Combi with unvented using a couple of extra zone valves

[Andzc]

Hi forum,
This is my first post and having looked on the internet I'm not sure just how feasible what I want to achieve is but I thought its worth an ask before I ask a heating engineer face to face and come away looking like an idiot.
I'm buying a 5 bed house with 1 family bathroom, 1 downstairs cloak/washroom and a master bedroom en suite, plus kitchen obviously. I'm also planning to run underfloor heating with a greener heating system being added later.
At the moment I live in a 3 bedroom house which is supplied by a Baxi combi boiler, my partners house is a system boiler and cylinder.

What I want to do is use a combi boiler to run an unvented cylinder with 2 coils, I have seen some notes written on this, and I'm assuming this is ok as a kind of 's plan' with the flow and return going through one coil with the other coil to be used for solar or air source later. And zone valves for the coil and heating depending on timers and thermostat settings of heating/hot water.

Can a combi be used to heat an unvented cylinder and pass the relevant regs? Yes or no? I've seen that this can be ok but want to make sure.

Now the reason why I want to use a combi and not a system boiler is, as my partner can run the best part of a cylinder off in one shower I'm trying to introduce a back up system for the running off of the water. Not just an immersion back up which we would have to wait for.

On cylinders you can have multiple thermostats, so what I propose is if I set 1 thermostat at a 'lower limit' once the cylinder has lost some/all of its heat then could this thermostat switch over 2 zone valves, one shutting the cylinder supply, the other opening the combi hot water supply allowing normal use as a combi( normally open, normally closed on the thermostat).

I understand that it possibly isn't as simple as this or not possible and non returns and other zone valves might be needed, that's what a heating engineer would know.
What I want to know is,

Can this be done? If so why is it not done often?

Thank you in advance.
Andy.

 

[picasso]

Yes it can be done, the combi is hot water on demand so you dont have to worry about a valve for that, the zone valves are to seperate the cylinder/ufh/rads, its a good setup the only draw back is the combi can only supply the taps its plumbed for, you cant "switch" it to supply hot water to other taps if the cylinder runs out.

 

[swbjackson]

The easiest way to do this is to select a combi that allows the use of pre heated water for the domestic side. You then feed there are commercially available valves that will divert the water from the cylinder through a combi once it drops below a certain point. The normal use for this is to use pre heated water from a solar cylinder but no reason why it couldn't be done in the scenario you are thinking of.

Get your installer to speak to Atag.

 

[Andzc]

Ahh right, so what I'm asking isn't feasible in that the taps would have to be plumbed to the taps it's plumbed too only and you can't tee the two hot supplies together with zone valves controlled by a lower limit set thermostat.

Right so really apart from perhaps gaining a kitchen tap, that can be supplied from the combi anything piped from the cylinder wants its run the water off its gone.
Thanks.
Andy

 

[Andzc]

That sounds more like it, so as the temperature drops the valve recognises this and automatically switches the water from the cylinder to run through the boilers DHW system, heating the water up! Sounds a lot less complicated than my scenario
Thanks
Andy

 

[dcawkwell]

The best answer to this is use an ACV cylinder. This is a tank in a tank
and can take much higher heat input more than even some of the bigger
boilers can supply. It will nearly always heat up faster than you can use it
provided you boiler can keep up.