What is the point of C-plan?

[Lieutenant Scott]

I think I'm using the correct terminology here, forgive me if I'm not....

I have S-plan - hot water comes out the boiler, through the pump, then divides into three (it was two, but I modified it!) to go through three independent valves. One goes to the heat exchanger in the hot water tank. Another goes to the radiators in the house, the other goes to radiators in the garage (I converted it to a parrot aviary, and electric heating is expensive!).

My question is, what is the point of C-plan - where you don't have a choice, you have to heat the water when you heat the house (or is it the other way round?). My parents boiler is set up so they have to heat the water when they heat the house. What if the house is too cold, but you aren't planning on taking a bath that day? I know the heat from the tank will eventually seep out into the house, or get used later as hot water, but it seems daft to throw heat into something you don't want right now.

[Tim Downie]

The "point" is that it's an archaic system that relies on gravity circulation to heat the water and consequently no motorised valves are required. As you've worked out, it means that you don't have fully independant control of your HW and CH systems *but*, it is simple and reliable with the pump being the only moving part outside the boiler. Pumps generally last a lot longer (or at least they used to) than motorised valves where problems with either the motors or the switches are relatively common.

In these days of higher fuel costs and possibly higher HW consumption, fully pumped systems with motorised valves make more sense for 99% of the population who want as stored HW system but the simplicity and reliability of the old systems may be valued more by folk who don't use a lot of HW.

Tim

[Lieutenant Scott]

Oh I see. Thanks. I've never had a problem with the two motorized valv= es in the 10 years I've been here. And a replacement valve is only =A32=

0 anyway (that's what I paid for the third one I just added - in fact ev= en less if you just replace the motor/switch unit).

[Tim Downie]

As you say, motorised valves aren't really a big issue for DIYers but they could be a major cause of expense and inconvenience to non-DIYers. You only need to read a few "My combis stopped working!" threads to realise sometimes reliabilty is more important that maximum efficiency for some folk.

Tim

[David WE Roberts]

two houses ago I was on 'C' plan and it worked very well. I had a Parkray anthracite bean boiler for heating the living room, hot water and radiators during the colder months. I also had a Raeburn cooker with a back boiler which we used for heating the kitchen and for cooking for most of the year. This also gave us "free" hot water.

Now with older style solid fuel back boilers you don't want any valves to restrict the circulation of the hot water from the back boiler to the hot water tank or you could build up a head of steam - a couple of times we forgot to shut the damper on the raeburn and managed to boil up the hot water tank in the middle of the night.

The Parkray was fine, because it had a thermostatic damper.

I can't remember now if the two gravity systems were linked or if we had two coils in the tank. I suspect the two boilers were linked up near the tank.

So perhaps no point on modern systems (apart from simplicity) but with less sophisticated heat sources the 'C' plan can work well.

Thinking on this, how are solid fuel stoves with back boilers linked into heating systems these days? Gravity feed to a heat store?

Cheers

Dave R

[John Rumm]

That makes it S-Plus Plan now then:

independent valves. One goes to the heat exchanger in the hot water

Well, C-plan if done completely:

have a choice in the sense that if the water cylinder is hot enough the zone valve that permits gravity circulation does not open.

However older installs probably don't even have the zone valve on the cylinder. (some other temperature sensitive / thermostatic valves were also around at one point to inhibit flow to a hot cylinder)

Indeed. One of the reasons such systems are deprecated. However, if you change it to be a genuine C plan setup as per the diagram above and the wiring:

you can use the programmer to not have the water heated when running the heating. It does mean there needs to be a two port valve in the water heating loop though.

[John Rumm]

Yup can be... sometimes a separate coil HE in the cylinder...

[Lieutenant Scott]

As I said, I've never had one fail. And surely, if the heating works an= d the water doesn't, to anyone with an ounce of common sense, this means= the problem lies with something which is not common to both.

[Lieutenant Scott]

That sounds far too complicated. I simply have a gas boiler, a pump, two valves (plus one for the garage heating but nevermind), and a bunch of radiators and a hot water tank. I can choose exactly what I want when I want. I actually hardly ever use hot water so I have a manual switch for that, and the heating I leave on 24/365 at the room temperature I want, and it simply doesn't fire up in summer.

