Any point in fitting a relay?

I'm trying to work out if there's any point in fitting a relay to allow for independent operation of the CH and HW.

I have a gravity fed system at my house supplied by a Potterton boiler.

I recently had my local electrician replace a prehistoric satchwell programmer (no battery back up, so losing the programme everytime there was an electric cut or if I turned off the electrics to do some works was driving me mad) with a Drayton programmer. At the time my electrician told me the hot water would need to be on to trigger the central heating.

Shortly thereafter I decided to replace our wildly inaccurate wall thermostat (needed to be at 35 C to heat the living room to 20 but then kept the heating on until we cooked!) with a nest thermostat, which was installed yesterday.

The Nest installer told me the hot water would need to be on all the time to allow the Nest thermostat to control the central heating. He told me fitting a relay would allow the Nest to operate the central heating independently of the hot water, otherwise the Nest would only control the heating when the hot water was on.

However, I have a (separate) hot water cylinder thermostat which turns off the boiler once the water in the cylinder is at the required temperature.

Let's assume I leave the hot water on 24-7 to allow the central heating to operate. Is there any point in fitting a relay to allow for independent control of the central heating as suggested by the Nest installer, given that the hot water cylinder thermostat would seem to prevent continuous operation of the boiler?

Apologies if this is a dumb question and thanks in advance for any thoughts you might have.

I would suggest converting the system to a 'c plan' by adding a motorised valve onto the gravity hot water circuit. You will need to make sure you get a 6 wire valve (has an extra white wire that is necessary for a c plan)

This would allow you to operate hot water and heating independently.

If you need more info google honeywell c plan

The cost of the Nest could have been put towards a fully pumped system!

Then it would be properly controlled and cheaper to operate.

Also your old boiler will be very inefficient. It would have been covered by the boiler scrappage scheme a few years ago.

Tony

Agile said:

The cost of the Nest could have been put towards a fully pumped system!

Then it would be properly controlled and cheaper to operate.

Also your old boiler will be very inefficient. It would have been covered by the boiler scrappage scheme a few years ago.

Tony

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The old programmer was a good bit older than the boiler - the boiler itself is a Potterton Kingfisher Mf installed in 2002 which is D rated (78% efficient). I totally agree it would make sense to replace it in due course, but I think I'll leave it the 3-4 years we anticipate keeping the kitchen as it's integrated into a unit.

I actually have a pumped system already (apologies, I know I referenced it as gravity fed - I think every system with tanks in the attic is gravity fed ), but it looks like the old wiring was left in place when it was installed, hence the CH requiring the HW to be on. Apparently this was something to do with the previous owners desire to keep the same old programmer when they installed the new boiler. Caused my electrician to scratch his head somewhat on first glance when swapping the programmer.

What I'm really trying to work out is whether fitting the relay would result in any savings over leaving the HW on all the time and relying on the cylinder thermostat?

TCCHeating said:

I would suggest converting the system to a 'c plan' by adding a motorised valve onto the gravity hot water circuit. You will need to make sure you get a 6 wire valve (has an extra white wire that is necessary for a c plan)

This would allow you to operate hot water and heating independently.

If you need more info google honeywell c plan

Click to expand...

Thanks for this - the nest installer suggested fitting a diverter valve as an option but told me that it could require removal of a couple of my current kitchen cupboards to achieve, as the boiler is in one of them and the pump is mounted alongside another (tucked behind a wall). He suggested a relay as an interim solution until we swapped out our kitchen (hopefully not for a few years at least).

What I'm trying to understand is if there's likely to be any savings in leaving the HW on all the time and relying on the HW cylinder thermostat vs fitting the relay?

Normally where there is a requirement to have the hot water 'on' in order to operate the central heating, the system operates as follows:

When the the hot water is 'on', it controls only the boiler. The boiler operates, and water is circulated around the boiler and hot water cylinder by gravity. Then when the central heating is required the pump is started and that circulates water around the radiators. The reason that the hot water needs to be left 'on' in this instance, is to keep the boiler running for the radiators.

The restriction in the control of the system is thus because of the limitations of the existing plumbing and absence of motorised valves to perform the necessary control functions.

You also refer to the system as being "pumped" Is this what is known as 'fully pumped' where the hot water and central heating are pumped, or is just the central heating pumped?

This relay that you mention, what would be its intended purpose; what will it be doing?

You also say that you have a hot water cylinder thermostat. What is that wired to and what does it do? The reason I ask this, is because it can perform a relay function, and keep the boiler going in the situation where the hot water is up to temperature but the central heating is still required. But if you already have this, why would you need a relay.......

I am still a little unclear from the evidence given exactly how the system is wired if there is no diverter valve!

It does sound like a fully pumped system but hard as I can imagine it then it sounds as if either the heating will not work when the cylinder is satisfied ( unlikely ) or its temperature will be uncontrolled when the heating is on. Again this sounds unlikely.

But unfortunately there are many bodged systems around.

I dont see how any relay will give any proper control of it if there really is no diverter valve.

