Power shower - can I feed an immersion tank from a combi?

[myxiplx]

Ok, this may well be a weird way to do it, any advice would be appreciated

We're planning an extension on the house and I'm thinking it would be a good time to take advantage and install power showers in both the main bathroom and the en-suite.

We just have a combi boiler at the minute, so I'd need to install a hot water tank / immersion heater in the loft. The problem is we've just come back from holiday in spain and the power showers were lovely, but ran out of hot water after the 3rd person. I'm a fussy bugger, so the last thing I want in my home system is any risk of running out of hot water.

So, I'm wondering if it's possible to feed the tank / immersion heater from a combi boiler?

With the power shower running I doubt the combi will be able to supply water fast enough to keep the storage tank full, but it's not going to empty in the time one person has a shower. The break between them having a shower and the next person starting should be enough to top it up, and there's then no risk of ever running out of hot water.

Is this a normal way to do things, would it work, or am I completely off my rocker?

 

[croydoncorgi]

If you put an indirect cylinder in the loft and fed it from the PRIMARY side of the combi (ie. as an additional HEATING load with a zone valve etc., in parallel with the radiators) that would work.

But I've a nasty feeling that's not what you meant. If you had some idea of feeding hot water from a combi to a DIRECT hot water cylinder, forget it immediately! DHW cylinders DO NOT work at mains pressure. An 'unvented indirect hot water cylinder' would be fed with COLD water from the mains, via a pressure control valve but sounds like overkill for what you're trying to achieve.

Another huge question mark is where the cold feed for the DHW cylinder is going to come from???.....can't be mains unless unvented. And presumably you have no cold storage tank because you have a combi.

 

[ChrisR]

Just pop a 300l unvented, mains pressure direct cylinder in the loft and heat it electrically at night.
Installation about £3000, new mains pipe out to the road maybe £1000.

Or a thumping great gas water heater instead of the cylinder. Hamworthy do a nice one, 30 litres/minute at a 45°C rise.

I'm being sarky, there is NO easy cheap answer

 

[myxiplx]

Ok, now I'm confused. This site:
http://www.tradeplumbing.co.uk/Cyli...l_c_455.html?gclid=CMmj5bKX6owCFRWOEgod00oe7w

Has unvented, mains pressure hot water cylinders starting at around £500. What's the problem with plumbing the feed for these into the combi boiler instead of to the cold mains?

It may well be overkill, but £500 to guarantee hot showers for the next 25 years is something I'm quite happy to spend.

 

[gas4you]

With some new up to date combis probably nothing as long as they are solar rated. Old combis or non solar rated ones do not like anything other than cold mains water going in on the inlet. What you are descibing is an Alpha Solarsmart system without the solar! If I understand you correctly

 

[myxiplx]

Think you've got me back to front. I'm feeding the tank from the combi, not the other way around.

 

[gas4you]

In that case use an indirect unvented cylinder and let the combi heat the kitchen and utility taps and the unvented supply all baths or showers

 

[croydoncorgi]

In that case use an indirect unvented cylinder and let the combi heat the kitchen and utility taps and the unvented supply all baths or showers

see above!

The way the Unvented HW Systems Regs are worded I suspect feeding an unvented cylinder with HOT water must be a no-no. As written, there HAS to be a 'positive disconnection' of the cylinder from the heat source once the set temperature has been reached. This is why you can't put an unvented cylinder onto a boiler Primary connection WITHOUT a spring-return Zone valve (3-port won't do). How this squares with two-coil types on solar systems (which are of course 'uncontrolled' and CAN get very hot indeed) I'm not clear. Maybe a solar expert here can explain what rules apply from panels to coil?

 

[Agile]

As far as I am aware the building regulations have not been extended to specifically cover Solar yet.

However, the two standard solar configurations take account of overheating and further protection is given by the cylinder installation which is covered in the BR.

This OP seems to think that he can just take the hot water output from a combi and store it in a cylinder.

Unfortunately he does not understand how a combi works and so he cannot see why that will not work.

What he could have is a vented or unvented cylinder heated by the boilers heat output with the appropriate controls and valves.

It never ceases to amaze me why people dont ask their local boiler engineer who would be fitting any new system.

Tony

 

[croydoncorgi]

...hmmmm
Never occurred to me that anyone would fail to realise that a combi only fires up when water flows - so at best the water in any kind of downstream storage would be tepid. Unless of course it's a proper setup like the Vaillant Ecotec 937 with a tank with a circulating pump to trigger the boiler as required.

 

[Agile]

It would work if he had an unvented cylinder with the heat topped up with an electric immersion element.

BUT....

Where it fails is the flow through the combi is restricted to about what it will heat so whatever he does he will not increase the flow above the 10-15 li/min his combi is designed to provide.

There no limit to the mad ideas some people come up with. They have these ideas and expect us to be able to implement them!

Tony

 

[myxiplx]

Aye, but at least I'll admit it's a mad idea

Haven't asked an installer as I wanted to sound the idea out here first to see if it was even vaguely feasible. I knew full well it's not a standard way to do things, but wondered what the actual problems with it would be.

Agile had it about right - an cylinder with electric immersion element was what I was thinking of, using the combi boiler instead of a cold water feed so the hot water in the morning would last longer

Where the idea fails is that it would need to be an unvented cylinder to work at mains pressure, and it looks like that means I have to be able to supply water in at the same rate it's going out. And that pretty much scuppers the whole plan. If the combi could supply water at that rate I wouldn't be needing the storage cylinder in the first place

I guess I'll have to stick with the traditional way of doing it & get a hot water cylinder that's big enough to cope with any expected demand.

 

[gas4you]

In that case use an indirect unvented cylinder and let the combi heat the kitchen and utility taps and the unvented supply all baths or showers

see above!

The way the Unvented HW Systems Regs are worded I suspect feeding an unvented cylinder with HOT water must be a no-no. As written, there HAS to be a 'positive disconnection' of the cylinder from the heat source once the set temperature has been reached. This is why you can't put an unvented cylinder onto a boiler Primary connection WITHOUT a spring-return Zone valve (3-port won't do). How this squares with two-coil types on solar systems (which are of course 'uncontrolled' and CAN get very hot indeed) I'm not clear. Maybe a solar expert here can explain what rules apply from panels to coil?

I never meant to imply that the hot oulet from the combi would fill the unvented cylinder I meant that it is heated indirectly as would be on a heat only boiler

The twin coil solar cylinder does not receive the hot circulated 'water' that comes direct from the panels! It is done via a pumping station that is temperature controlled by demand with 2 ntc's on the cylinder and 2 on the panel.

I do see your point though as you would have to have the 2 port valve on the flow from a boiler.