Solar PV but no immersion heater - dual tank options

[Chris X]

Our house has a solar PV array and in weather like this will generate 24kWh per day most of which we don't use.

 

For hot water we have a gas boiler heating the 190 litre indirect vented tank - but there is no immersion heater.

 

To eliminate gas usage during summer months I can think of a few options for either replacing the in-place tank with one that has an immersion heater (ideally at the bottom - as those top mounted ones only seem to extend half way down) or providing a secondary feed tank - with immersion heater(s).

but cost is a big factor - even with current energy prices the potential saving is only perhaps £150-£200 per year so £1000+ to get a new tank fitted doesn't seem so attractive.

 

I'm thinking more about self fitting a secondary cylinder in utility room downstairs?  Something from Ebay could be in the £100-£200 range plus similar for fittings.

 

But then what would be best option for plumbing it in.  Serial or parallel?  Other options?

 

By serial I mean take cold water feed down to the bottom tank and then connect top of this new tank to the bottom of the existing one.   The immersion preheats the bottom tank and the existing tank arrangement is largely unaffected.  Gas heating might still be needed for top up or if the weather takes a turn.

I like the idea that the hot water capacity might be doubled - 24kWh should heat more than a single tank - so this capacity could give some buffer against a following cloudy day - but only if the heating effectively extends to more than a single tank.

The biggest negative I can see for this arrangement is that there is no loop for gravity flow of hot water into the top tank.  If the top tank is cool and the bottom is hot can the water flow up (and down) effectively through the single pipe?  I imagine not ... perhaps others have advice on that one.  Of course it will get drawn up as soon as the power shower is turned on - with potential scenario that the water in the shower gets hotter the more it is used.  Which would take a bit of getting used to ...

 

By parallel I mean connect top to top and bottom to bottom.  Now there is a clear circuit for gravity flow - so if bottom tank is hot then the colder water from the bottom of the top tank will be pulled down to displace it.  Do I then just have a really big Willis heater?  And best to leave cold water feed to top tank? (Also I'd put a none return valve on cw feed.)

Is there less water mixing in this approach - so risk hot water feed would be too hot? 

And when the shower is on would it draw water from both tanks at once?  But can't see that would be an issue ...

 

Would like to hear views on either of these options or indeed other ideas - so thanks in advance.

 

[JohnMo]

All you need is a Willis heater. It's an immersion designed to add on to cylinders that don't have immersion, ties into the inlet and outlet piping.  Have search on Google (or other search engine)

[SteamyTea]

As @JohnMo says, fit a Willis.

 

I think you mean thermosyphoning rather than gravity, which generally relies on large diameter pipework.

 

As a general rule, you put a PV powered element low down in the cylinder, that way, even when there is not enough PV to fill the cylinder, you at least get something.  That can help offset gas usage.  I think you can get extra long immersion elements, that one is a yard long, to overcome the problem, others may know better.

 

Worth noting that the temperature in a cylinder is a variable gradient, once settled, after heating, it is not stratified, which implies that there are layers at a fixed height and a fixed temperature. Generally the base is close to room temperature and the top is higher, sometimes higher than the thermostat set point.  Secondary point is that DHW needs to be hot, no one wants a luke warm shower or bath.

Edited May 23, 2023 by SteamyTea

[Chris X]

1 hour ago, JohnMo said:

All you need is a Willis heater. It's an immersion designed to add on to cylinders that don't have immersion, ties into the inlet and outlet piping.  Have search on Google (or other search engine)

Yes  - seems like a good idea and I did read up on this and downloaded a data sheet.  But got put off as it clearly says not for use in hard water area and implied it was a sealed unit so couldn't easily swap out the element.  Also the current pipe work is very tight for space and that wouldn't help access for replacing - although I think they can be mounted lengthways which might help.

[Chris X]

Hi @SteamyTea.  Thanks for those points and new terminology.  Most new tanks seem to have 22mm connections but I guess it might help the thermosyphoning to convert to 28mm for the length of the connection.

[JohnMo]

Willis - Have a look on eBay, you can just buy the casing, so you can fit any elements you want.

 

[TonyT]

check with you boiler instructions as some boiler can only accept pre heated water on the inlet to a certain temperature, 

[Chris X]

28 minutes ago, JohnMo said:

Willis - Have a look on eBay, you can just buy the casing, so you can fit any elements you want.

 

Ah that sounds much more convenient.  I can find an element rated for hard water.  And to make space for maintenance I'm thinking this/these could live downstairs in the utility area where I envisaged a second cylinder might go.

This does mean they would be well below the tank level  - which is against the installation instructions I have downloaded.  I presume this is to avoid thermo-syphoning hot water into the cold water feed?

Could a non return valve on the cold water feed eliminate that issue?