Vaillant Combi & Hot Water Tank ... Possible? Yes but how best?

[TeachMeObiWan]

Hi,

I'm hoping from some sage advice, from you lovely people.

I have a Valiant Ecotherm 832 Combi boiler that was put in about 4 years ago. I'd just moved in, and the place had an old oil boiler that sounded like a jet engine, and an uninsulated hot water tank that was taking up tons of room. The plumber suggested the combi to replace the boiler and it would also let me get rid of the tank. I've been very happy with it, all is good.

However, the house also has PV solar panels, and was apparently using them to heat the hot water when they were generating, essentially free hot water. I hadn't realised this at the time, being new to solar, I just thought it reduced my electric bill a bit. Anyway, whilst it's not been an issue with the price of gas and electricity going through the roof I thought it might be a good idea to make the most of what I have, as essentially I only get paid for 50% of the electricity I produce and unless I'm at home in the day and using electricity the rest gets "wasted".

From what I've read on here it is possible to add a hot water cylinder to a combi, so that's good, but it seems people want to do it to get over flow rate issues to provide for additional bathrooms, that's not my issue. I have one bathroom and an ensuite, I'm more than happy for the combi to supply the hot water to the kitchen and ensuite, but as I'm doing the bathroom up I'd quite like to fit a power shower and a deep soak tub, both of which I feel would benefit from a nice large tank.

Circumstances mean the tank would either have to be on the same level as the bathroom (which is downstairs) or in the eaves just above it if I can find a shorter wider one.

So my question (sorry to ramble, but we got there) is should I be looking at an unvented system or an open system? Can I even have an unvented system on the same level? I've always known those as gravity feed but I guess they can be pumped? I have good mains pressure, so should it just be unvented on the same level, mains fed for top up with a pump for the power shower and a direct 22mm feed to the bath for a nice quick fill up? Also, any suggestions as to good well insulated tanks would be great.

I'd like to contact some plumbers once I have things a bit clearer in my head so that I can discuss it with them properly and see what they suggest.

Many thanks for any advice, suggestions or tips you might have.

PS. Happy to let the combi carry on handling the heating

 

[ericmark]

I have looked at the same issue, the problem is installation cost, the water tanks are not cheap. Also there is the time it takes for hot water to get to taps.

So where do you use the hot water, and where is the boiler and the water store going to be in relationship to the place where hot water is used?

In the main I use hot water in the kitchen, and if I fill a bowl then I can simply turn on the hot tap, but to half fill the bowl need to run some water off first.

A under sink water heater like this holds 7 litres, and I am sure one could set up a system where it heats water when solar panels are exporting, but the big question is by time you consider the cost to fit it, would it be worth it?

In the main people look at the battery packs, but the question is for how long will they save money?

Back in the 70's we had economy 7 central heating, not sure how they got away in calling it central heating, as in the main the heat was not centrally stored, but stored room to room, but the electric was around 1/3 of price with off peak, but today lucky to get it at 2/3 of price.

My point is for how long will we need a energy store? All it takes is for the government to change their mind and change the tariffs, and all the work you have done if for nothing. Or of course you get an electric car which uses the spare power.

Yes can be done, but is it worth it?

 

[TeachMeObiWan]

Hi Eric,
Thanks for the thoughts. Yes the installation isn't cheap, I'm thinking perhaps a vented thermal store, and that's likely to cost £1,000 on its own, bore fitting. However, it's a lot cheaper than a battery solution to store the excess electricity. I'm lucky in that I have a space directly next to my bathroom for the hot water tank so pipe runs would be very short, and as I say I'll keep the kitchen hot attached to the combi.
Cheers

 

[DP]

Use your boiler to supply hot water to often used hot taps and an unvented cylinder to be heated by the boiler and or the PV panels to supply water to other taps

 

[TeachMeObiWan]

Thanks DP. I'm a bit wary of unvented cylinders. I know if fitted correctly with all the safeguards they should be OK, but the potential for a dangerous explosion exists whereas it doesn't with an unvented system.

