high point in the central heating (flow) pipes to locate the header tank somewhere else.

Why does the header tank have to sit on top of the TS, Can you run pipework to locate the header tank elsewhere, (e.g. the loft, but inside the insulated envelope) either dedicated pipe or teed off the CH (if allowed where you are)

Posted By: ComeOnPilgrimThanks peter and dhj. There's plenty of space in the attic, just not much space higher than the heat store if it is indeed 2250 high. Ie, a wide flat tank might fit somehow.

I've been guessing that a tank 2250 high is a 2500 L Akvaterm. Which would weigh pushing three tonnes when full, so probably not best suited to an attic.

Perhaps if you explained more about your situation, we could be more helpful.

Posted By: djhI've been guessing that a tank 2250 high is a 2500 L Akvaterm. Which would weigh pushing three tonnes when full, so probably not best suited to an attic.

I presumed that just the header tank would be in the attic.

On the other hand stuffing a 2500 L Akvaterm up through the average loft hatch would be an interesting weekends entertainment.

My thermal store is located on the ground floor and the header tank is in the attic. Works ok.

Posted By: djh

Posted By: ringiE.g why does it have to be vented?

Personally, I look at the question the other way around. Why on earth would I want a store that isn't vented?

Why do you want a pressurised bucket of very hot water that has to be serviced annually?

A lot less issues with air locks and no need to find somewhere high up for the header tank.... There is nothing stopping someone having two sets of safety values, if they don't trust a single test. The annually service is no more then testing the pressure/temperature safety value and could be done at the same time as a wood boiler is serviced.

But if the heat source is a wood burning stove with back boiler that could boil away a lot of water, I would want a large header tank at a height it can't fail!

@Peter - Yes, getting a large heat store through a loft hatch would be pretty entertaining! At the moment we don't have any stairs, so there is a massive (and slightly unnerving) hole right through the middle of the building. See my other thread on the question of getting the heat store up into the attic. I'm hoping that we can hoist it vertically up from the steel roof beams that are directly above the hole.

@ringi, we will have solar thermal and a stove with a back boiler.

Posted By: ringiA lot less issues with air locks and no need to find somewhere high up for the header tank

I don't even know what possible problems there are to do with air locks, so I suspect there are none. My header tank sits immediately above my thermal store and the whole lot sits below my normal ceiling height.

Posted By: ringiUsing a thermal store for DHW is questionable, compared to a separate DHW tank heated by the thermal store. With a separate small DHW tank your loses are a lot lower outside of the time of year you need space heating hence may allow you avoid lighting your stoves as often in warm weather.

+1

Posted By: ComeOnPilgrimthanks @ringi - can you explain why a thermal store heating another DHW tank makes the system more efficient?

Because for about 8 months of the year you ONLY need to heat DHW, the thermal store being large will lose a lot of heat into your home. This lost heat is not useful unless you need to heat your home.

The issue is getting enough heat from solar (PV or thermal) to give you all the DHW you want without having to light the stove, come Oct or Nov you will not have enough solor to waist on the loses from your thermal store.

Also if you want your DHW at 60c you need your thermal store at about 70c, but you only need to heat a DHW tank to 60c.

I have a thermal store (TS) and a separate DHW tank. The DHW tank is mains pressure and indirect. The DHW is heated from the TS. The TS has 3 loads, 2 houses and the DHW (that serves both houses). Each load has their separate controls and are thus independent of each other. the TS is heated by a gasifying boiler. The DHW tank also has an immersion heater.

With this setup the DHW is heated by the TS but will stop when the TS drops below 65-70 deg. at the top, the CH will continue without a lower limit. The DHW tank is 300lts heated to 60 deg. During the heating season the boiler is lit when the TS gets to about 45 - 50 deg, (at the top) The DHW tank will have enough residual heat to manage until the TS gets back to 65 - 70 deg.

During the non-heating season I can either light the boiler twice a week to provide DHW or if I am lazy use the immersion.

Without a separate DHW tank I would have to light the boiler when the TS got to 65 - 70 at the top which is a lot more restrictive than being able to use the TS down to 45 - 50. In addition in the summer I would have to heat the TS with an immersion heater (when I'm lazy), - which would not be very economical.

Posted By: ComeOnPilgrimI'm still wondering whether there is a solution to my original question about a vented system with limited headroom. Is there any guidance that anybody could point me in the direction of? The Akvaterm documentation send assumed at an unvented system and is not very illuminating.

With a vented system, you do need a reasonable amount of headroom for a header tank, so you either need shorter tank(s) or a pressurised system.

I mentioned the possibility of using multiple tanks earlier. That is something that Akvaterm in particular support, I believe. So you could use one tank in summer (and whatever fraction of spring and autumn) just for DHW, and bring one or more extra tanks into use when you start using your stove to heat the system.

Whether you configure it as a DHW tank and a CH tank(s) or view the system as a whole is up to you.

Well I had a similar problem, indeed there wasn't even room for my tank initially - I dug a hole in the floor to solve my problem :) If that isn't possible then you just have to find a route upstairs for a pipe - either to the loft or to a cupboard - even if that means going through the wall to outside.

You just have to be logical about it and think outside of the box (or room in this case!).