Converting CH to C plan or something else ... a few questions

[The Night Tripper]

Hi all I have a plan ... a yen really ... to improve our CH heating system. It currently has a 4-port back boiler running a gravity feed HW, pumped CH system. I installed a Sunvic select (207, I think) programmer a while ago, and replaced the pump with a Grundfoss when the original died.

We do suffer from lack of HW after someone has had a bath etc. I don't want to change the back boiler, even though I realise it's old-skool.

I have in mind to either:

a) add a Cylinder thermostat and 2-port valve, and convert to C-plan. The valve will have to be on the return from the boiler, there's no room in the flow pipe. b) add a mid-position valve and convert to Y plan (no zoning envisaged).

I've read lots about doing this sort of work on the archives of this fine newsgroup, and on the wiki. I've also identified all the pipes in the current arrangement and understand the need for siting the valve with respect to the flow & return to the F&E tank etc. I do still have a few questions which might help me decide the route to take. Answers gratefully received.

1) with my devil's advocate hat on, would there be any real benefit of a C- plan over our current arrangement?

2) Y-plan seems to be talked of slightly disparagingly in comparison to S- plan, due to reliabilty problems, difficulty with problem diagnosis, and lack of zone expansion capability. Given that I don't intend to have any zoning, are the other problems real (eg. these days)?

3) If I were to change to Y-plan, I understand that I'd need to cap off part of the boiler. As I mentioned, the back boiler has 4 ports - 2x22mm (currently CH), and 2x28mm (currently to the heat exchanger for HW). Is this just a matter of capping off the two 28mm pipes (and moving the feed and vent pipes) ?

3a) I assume that the reason that the pipes from the boiler are different diameters for the two circuits is that the larger 28mm dia. gives less resistance and better allows the HW to be unpumped - is that right?

4) what, exactly, should I be looking for spec-wise in a gate valve, apart from the pipe diameter? I have a part number for a 2-port Honeywell valve but I'm curious about what the spec actually calls for. There are a lot of valves for sale on eBay ...

Thanks a lot in advance

regards Jon N

[The Night Tripper]

Slight correction to my posting:

The valve will have to be on the *flow from* the boiler, there's no room in the *return* pipe.

[John Rumm]

Faster water reheat probably, plus stopping the water getting too hot - however on the speed front the hot water tank may be a limiting factor - many can't swallow heat at more than about 5kW using their indirect coil. Obviously changing for a fast recovery one or using a PHE could change that.

Not really. Y plan with mid position makes sense with a relatively low input cylinder. W Plan (i.e. three port diversion rather than mid position valve) can make sense with a very fast cylinder.

Usually yes. You can get plugs to screw into the sockets usually. With most 4 port boilers there is no real difference between the two sets of ports.

Yup, and the bigger mass of water makes for more effective convection.

One that matches you intended design...

[YAPH]

It would stop the hot water getting too hot, but wouldn't make it heat up any quicker.

Y-plan is actually a special case of 2 zones: CH and HW. As for reliability there must be millions of them in service, and 2-port (S- plan) valves fail too.

gate valve? you mean 3-port valve? should be some blather about it on the wiki.

Your other option would be to put a pump on the HW side, but that would require some savvy with the controls wiring: specifically, using a relay (or two[1]) to wire-OR the CH and HW demand signals to control the boiler. You'd probably also need to change the pipework to get the feed and vent pipes close together to prevent pumping over with your newly pumped HW circuit.

[1] it can be done with one relay but easier to understand with two

[Tim Lamb]

Also consider the need for the cistern to act as a heat dump if using solid fuel. IANAP:-)

regards

>

[Roger Mills]

The advantages are that the HW temperature will be controlled - so you can run the boiler hotter for the CH if desired - and you will have "boiler interlock", stopping the boiler cycling on and off when all demands are satisfied.

However, since it will still be a gravity HW system, the water won't get hot any more quickly.

You pays your money . . . Personally I prefer S-Plan but Y-Plan is ok. It's just a bit more "all your eggs in one basket".

