Best UVC for ASHP and solar PV divert

[Bramco]

Apologies if this has been asked before and if it's in the wrong category, but our M+E consultant is recommending a Vaillant ASHP which I'm fine with but when I look at the Vaillant UVCs, I'm concerned that they won't get the best out of the solar PV diversion.

 

Most solar PV diverters have two outputs, so that once the first output has stopped accepting energy the second output is activated.  So if you are doing what most people do, diverting to an immersion in the UVC for hot water, you'd ideally want to have 2 immersion heaters in the UVC, so 1st heat the top of the tank, then when that's hot start to heat the bottom of the tank.

 

The Vaillant UVC has one immersion in the middle of the tank - so only the top half would benefit from the solar diversion. An alternative would be say the Gledhill StainlessLite Heat Pump cylinder but that only has a single immersion heater point which is at the bottom - theoretically better because on a good day you could heat all the tank but not on a marginal day as you'd only warm the whole tank up a bit.

 

Anyone know of a cylinder suitable for ASHPs with 2 immersions - middle and bottom?

 

Thanks in advance for any help.

 

Simon

 

 

[ProDave]

Telford Heat Pump cylinder here with single immersion heater near the bottom of the tank.

 

Even in summer ASHP heats the tank to 48 degrees, surplus solar PV takes it hotter.

[PeterW]

Telford. 
 

Contact Trevor at Cylinders2Go, mention the forum and ask for a Telford Heat pump cylinder. Then spec it with high and low immersions (mine have two) and all the pipework where you want it.  
 

If you’re going ASHP go for 400 or 500 litre if you can as the cost difference is negligible. 

[Bramco]

@ProDave thanks, that's the route we were heading down.

 

@PeterW  also thanks, especially for the contact at Cylinders2Go.

 

Not sure though why you would want such a large cylinder - there will normally only be the 2 of us and it seems wasteful (and expensive) to have the ASHP heating 4 or 500 litres in the winter when the PV won't be delivering much.  We were thinking of a 250 litre tank with maybe a couple of Willis heaters in parallel for the rare times that both sons are home with their partners. Under normal circumstances you wouldn't need the Willis heaters and 250 litres should be enough for the 2 of us.

 

Simon

 

[PeterW]

8 minutes ago, Bramco said:

@ProDave thanks, that's the route we were heading down.

 

@PeterW  also thanks, especially for the contact at Cylinders2Go.

 

Not sure though why you would want such a large cylinder - there will normally only be the 2 of us and it seems wasteful (and expensive) to have the ASHP heating 4 or 500 litres in the winter when the PV won't be delivering much. 

 

 

 

 

OK - was on the basis of a 4 bed / 2 bath shower type setup and people wanting lots of hot water. Losses are small and they are into the fabric of the build so it doesn’t make much odds so I would go with a 300 litre which I think is about the smallest you can get a large surface heat coil and two immersions in to. 
 

8 minutes ago, Bramco said:

with maybe a couple of Willis heaters in parallel for the rare times that both sons are home with their partners.

You’ve got a pair of immersions in the tank - just use them to boost. No Willis required. 

[Bramco]

26 minutes ago, PeterW said:

You’ve got a pair of immersions in the tank - just use them to boost. No Willis required. 

 

Duh!   Good point!!

 

Simon

[Spoogster]

New build developers are aiming for 110 litres hot and cold water per day per person. Surely on this basis 200-300 litres would be more than adequate and not wasteful.

[PeterW]

4 minutes ago, Spoogster said:

New build developers are aiming for 110 litres hot and cold water per day per person. Surely on this basis 200-300 litres would be more than adequate and not wasteful.

it’s a 3 or 4 bed house so catering for 2/1/1 would mean having hot water that can cope with 4 showers x 8 minutes at 12l/min so you will need all of that 250 and more. ASHP can’t recover a tank as quick as a gas boiler so upping the size is negligible. 
 

The “waste” is an additional 7W/hr between 250 and 400 litres so 168W/day or £9/yr assuming direct electric. Heat would also be lost into the house so would directly reduce the heating demand of the house by the same amount. 
 

Developers work to a bare minimum to maximise profit - I would never use a developer house as a benchmark to aspire to ..!!
 

[SteamyTea]

2 hours ago, PeterW said:

168W/day

That will be 4 kWh/day if it looses, on average 168W.

That is £45/year at 3p/kWh.

 

(168W/day is 168J/s/day. A day is 86400s, so that would be 0.00194J)

Edited March 27, 2021 by SteamyTea