Gravity fed combi

[europhil]

I'm trying to figure out a problem for a guy with solar heating that connects through a cylinder and then to the DHW inlet at his combi.
The warm water from the solar panel is a closed circuit which warms the water in the cylinder through a heat exchanger, the water in the cylinder is static, it warms up, then there is a second heat exchanger that's mains fed to the DHW feed to the combi.
I'm thinking that it would run more efficiently if i fed the combi by gravity using what is static water at the moment, therefore doing away with the second heat exchanger.
i know it looks pretty complicated, so let me ask the simple question.
would the DHW on the combi work ok gravity fed rather than mains fed?
Other than that it'll possibly be an unvented cylinder setup.
i'm looking at all the options, so any suggestions would be appreciated.
thanks.

 

[cr0ft]

Are you a plumber?

 

[lame plumber]

doesnt sound like it, the set up described sounds reasonably installed and safe and best to leave well alone.

 

[snowhead]

Google Thermal store

 

[hammers4spanner]

probably not a combi more likely system

 

[lame plumber]

main thing is it sounds like a well set up thermal store whatever the boiler and needs to be left as is imho

 

[hammers4spanner]

question for the OP how much more efficient do youy think you can make it??

 

[Whyme]

Am i being thick, i dont understand the 'problem'.

 

[hammers4spanner]

looking to make system more efficient

 

[Whyme]

Mmmm good luck.

 

[europhil]

Yes i am a plumber.
I'm trying to make the system more efficient, and looking for suggestions.
Yes it is a thermal store, a very inadequate one.
This system should as it is, isn't combi friendly.
After spending a fortune having the system fitted, he was told by the installers that it should be ok as it worked in spain. other plumbers have been and can't figure out how it works, others just don't turn up.
I have an engineering background, it maybe as a result of that, that i like to figure things out and on occasions redesign an over engineered product/system.

 

[Kev the gas]

What combi is fitted?
Min operating pressures vary, from 0.2 bar up to 1.2 bar just to get the boiler to fire up.
Historically combi boilers are designed to give a 35 degree temp rise, send water thro a combi @ 25 degrees and it will get confused! As it is expecting it to be around 10/12 degrees.
Worcester say their boilers are not suitable for this type of system.
Best solution in my opinion, have the cylinder replaced with a solar twin coil unvented cylinder and repipe the combi as a system boiler, with zone valves ect.
You will need G 3 quals for this tho.

 

[hammers4spanner]

the thermal stores i have installed have not had any boilers fitted ,,,,,confused !

 

[cr0ft]

As Kev said, combi-boilers are not designed for hot water coming in the cold feed pipework to the boiler. Whilst it 'may' work it's certainly not recommended by the manufacturers. This assumes that the coil supplying the combi is at mains pressure. If its gravity fed from a tank then forget trying to run a combi with it unless the tank is at least 10 metres above the combi boiler. Even then, not likely to work properly and the combi-boiler will most likely spit it's dummy out of the pram if the tank empties or anything like that.

Any efficiency savings you could get by using the water inside the cylinder to feed the combi would be so minimal they wouldn't really be worth considering, certainly not worth wasting your time trying to use that.

One option you may want to consider (assuming it's a standard dual coil solar HW cylinder is to actually use the HW inside the cylinder for the house hot water. Re-pipe the combi as a system boiler (i.e. no cold feed and DHW pipework) and use that to heat the cylinder like a traditional system boiler with hot water cylinder. You will have to check with the manufacturer on whether there boiler can be used this way. I know some combi-boilers (Broag being one) that will work fine with just gas, flow and return connected but I do believe that some combi-boilers don't like this setup at all.

If you can do this it would obviously save having to buy a new system boiler and it would be the cheapest proper solution for the customer.

I will caveat this with if the thermal store is unvented then you definitely should not be drilling into it and adding new tappings and you will need your unvented ticket to work on it legally. All of which I'm sure you know!

 

[europhil]

thanks for the replys chaps.
I went for the easy solution to see what happened. I fitted a pressure reducing valve before the blending valve as too much cold was getting into the hot water, He's tried several blending valves and they've all failed after a very short time.
Worcester have checked the job, and they're happy with the setup.
They are also happy with the warm water feed going into the cold water inlet on the combi. which was pretty suprising.

 

[hammers4spanner]

Arnt you supposed to fit prv on the cold supply into thermal store anyway??

 

[stevetheplumber]

no prv needed with a thermal store unless you have excessive preasures mine works great with up to 4 bar hot water

 

[stevetheplumber]

feeding warmed water into a combi is just what the new vaillant device does name of which escapes me