Combi or Unvented or Vented HW system?

[outmyway]

I seem to be going around in circles with quotes and advise from various plumbers re. combi / unvented / vented HW.

Property = 4/5 bedroom 3 bathroom house.

Trying to loose airing cuboard to recover space to bathroom- or at least some of it - currently has header tank and vented cyliner.

I'm confused Re. Combi Vrs Unvented flow rate / pressure.

Am I up against the same flow rate/pressure issues with an unvented cycliner as I would be with a combi. Both can only be as good as the mains supply?

Is it safer and simpler to just stick to a vented cylinder and keep existing shower pumps? Or are unvented in anyway more efficient?

 

[ianniann]

Unvented cylinders do not offer any fundamental efficiency improvements over a vented cylinder, although a new system may well offer better insulation and better controls. Presumably you'll be doing a boiler replacement will should be more efficient, although it is difficult to get accurate information about the efficiencies of water heating systems.

Presumably you have a pretty good idea of the merits of a vented system. Stored pre-heated hot water, nice warm airing cupboard, and good flow rate on the showers once you've fitted a pump.

Unvented cylinders store hot water at "mains" pressure, limited in practice to 3 bar (more?). They have some of the pros and cons of an unvented cylinder. Water must be heated in advance and only a limited amount is available. Although you can lose the cold water header tank, unvented cylinders are often larger than the cylinders they replace. You keep the high flow rates that are possible from an unvented cylinder, but you get 3 bar without needing a pump on your showers. Unvented cylinders are becoming popular combined with solar water heating. Installation is somewhat specialised and since they are relatively new you might find that many fitters don't have the expertise or experience and so won't recommend them even if they're right for your situation.

You wouldn't generally put a combi in a three bathroom house unless you are very sure that the hot water usage pattern will be suitable. In short, one shower at a time and ideally nobody else using hot water during the shower. Think very carefully if you are going to need two showers at once or even a shower and other hot water use, because most combis won't do that very well. Possible workarounds include an electric shower in a bathroom that may not be used often, storage combis with small internal hot water tanks, multiple combis in very large houses, etc. The advantages of a combi boiler are no tanks or cylinders at all, unlimited hot water always available "immediately" (within seconds), and mains pressure hot water so no shower pumps. The only real disadvantage is that the flow of hot water is limited to about what one good shower needs (depending on water temperature and boiler size). You can't use a shower pump with a combi, partly because the water is already at high pressure and partly because you just can't increase the flow rate beyond the boiler capacity. Some people don't like the idea of not having a tank of water on hand in the case of emergencies. Some combis can accept warm water inputs for compatibility with solar water, but most won't.

 

[outmyway]

Thanks ianniann - very detailed reply.

I am having the boiler and whole heating system replaces (pipes + rads, replacing old single pipe system).

I had decided on a combi - had one before in a 3 bed house - I had one plumber measure the flow and pressure who said it was OK, but yesterday a plumber coming over to quote for a combi looked at my kitchen cold tap pressure and aid 'think carefully' I have been looking at the Worchester High Flow 550's which 'could' supply 30ltr/min.
He is Worcheser/Bosch approved, and probably has a lot to gain by recommending/selling me this boiler - I appreciated his honest opinion, he said look at unvented ... then get back to him. Very 'non hard sell'!!

I am now concerned that if I fit an unvented and have 2+ showers running that it I wont get what I would get with vented and pumps? Is an unvented only as good as the rate and pressure it can be refilled? So if lots of taps are on the pressure and flow is shared, and ultimatley reduced at each tap?
ie. If the unvented cylinder is draining at a quicker rate than it can be refilled then I will loose out? Or they keep the mains pressure as they start empty?

I saw thse combined unvented cylinders, which would save me airing cuboard space (currently has header tank to one side taking up lot of space).
http://www.plumbnation.co.uk/site/n...nted-foam-lagged-copper-hot-water-cylinders-/
Seems an intersting space compromise?

I have considered solar/unvented - My roof is ideal, very shallow apex and south facing, but dont think its worth the investment for teh cost of heating water with gas.

 

[ianniann]

The WB Highflow boilers are storage combis. They maintain a small internal tank of hot water and then also heat as you draw water. The combination of the two sources can provide the 30l/min figure, but once the tank is empty it drops to less than half that. The tank is 50l I think and may empty in as little as 5 minutes if you draw flat out so think whether that is going to work for you. After that it is simply a typical combi of around 30kW.

Any pressure loss while using an unvented system will depend largely on the pipework. Just as your neighbour having a shower doesn't (shouldn't!) kill your mains pressure because there are nice fat pipes, one shower running from your pressurised tank shouldn't impact the pressure available for another shower providing it is installed properly. There also shouldn't be issues with maintaining the contents of the cylinder although eventually it will be full of cold rather than hot.

The link you gave shows a combined vented system. Is this what you intended? I'm not familiar with those, but that's not a lot of stored hot water if you're thinking of drawing two showers from it.

 

[outmyway]

If a combi and an unvented cylinder has a 22mm mains supply to it wouldn't they both run out of 'puff' at the same time?