 

[oldbutnotdead]

The simple (and completely DIYable if you have the skills) solution is either a conventional vented 150ltr cylinder or go a bit bigger to 250 or 300 ltr if you have that much excess pv energy and a use for that much dhw.
The thermal store route again is diyable- if you keep it vented- but there's probably not much point in transferring the room heating to the thermal store since you won't get a lot of pv energy in the winter.
Installation wise the cylinder is just another radiator as far as the boiler is concerned. If you can, have separate 22mm feed and return to the cylinder (teeing off near the boiler) and use a motorized valve and cylinder stat to prevent incidental heating of the cylinder- also either fit wireless TRVs and a suitable controller for heating or fit another motorised valve to the room heating circuits
Preheating the water supply into the combi is a bit hit and miss plus you'd lose the high flow rates possible with vented (or unvented) cylinders. If you were minded you could do cunning things with a small plc and a couple of potable motorized valves such that if cylinder temp is above x then taps supplied by cylinder water otherwise taps supplied by combi. But that's more cost and complexity, probably best to keep it simple- kitchen on combi, bathroom on cylinder

 

[TeachMeObiWan]

Thanks for the advice OBND.
I much prefer the idea of an unvented cylinder, and it's something that I have fitted before so am happy to do that. Forgive me though I'm not quite clear about your suggestion to connect the combi to the cylinder. I currently have a Nest that decides if the heating comes on and each radiator has a TRV. Are you saying that by connecting the combi to the tank I'd get additional hot water heating if A) the cylinder stat enabled the motorised valve and B) the heating was on anyway? If the heating wasn't on and the cylinder stat opened the new valve then I presume the boiler wouldn't kick in as no pump would have been kicked off, is that correct. So it's a sort of backup if the conditions were right. Or I could of course just turn the heating on if I'd run out of water.
I appreciate the option to also switch tap supplies but I'm a great believer of the KISS method
For me the devil will be in the detail of how the immersion is controlled. If it's simply by thermostat then it will heat whenever it's required, but I may not want that, I may want to wait until the next days solar kicks in. I can use timers, dual immersions and a Solar Boost unit to sort that out or go the whole hog and have something like a Mixergy cylinder fitted that has controls up the wazoo and will accept heat input from practically everything short of a BBQ.
Anyway, thanks for the ideas.

 

[oldbutnotdead]

Couple of notes...unvented cylinders are not DIYable due to it being a pressurised vessel. Vented (gravity feed, open vent) are DIYable.
Heat control- if you have dumb TRVs then (without any other pipe valves) if the cylinder stat calls for heat some of the rads will get hot, specially if one is set up with no trv (usually in the room where the room stat is). You can now get several different brands of WiFi TRVs that allow you to zone your heating from your phone and pretty much eliminate the idea of a room stat. Mine are set up so rooms I don't use much get an hour's warm in the evening (when everything else is set to run) and then shut off except for frost low limit.
Advantage of having cylinder set up as a separate zone is much finer control over when it is heated by gas. You could set a timer for, say, 4pm for gas heat to be enabled- if solar has done its job then cylinder stat won't call for heat. You could use a current transformer on the supply to the imm heater (if imm is being powered by solar then inhibit gas heating). All sorts of options. Only thing to be wary of is the cost of all these mods pushes payback from 'free' power down the road- your boiler slinger did you a favour getting you off oil but no favour ditching the cylinder and tankage

 

[JohnD]

Heat control- if you have dumb TRVs then (without any other pipe valves) if the cylinder stat calls for heat some of the rads will get hot, specially if one is set up with no trv (usually in the room where the room stat is).

No, because you will install the cylinder as if it was a separate heating zone, with a motorised valve and a programmable room stat.

 

[DP]

Thanks DP. I'm a bit wary of unvented cylinders. I know if fitted correctly with all the safeguards they should be OK, but the potential for a dangerous explosion exists whereas it doesn't with an unvented system.

Not heard of one take off for the moon here.
The cylinder has three stages of safety on them
That is just the cylinder. The heat source, ie the boiler is never going to get to 100 degree C with multiple safety components on it.
One would need to bypass at least half a dozen safety components and keep the heating of secondary water going for the cylinder to get critical

 

[TeachMeObiWan]

Couple of notes...unvented cylinders are not DIYable due to it being a pressurised vessel. Vented (gravity feed, open vent) are DIYable.
Heat control- if you have dumb TRVs then (without any other pipe valves) if the cylinder stat calls for heat some of the rads will get hot, specially if one is set up with no trv (usually in the room where the room stat is). You can now get several different brands of WiFi TRVs that allow you to zone your heating from your phone and pretty much eliminate the idea of a room stat. Mine are set up so rooms I don't use much get an hour's warm in the evening (when everything else is set to run) and then shut off except for frost low limit.
Advantage of having cylinder set up as a separate zone is much finer control over when it is heated by gas. You could set a timer for, say, 4pm for gas heat to be enabled- if solar has done its job then cylinder stat won't call for heat. You could use a current transformer on the supply to the imm heater (if imm is being powered by solar then inhibit gas heating). All sorts of options. Only thing to be wary of is the cost of all these mods pushes payback from 'free' power down the road- your boiler slinger did you a favour getting you off oil but no favour ditching the cylinder and tankage

Totally agree, I wouldn't attempt an unvented system myself. I'll have a look at the WiFi TRV's though they sound very useful, cheers.