Gravity System But Programmer c-plan???

Hello Everybody,

I just cost myself a call out trying to work out the heating system in my new house but oh well!

I have an old Worcester boiler (oil fed) with a gravity fed system.

So, I wanted to install a Google nest third gen and went into the Honeywell programmer expecting to see the writing set up as a. Gravity system but it appears to be set up like a sundial system. I note a link between terminals 8 and 5???

On operating the system I have to have the water on to use the heating. If the water is not on the pump runs but the boiler does not fire up.

I'm really confused about why the programmer is wired up like this. Any help would be really appreciated. I just want to wire up the nest to control the heating schedule and know that I will have some significant limitations due to my physical system.

Thanks alot.
Adam_Fozz
(Programmer wiring photo attached)

 

you have a wire going into terminal 4 , do you have a MOMO valve, show pictures of what motorised valves you have

 

C Plan has changed over the years, the basic C plan had no tank thermostat or motorised valves, however often a tank thermostat is included so in summer you can leave hot water on and it will just fire up when required, and also a version with a motorised valve so DHW temperature can be controlled independent from central heating.

The change over contact in Nest is wired with common to boiler, the reverse to normal, so the boiler can be run with or without the pump running, with pump for central heating, without for DHW only.

I found my heat would random start up without being programmed to start, I found there is anti legionnaires software built into Nest to ensure water gets hot enough, I had to disable this function, I found ½ hour 5 days a week worked about right for me in summer, missed Monday and Friday, I found the boiler runs for 20 minutes in summer, then off for 5 minutes and back on for 5 minutes when it runs out of time.

Mine is not quite as above as two zone valves on CH one for flat and one for main house, but I have no zone valve on DHW, I did consider fitting one, but the heating guy advised against it, as there is not cool down run on with my boiler, so being to transfer heat to DHW after switch off he says is good, I am an electrician not a plumber.

If you want to see other plans or how mine is fitted you are welcome. Quote me then I will get an alert.

 

There is no connection in 4 but a pic of valves would be good.

 

Hi there,
There isn't a wire going into terminal 4. That is a link wire looping from 5 to 8.

There is also a link from L to 5.

I don't see any signs of electronic valves and no thermostat on the tank.

This is why it's really weird that the programmer appears set like c-plan. I didn't mention it looked wired like c-plan earlier because I wanted to see what other people thought about the programmer wiring.

Any thoughts on how I could wire this for the nest?

 

The plan I showed is based around Nest Gen 3, there are two ways of wiring the C Plan, thermostats like Hive you need a software setting so when the CH contacts are made it also makes the DHW contact, but with Nest Gen 3 the CH contact feeds the normal closed DHW contract so fires up boiler as well as direct connection to pump, so both the N/O and N/C contacts are used.

With a separate thermostat and programmer this is the plan, but will not work when the thermostat and programmer are combined, so this will work, this is mine there is an error in mine, it needs a second relay, but only because of the zone valves to separate flat and main house heating this should work fine with your set up.

With your old programmer there are switches and interlocks so you can't turn on central heating without the hot water.

 

Thank you Eric, this may be a stupid question but do you have any idea of how to translate my current Honeywell pins to the new schematic you have provided? Part of the problem with me relalizing it had been set up to C-Plan was that I am not 100% sure if I need to open up the boiler to grab new connections or if I can do it all with the existing cables in the Honeywell programmer.


Adam

 

Sorry can only guess how the old thermostat is wired, but this seems likely.

L = L Red likely linked to Nest 2.

N = N Blue

3 N/O CH = 3 N/O CH Red Likely goes to thermostat but needs to go to pump plus link to Nest 4.

4 N/C CH = 1 N/C CH Not used.

5 Com CH = 2 Com CH Red likely linked to L.

6 N/O DHW = 5 Com DHW Black Likely goes to boiler.

7 N/C DHW = 4 N/C DHW Link to 3

8 Com DHW = 6 N/O DHW Black

The problem is there is a bit of guess work as I can't ring out wires to verify where they connect.

 

Can you switch the CH on without the HW also coming on ?

 

Hi Eric,
Thanks for your reply.
I think you are bang on. I managed to find this schematic in a Honeywell manual in paperwork left by the previous owner and I believe it matches your assumptions-


Ian, no CH only activates the pump if the hot water is on.

Thanks, I'll let you know how the rewire goes. If you have any insight into any settings that need to be activated in the nest for this type of system let me know.

Thanks again,
Adam

 

So I connected it up as per this photo and the boiler works for hot water but when I put the heating on it only starts the pump and not the boiler itself.

I have set the water to permanently on in the nest software but still not dice. Are my connections wrong?

 

If 5 and 6 swapped that may cause problem.

 

you will have to select Gravity HW in the nest settings menu

 

Unlike Hive, unless Nest have introduced it recently, it doesn't have a gravity system option. The instructions state that it is suitable for:

• Combi boilers • System boilers with hot water control • Heat pumps (heating only) • Zoned systems • District heating with electrical control valve

As you can see, no mention of gravity circulation systems. However I worked out a way to wire Nest's up such that the boiler is automatically started and heats the hot water, when the receiver turns the heating on. Here's how I do it:



On the other hand, if they have now added a 'gravity' function, I would be pleased to hear that.

EDIT it appears to be the same as per the diagram posted by @ericmark so that's encouraging. and yes it's crucial that the the boiler is connected to the hot water 'common', and not the 'call for heat' as you would normally expect. That's what links the boiler to the pump via terminal 4 so that the boiler starts when the heating only is selected.

 

As @stem points out since volt free contacts it can be wired as gravity, and mine is wired that way, however every so often it does lock out, I think a faulty fuel shut off valve, waiting for the boiler guy to change it, it seems caused when boiler is reactivated after a very short off time, I think this short off time may be caused when the schedule for domestic hot water turns in on when boiler already running, but can't confirm this is the problem, so in winter I turn off the schedule for domestic hot water as not really required, it is auto heated when central heating running.

@adamfozz has posted a sundial C plan, this has a cylinder thermostat in the plan so in summer you can leave the DHW on all the time, if you have a cylinder thermostat then it needs the diagram changing. You use the change over contacts on the cylinder thermostat not Nest, if you have a cylinder thermostat then I will try and build a modified plan to include it.

The C Plan has been altered over the years, I know I added the motorised valve and wireless tank thermostat to my dad's many years ago. I have assumed yours is the basic.