[David WE Roberts]

*That sounds far too complicated. I simply have a gas boiler, a pump, two valves (plus one for the garage heating but nevermind), and a *bunch of radiators and a hot water tank. I can choose exactly what I want when I want. I actually hardly ever use hot water so I have a *manual switch for that, and the heating I leave on 24/365 at the room temperature I want, and it simply doesn't fire up in summer.

Err......fine if you have mains gas. If you are not connected to mains gas then your options are more limited If you chose solid fuel with back boilers on stoves and/or cookers then you need a different system.

[ARWadsworth]

A C Plan is independant HW and CH and does use a motorised valve. See

HW is not called for via the cylinder stat or programmer then the valve for the HW is closed.

[Lieutenant Scott]

I'd choose an electric boiler, or to save money a heat pump. I like the operation to be simple.

[Lieutenant Scott]

> When HW is not called for via the cylinder stat or programmer then the valve

Can you therefore have:

HW only heating only Both?

[Tim Downie]

Well obviously everyone's mileage with vary. I've had two motors fail = in the last 15 years.

Tim

[ARWadsworth]

>>> When HW is not called for via the cylinder stat or programmer then

You can have HW only or CH only or both at the same time with a correctly installed C Plan.

[Roger Mills]

>>>

YES! That's precisely the point of it. With a 'conventional' gravity HW and pumped CH system (which was what you initially described), you can't

- you have to run the boiler to make the CH work, and that heats the HW whether you want it or not.

BUT, convert it to a C-Plan, and that all changes. A C-Plan system has a motorised valve in the gravity HW circuit and a thermostat on the HW cylinder. The auxiliary switch on the valve (which needs to be of the change-over variety) is wired in a cunning way which provides the following: HW only: Valve opens, boiler runs until cylinder stat satisfied, then boiler shuts down CH only: Valve closes, boiler and pump run until room stat satisfied, then boiler and pump shut down Both: Valve opens, boiler and pump run. As soon as HW or CH demand satisfied, system operates as per CH only or HW only, as appropriate. When both demands are satisfied, the system shuts down.

I don't think that anyone would install a C-Plan system from scratch (in preference to an S-Plan or Y-Plan) but it's good way of converting a gravity HW system in order to provide independent control of HW and CH, plus boiler interlock, without too much upheaval.

[cynic]

vides into three (it was two, but I modified it!) to go through three indep= endent valves. =A0One goes to the heat exchanger in the hot water tank. =A0= Another goes to the radiators in the house, the other goes to radiators in = the garage (I converted it to a parrot aviary, and electric heating is expe= nsive!).

ce, you have to heat the water when you heat the house (or is it the other = way round?). =A0My parents boiler is set up so they have to heat the water = when they heat the house. =A0What if the house is too cold, but you aren't = planning on taking a bath that day? =A0I know the heat from the tank will e= ventually seep out into the house, or get used later as hot water, but it s= eems daft to throw heat into something you don't want right now.

"Proper" C Plan uses a motorised valve in the gravity primary which limits the temperature of the hot water cylinder. See

A dump radiator or towel rail is the norm to keep towels dry and accomodate boiler heat dissipation. I think you are talking about a simple gravity primary with pumped radiator circuit

[Graham.]

When I moved into this house in 1979 I realised that the two motorised valves installed made the DHW and CH capable of independent operation. The only thing that prevented the DHW from running without the CH was the mechanical Randall timeclock (oh look, they still make them

) So I augmented it with two rotary 3 position switches, DHW=timed-off-constant & CH=timed-off-constant A toggle switch on the same panel marked BOOST shorted out the cylinder stat before bathtime.

[ARWadsworth]

You probably could not install one on a new system due to Part L.

But as you said, you would be able to install a C Plan as a conversion on an existing gravity (uncontrolled?) HW system.

[Lieutenant Scott]

Hhahahahaahah!!!!

G-plan is ok.