Old danfoss for 975 replacement, baxi back boiler

[deancwb]

Hi guys, wouldn't mind some expertise!

I have a danfoss fp975 controller attached to my baxi back boiler in a Property I recently purchased.

I'm wondering which controller I can buy to replace the fp975, I can't make much sense online as to what would be compatible.

For reference I have attached the terrible current wiring to the fp975.

Again any help is appreciated. Ideally I was wanting a room stat in the hall, not sure if capable or possible.

I do see that some of the digital stats have L,N then 1-4 for 2 channel and thought this would work, please shoot me down if I am as i dont want to waste money!


Cheers all

 

[Deleted member 267285]

If there’s no existing room thermostat then a wireless version would be easier to install. There’s quite a few that would be compatible just changing the wire terminal positions over, such as Hive, Tado, Honeywell, Drayton, ESI, EPH et al. Any you had in mind?

 

[DP]

Your programmer is voltage free switching but it does not matter what you select in this case as CBW says. Mind you, your pick will need to work for gravity hot water option if that is what is installed. If you do have gravity hot water system ( ch switching fires hot water on the programmer), then Honeywell Lyric will fit the bill

Honeywell lyric will give you smart thermostat with multiple smart features
If you cannot control hw temperature, upgrade here would be prudent

 

[deancwb]

As far as I can work out, I have two tanks in the loft space, one for hw and one for ch I would assume.

I then have a hot water tank with temperature control on the front of the unit 50,60, 70 etc in an upstairs room, that's as far as the control goes there..it's direct off the element.

You are also correct CH does fire HW.

I'm not bothered as to which I go for in all honesty as long as it would control both and I could have a stat to turn on and off @ temp.


Thanks for the replies so far, greatly appreciated

 

[stem]

The important Danfoss FP975 terminals are (1) with the red wire in to control the hot water, and (4) with the yellow wire in to control the central heating. The other wires are just there to provide power to the programmer and its functions. With a few exceptions,most new programmers don't need the extra links between (L) (2) and (5) as they have them built in.

You probably do need to confirm which type of system yours is controlling though, and it's difficult to tell from your description. With an old system having gravity circulation of hot water, the red wire controls the boiler, so whenever the hot water is set to be 'on' the boiler fires up and the hot water cylinder is heated by natural circulation, in that warm water rises, cooler water falls.

When central heating is required the yellow wire would become live and that start the pump. This then circulates the hot water around the radiators. (It can be linked to a room thermostat) However, the hot water has to remain 'on' as well, to keep the boiler running. So that's why the hot water comes on with the heating regardless.

The problem is that you make reference to a thermostat on the hot water tank......a basic gravity hot water system doesn't usually have this because then when the hot water got hot, the boiler would go off and so would the radiators. This is a restriction enforced by the antiquated plumbing......However, I suspect from the language that you use that this thermostat may be related to an electric heating element (immersion heater) and if so, has nothing to do with the FP975 or the boiler.

At the end of the day, if this is the type of system you have then any programmer that has the facility to control a system with hot water heated by gravity circulation will do. They ensure that when the central heating is switched on, the hot water also comes on automatically with it otherwise the boiler won't start. You can wire in a room thermostat to control the heating.

Many programmers have the function to control the old 'gravity' and newer 'fully pumped' systems. The FP975 you have has a switch at the back for the selection of 'pumped' or 'gravity'

Fully pumped systems make use of motorised valves and can provide independent control of the heating and hot water. But as you don't make mention of any of these the gravity system sounds most likely.

 

[ianmcd]

I'm not bothered as to which I go for in all honesty as long as it would control both and I could have a stat to turn on and off @ temp.

regardlesss of what you fit, if you have a gravity HW system as you have indicated, you can not have the CH on without the HW also coming on without pipework alterations and additional controls installed

 

[deancwb]

regardlesss of what you fit, if you have a gravity HW system as you have indicated, you can not have the CH on without the HW also coming on without pipework alterations and additional controls installed

Yes this is the case and thought as much, I don't mind this although.

