Advice needed on system with old combi

[JonB]

Hi,

I'm planning on putting in a system later this year / early next, but have a few complications.

1. A 10 year +  Valiant combi in the attached garage runs the hot water on incoming mains.
2. Hot water feeds up into the bathroom, joins the old pipework, feeds the bathroom and drops into the kitchen.
3. Where the airing cupboard used to be has the pipework underneath for back boiler etc. which I want to use with a back boiler from a woodburner once I've sorted the solar.
4. The support is in the loft for the old header tank, but no tank or pipework left.
 

My plan would be to T off the CH circuit from the boiler and have a 3 coil thermal store (this should get around the header tank problem?) Have the HW from the boiler running to one tap in downstairs loo.

Would appreciate feedback on this plan, and any advice from people who've done something similar.

Thanks

JonB

[Ivan]

Yes,  you should be able to turn a combi boiler into a normal boiler by just using he CH output and a three way valve and standard CH controller to operate it +boiler.

Or you can preheat the water going into the boiler, if the manufacturer allows it.

[ajstoneservices]

Its better to use an S plan rather than the Y plan when when using the combi to heat a cylinder. If you do choose the Y, then you will need a relay as well because most combi's switch volt free.

Tony

[David]

Have the HW from the boiler running to one tap in downstairs loo.

To minimise heat losses pipe runs should be as short as possible from the heat source. Depending on the pipe runs it may be better to keep the kitchen hot water supply from the combination boiler too. This may not be ideal though, as the kitchen tap is the most used one in most houses.

[JonB]

The kitchen HW comes off the bathroom supply, so it would no further from a thermal store.
I've emailed Valiant to see if the boiler is solar compatible (not holding my breath)

It seems my thinking is on the right lines - just the hard work to do (or pay someone else to do it)

Thanks for the help

JonB

[browbeaten]

One approach to the problem is a simple design (not yet implemented) under the thread "SFD in parallel with dreaded combi....

I am having a problem finding a thermostatic mixer valve that will blend down to 20/25 degrees -most combi's do not like high input temps ( there is a link to list of manufacturers detail as well)  so if you have space for the Thermal store it is better 

[Ivan]

I doubt you'll find a mixer that works down to 25C! It really isn't going to offset much of your hot water energy at that level. Thermal store/SFD in parallel is a much better idea.

[rob26440]

This TMV will adjust down to 25C.  It's a 28mm job.  Horne make smaller ones but they only go down to 35C  (I think).

http://www.horne.co.uk/Documents/H25Brochure.pdf

Rob.

[Ivan]

Wonder what it's used for? I can imagine down to 35C useful for underfloor heating, but 25C?

[rob26440]

Good point.  25C is very low.  I'm using one wound up to 50C (the max) on my DHW.  Terrific flow with v. low head (about .2 Bar).  I might give them a call on Tuesday and ask them the reason for the 25C setting.  They are based in Scotland .  Maybe they are hardier up folk there and like to keep it cooler.  Or maybe they just want to keep a tighter control on the pennies.....   

[browbeaten]

Perhaps I am trying too hard, the 25^o mixer was intneded to make use of the solar input on (spring/ autumn) days when the SFD does not get up to the 43^o needed for DHW. 

[rich21044]

I'm thinking about adding solar (SFB based system probably) to an existing combi system. In this case its a Oil fired 2005 Worcester Bosch Heatslave. From what I understand of the operation of the heatslave it already contains a heatstore through which mains pressure water is heated, the boiler just maintains the temperature of the heatstore and feeds the CH when needed.

I think all I need is a pressurised tank with a single solar coil so that I can preheat water before its passed through the boilers heatstore. A TMV on the output of the tank to ensure that the heatstore input temperate doesn't exceed say 65C and some sort of heat dump arrangement if the tank gets too hot. I've sent an email to worcester bosch to find out if I can feed hot water through the heatstore but haven't had a reply yet. I may use a double coil pressurised tank because I'm also toying with the idea of a woodburner with back burner.

Is it really that simple of have I missed something?     

[rich21044]

Just had a call from Worcester Bosch tech dept (on as Saturday?) in response to my email. Looks like it was too simple in that I cant just put the hot water from the solar tank into the Heatslaves store. I didnt ask why and after i put the phone down I wanted to know why because its really just passing hot water through a heat exchanger with potentially hotter water in it. I expect its more to do with having to ensure that the boiler heat exchanger is not heated by the solar tank water beyond limits and warranties etc . He suggested that an alternative is to run a single tap off the boilers heatstore (presumably to avoid dead legs - legionella etc) and use the CH connections as an auxilliary heat source for the solar tank on a second coil. I don't think this is going to make best use of my solar energy so are there any alternative schemes?

[Loir]

One of the problems with Combis and solar is that many boilers have the odd bit of plastic on the mains water input that might(!) melt if the incoming water temp is too high. This may be that it is just out of their specification, has not been tested etc. It may(!) not be a real risk but you need to be certain.

You mention a mains pressure tank as a possibility. Do remember that there are quite rigorous regulations about mains pressure tanks; they must be fitted by a qualified person and must be checked by such a person annually.

You can use a low pressure heat store with multiple coils; solar at the bottom, then boiler and wood burner. If you want mains pressure water then another coil at the top to run mains water through or a heat exchanger on the side. Various ways to do it!

Loir

[rich21044]

I pretty much discovered the problems with mains pressurised tanks today -if its DIY it has to be a vented system.