Many old timeclocks would only allow CH when the HW was on. And to ma amazingly many of these have been left fitted to fully pumped and controlled systems.

So back to the OP. Are you so sure that you dont have any diverter valve?

If you do then all you need is a simple timer to time the HW. And not a relay!

Of course a Nest was a poor choice in that case as it only controls the CH. You should have got a Cosy or Hive or Salus 500 which can control both CH and HW.

Tony

Agile said:

I dont see how any relay will give any proper control of it if there really is no diverter valve.

Click to expand...

It doesn't. It's sort of a halfway house, because it only works properly in the summer.

In the summer the thermostat on the hot water cylinder that turns the boiler 'on' and 'off' so that the boiler does not continue to 'cycle' once the hot water is satisfied, so all is well.

In the winter, this causes a problem because if the radiators are 'on' the boiler also needs to be 'on', but if the hot water is satisfied, it won't be. So, a relay wired across the pump terminals can be used to switch the boiler 'on' when heating is required. The downside then is when the radiators are 'on' and the relay energised, thermostatic control of the hot water is lost, because it's overridden by the relay.

Whilst I have seen this once, the same functionality can be achieved without a relay by wiring the cylinder thermostat thus:


The confusing bit for me is when the OP says that he has a cylinder thermostat already, so to me that suggests he already has something akin to this installed. Hence my question asking what the intended relay was for.

Also, this only works with pumped heating and gravity hot water as turning the pump 'on' and 'off' is what controls the radiators. The OP says his system is "pumped" which doesn't really define what he has, maybe it's 'fully pumped', or maybe gravity hot water with just the radiators pumped.

As you surmise, there maybe a motorised valve somewhere, but everything is unclear. Hopefully the OP will come back with some answers for us.

Hello all again, apologies for not replying sooner and thanks for posting responses here. Further apologies that my own lack of knowledge is contributing to making this harder to discern what's going on. It may be worth me flagging the bits about the system I do know and then explaining its current behaviour.

What I know about the system:

The system has a potterton boiler installed c 2002 into a kitchen cabinet (this gives it access to a Chimney). Here is a picture (apologies this picture is upside down!):

The pump is installed next to the boiler, not in a kitchen cupboard but accessible through a cut out in a cupboard (apologies again this picture is upside down!).

Historically the system had an old Satchwell/Sunvic Programmer (seems likely to predate the boiler and is now removed) and a wall thermostat installed :


The system has a cold water tank in the loft and a hot water cylinder in the airing cupboard. The cylinder has a thermostat of its own installed.

Subsequent to all of this, the Satchwell programmer and thermostat have been replaced with a drayton 722 programmer and Nest thermostat.

Behaviour in operation:

Prior to the replacement of the programmer and thermostat, the hot water and CH operated independently, though I'm not enough of an expert to say whether that's because the old programmer simply tripped the hot water whenever the CH needed to be on or whether truly independent operation was in effect. My electrician implied it was operating independently, though I don't understand how that could of been the case.

Post the replacement of the programmer, the system operated by the CH turning on the HW each time it was required (the CH was subject to limited control by the room thermostat, which was either defective and/or poorly positioned). The nest is wired independently of the programmer, but the boiler seems to require the HW to be on at the programmer for the CH to fire up upon request by the nest. It seems important to note the only requirement seems to be that the HW circuit is on rather than the boiler being fired up to provide hot water - so my central heating works even if the boiler isn't fired up for HW, it just needs the HW to be "on".

However, the cylinder thermostat limits the amount of time the boiler is actually fired up. If the Nest is not calling for heat the boiler is off for the most part (except in the mornings after a shower etc).

What I'm looking to understand:

The nest installer recommended the installation of a relay (for approx £120) to allow for independent control of the CH by the Nest. I don't need the HW to be firing the boiler to get the CH working, so all I do now is leave the HW on (as in switched on at the programmer) and rely upon the cylinder thermostat to prevent the boiler heating HW all the time.

What I'm trying to understand is whether the installation of the relay will actually achieve any savings over leaving the hot water on and relying upon the cylinder thermostat? I believe there could be some savings as my household isn't a heavy hot water user (so why keep the tank full all the time), but I'm looking for confirmation of that.

If it helps, I'm very happy to take the front plate off the junction box next to the programmer and provide pictures.

All the best and thanks again.

That suggests to me that the original programmer has been removed and the Nest is being used to switch both the central heating and hot water together. But if so, this doesn't make sense. How will you have the hot water only on in the summer?

Usually the old programmer is left in place and used to control the hot water, whilst the Nest looks after just the heating. Unless there is some limitation on the system. So we're back to not knowing how the flow of the water from the boiler to the cylinder and radiators controlled and distributed.

I'm still unclear about what control you have, unfortunately the photos don't help any, I know what the various components look like, it's how they relate to each other that is important.

Having said all of that, if you can have the hot water on without the central heating and if the cylinder thermostat is actually stopping the boiler firing when the cylinder has heated up, then I don't see how you will achieve any savings.