You'll need to have just one flow and one return to the boiler, with the other connections capped off. How easy the conversion is will depend on the current pipe layout. You need a convenient place to put the pump and valve(s) - with the two circuits splitting after the pump and before the valves. If starting from scratch, this would normally be in the airing cupboard.

Yes.

You need a 28mm valve which is normally closed, and opened by a motor and closed by a return spring when power is removed. It needs to have auxilliary contacts in the form of a *change-over* switch (i.e. NO, NC and COM contacts) which changes state when the valve is fully open.

As someone else has suggested, you could consider adding another pump, for just the HW circuit. You would then need to devise some relay logic to fire the boiler *only* when there is a demand from either or both circuits.

[Roger Mills]

Be very careful! You must have a clear unrestricted path from the boiler to the vent pipe - with no vales in the way

[David WE Roberts]

It isn't clear to me if your problem is the slow reheating of water, or just that your hot water cylinder doens't have the capacity to fill the bath and still have a bit of hot water left over.

If the water heats up reasonably quickly then perhaps you just need a larger hot water cylinder?

[The Night Tripper]

Hi Harry

[...]

I mean behind a gas fire. does that make it a 'gas boiler'? I wouldn't have thought so, but ...

This is a fair point - we live in a hard-water area (south Coast) and there may be a fair bit of scale. As I say, it's partly a 'yen' ...

Yes, it's well insulated, thanks.

J^n

[The Night Tripper]

Hi Roger

Yes, I appreciate that (thanks in part to your previous postings on the subject) - thanks.

Actually, I may post links to a few pictures. Some of the current plumbing is a bit ... novel ...

Cheers J^n

[The Night Tripper]

Hi Harry

[...]

I have no knowledge that it worked (any better) in the past. And when all's said and done it will have the limitations of the topology installed.

J^n

[David WE Roberts]

If it is behind a gas fire it will run on gas, and work even when the gas fire is not lit. This makes it a gas boiler. Sounds like the Baxi back boiler we had in Berkshire. Is it, perchance, a Baxi? How old is the system? When we had ours replaced (probably installed a long time ago - '60s?) the plumber drained the CH but had to drain down everything eventually because the heat exchanger was leaking and kept filling the heating up again from the hot water header tank. He needed another person to help remove the hot tank because it was so full of scale and gunge.

My lady wife sold the tank for scrap to a couple of door knockers and you could see the trail of scale where they had staggered from our front drive. I don't know how you clean up such a cyclinder to sell for scrap - just weigh it in and hope the yard doesn't notice it is paying copper prices for limescale?

So - unless it is a very recent installation and has also been always treated with central heating inhibitor products there could be problems on the heating side. In a hard water area, unless you use a water softener, there is likely to be a build up of scale over time in the hot water cylinder. As advised elsewhere it is a good idea to take out the hot water cylinder and give it a thorough checking. If it is gunged up and/or leaking at the heat exchanger you may solve most or all of your problems by fitting a new more efficient (and possibly larger) cyclinder. You may be 'polishing a turd' if you just modify the pipework without confirming that the hot water tank is functioning correctly.

HTH

Dave R

[Dave Liquorice]

In your current arrangement the HW is heated whenever the CH is on. The water circulating in the CH circuit should be around 70 to 80C, this rather hot (scalding hot) for HW which would normally be around

60C.

Whenever the HW is on the boiler will cycle to keep the boiler and gravity loop hot. Both of these "features" waste energy.

A tank stat could be fitted that would solve the last but not the first.

[The Night Tripper]

[lots of good stuff snipped, thanks]

fair point, well made. In some ways I would like a stop gap until the back boiler needs replacing and we (probably) re-do with a new boiler in the loft. I'm conscious that my plans are probably over-ambitious for this stopgap situation. I *really* don't want to take the tank out though ;-(.

Hmm...

Cheers J^n

[YAPH]

The scrap merchants just give you a standard price for a cylinder. They know it's useless weighing it.