Adding unvented tank and diverter

[dave1967]

Morning

Looking for help regarding this. Still paying. Approx £100 per month gas just for showers. Have 2.5kw pv and summer months we seem to have about 80 to 100 kw/ month going back to the grid. Looking for thoughts on tank sizing to maximize savings. 

Thank you for any advice.

[ProDave]

How is your hot water heated now?  I am guessing combi boiler if you don't have a HW tank?

 

What are you expecting?  To stop using the combi and just use HW from an UVC?  Or fit a pre heat tank to soak up some of the surplus PV (some combi boilers can take pre heated water in, but someone else will have to advise if yours is one)

 

That's only about 3kWh per day average  going spare, that on it's own won't do all your DHW needs.

[dave1967]

Hi Dave. Just looking for a preheat boost to a new viesmann 100 combi.

[S2D2]

1 hour ago, ProDave said:

That's only about 3kWh per day average  going spare, that on it's own won't do all your DHW needs.

To offset what, 31kWh of gas per day? Sounds like you need to limit the shower flow rates first!

[dave1967]

Approx 20kw per day.

[Nickfromwales]

4 hours ago, dave1967 said:

Hi Dave. Just looking for a preheat boost to a new viesmann 100 combi.

You could also use the immersion to heat the cylinder off cheap-rate electricity (overnight Octopus etc) if available? Cheaper than gas is great, but even better is going same price but zero wear and tear on the combi too!

[dave1967]

Cheers Nick. Any idea on tank size and if pre heating for a viessmann 100 is ok. 3 females in the house and dont like to be rushed.

[JohnMo]

2.5kW of PV will not heat a big cylinder much, so you may be better going small say 50L.

 

Alpha boilers do a cylinder set up out the box for this. An Alpha Superflow 50 and 25. You just need to add a 230v 1-1/4" immersion and your diverter. 

 

Some bedtime reading and images

 

Combi-SuperFlow-White-Paper-v1-2-4.pdfCanetis-SuperFlow-Product-Sheet-WE-050318.pdf

FlowSmart Inst & Serv Inst 17-6-08.pdf

[Nickfromwales]

5 hours ago, dave1967 said:

Cheers Nick. Any idea on tank size and if pre heating for a viessmann 100 is ok. 3 females in the house and dont like to be rushed.

 

It's down to available space tbh, but I'd say it would be useful to know the possible tariffs p/kWh before deciding on capacity. You're burning natural gas which is reasonably efficient and cheap enough (you could be on LPG for eg) so this is just down to the amount of DHW that you go through. Typically I'd say a 210-250L tank would be suffice, but if the maths can be swung by a good cheap rate of electricity, then you could be up at 300-400L with dual immersions without being silly. Not much uplift for the larger cylinder and the installation costs should be pretty much like-for-like.

Viessmann tech support will confirm, I do little to nothing with gas these days sorry and even less around those particular boilers. I was a Baxi and Vaillant man I'm afraid.

[dave1967]

On octopus standard rate at the minute, could go for agile with them. Also getting 68p per kw from 2011 tarrif from pv

[Nickfromwales]

9 minutes ago, dave1967 said:

On octopus standard rate at the minute, could go for agile with them. Also getting 68p per kw from 2011 tarrif from pv

You'd need to do the maths meticulously to be honest. Tariffs seem to be starting to favour the consumer these days, and I expect that to continue to improve.

If you install any cylinder you'll benefit immediately from excess diversion to DHW, so the question then is how much bigger do you go to get maybe 24hrs of DHW from an cheap nights (or days) grid juice. If you're able to track agile and use that to your advantage, then a dual immersion setup will allow you to rapidly heat the cylinder from much shorter windows of low rate availability. 

I'm assuming you'd need to buy the diversion controller too, so you'll be happy to know that the My Energy Eddi will give proportional power out to 2 loads, consecutively. So you could heat the upper immersion first, and then the lower one after that has been satisfied. They only do 1x 3kW throughput over 2 outputs, but not simultaneously. A relay card can be added but that is 100% on or 100% off, vs the proportional output from the factory offering. 

[dave1967]

Thanks Nick. Plenty to think about there.

[JohnMo]

Cool energy diverter is also worth a look, it comes with a built in timer function and everything else Nick explained above for the Eddi and is quite a bit cheaper.

[Nickfromwales]

14 minutes ago, JohnMo said:

Cool energy diverter is also worth a look, it comes with a built in timer function and everything else Nick explained above for the Eddi and is quite a bit cheaper.

Thanks. I'll have a look at those for my house if the price is better.

[JohnMo]

Cool energy have an outlet store the prices are cheapest there. I paid £299 delivered.

[JohnMo]

Doesn't have the same WiFi features but...

[Nickfromwales]

24 minutes ago, JohnMo said:

Doesn't have the same WiFi features but...

All I need is dumb, proportional output. I'll have a look to see if it's a single or dual 'channel' as per the Eddi.

[dave1967]

Viessmann say preheating pre boiler is a no so need a new plan. I am not sure of the benefits of having an unvented tank after the boiler.

[JohnMo]

Have you read the alpha boilers document? They only allow water at 30 degs to go to the boiler via a mixer valve.

 

Now think about a hot summers day, water at well over 20 can enter the boiler with no ill effects. Your heating system water does the same. Just use a mixer valve to control the temp to sensible limit. So you save and also get an uplift in DHW performance all year round.

[Nickfromwales]

31 minutes ago, dave1967 said:

Viessmann say preheating pre boiler is a no so need a new plan. I am not sure of the benefits of having an unvented tank after the boiler.

OK. You can turn the existing combi into a system boiler by adding 2-port zone valves to the CH output of the boiler. One goes to space heating (CH) and the other would heat an UVC. The DHW output of the combi would just get redirected to the least / shortest run outlet in the house, cloakroom WC for eg, and then the rest of the house comes off the UVC. Then you'll have much better showers, and somewhere to dump your solar excess. Probably a 150L to 180L cylinder would suffice, as gas re-heats an UVC PDQ  

[Nickfromwales]

3 minutes ago, JohnMo said:

Have you read the alpha boilers document? They only allow water at 30 degs to go to the boiler via a mixer valve.

 

Now think about a hot summers day, water at well over 20 can enter the boiler with no ill effects. Your heating system water does the same. Just use a mixer valve to control the temp to sensible limit. So you save and also get an uplift in DHW performance all year round.

If the combi was a retro fit then there may be a 22mm hot pipe there to tag the UVC on, and do as I just suggested above.

The Caleffi valve does what you state, but it's a few hundred pounds. I used to fit them to the Sunamps / Therminos so they could pre-heat any combi. Some manufacturers capped incoming temp at 25^oC, others 30^oC, and some just stated chuck in what you like!

The downside to tempering at that low a temp is that you get very little reward from what will be a 4-figure upheaval. Going to a system boiler config and taking the water from the UVC > outlets will give the highest rewards for the lowest outlay. You can get most sizes with dual immersion, but the smaller you go the more likelihood there could only be 1x installed due to space limitations inside the vessel. At 210 - 250L I think dual immersion will be fine. Ask Trevor at cylinders2go and mention my username and the forum and he will look after you with a good price on the cylinder.

[dave1967]

_{Thank you both for this.}