The important Danfoss FP975 terminals are (1) with the red wire in to control the hot water, and (4) with the yellow wire in to control the central heating. The other wires are just there to provide power to the programmer and its functions. With a few exceptions,most new programmers don't need the extra links between (L) (2) and (5) as they have them built in.

You probably do need to confirm which type of system yours is controlling though, and it's difficult to tell from your description. With an old system having gravity circulation of hot water, the red wire controls the boiler, so whenever the hot water is set to be 'on' the boiler fires up and the hot water cylinder is heated by natural circulation, in that warm water rises, cooler water falls.

When central heating is required the yellow wire would become live and that start the pump. This then circulates the hot water around the radiators. (It can be linked to a room thermostat) However, the hot water has to remain 'on' as well, to keep the boiler running. So that's why the hot water comes on with the heating regardless.

The problem is that you make reference to a thermostat on the hot water tank......a basic gravity hot water system doesn't usually have this because then when the hot water got hot, the boiler would go off and so would the radiators. This is a restriction enforced by the antiquated plumbing......However, I suspect from the language that you use that this thermostat may be related to an electric heating element (immersion heater) and if so, has nothing to do with the FP975 or the boiler.

At the end of the day, if this is the type of system you have then any programmer that has the facility to control a system with hot water heated by gravity circulation will do. They ensure that when the central heating is switched on, the hot water also comes on automatically with it otherwise the boiler won't start. You can wire in a room thermostat to control the heating.

Many programmers have the function to control the old 'gravity' and newer 'fully pumped' systems. The FP975 you have has a switch at the back for the selection of 'pumped' or 'gravity'

View attachment 284135

Fully pumped systems make use of motorised valves and can provide independent control of the heating and hot water. But as you don't make mention of any of these the gravity system sounds most likely.

I did mean immersion with a element in it. Just a tiny nob to turn to change top temp. Your assumption is correct!

If I can get something like the honeywell just to keep the temp at a particular level, that would be great. It It will work I'll just order it. Does the hot water being linked to the ch stop me from doing so I suppose is the question, from reading above, I don't think do I just have to be prepared for the hw to be on when I have the ch on?


Cheers

 

[stem]

You can add a room thermostat to control the central heating, but whenever the central heating is on the water will on too. Unfortunately, that can't be changed without altering the plumbing to convert it to a fully pumped system. which will have to be done at some point, as it is a building regulations requirement when the boiler is changed.

For now any 2 channel programmer will replace it provided that it has the gravity option. The Drayton LP722 for example. You can wire in a room thermostat to any of them.

If you wanted a 'smart thermostat' the Hive Dual Channel version has the required gravity option It includes a 'receiver' that replaces the existing programmer and has a separate wireless room thermostat you can put in your hallway.

 

[ianmcd]

I did mean immersion with a element in it. Just a tiny nob to turn to change top temp. Your assumption is correct!

Your back boiler is already heating the HW using gas , so to use electricity is daft , electricity is far more expensive than gas, if you want to regulate the temp of the HW you do that by adjusting the thermostat on the back boiler, but as others have said you can use different controls for your CH but without alterations the HW will be on when the CH is on

 

[deancwb]

Your back boiler is already heating the HW using gas , so to use electricity is daft , electricity is far more expensive than gas, if you want to regulate the temp of the HW you do that by adjusting the thermostat on the back boiler, but as others have said you can use different controls for your CH but without alterations the HW will be on when the CH is on

Ahh yes it's just my poor explanations, this is exactly it. And like the other post says I think I'll order a gravity fed smart stat like the hive. Thanks guys

 

[ianmcd]

Ahh yes it's just my poor explanations, this is exactly it. And like the other post says I think I'll order a gravity fed smart stat like the hive. Thanks guys

Sounds pedantic, but when comparing controls look for one that is suitable for GRAVITY HOT WATER, common problem is when people say they have a gravity fed system, they are absolutely not the same thing

 

[deancwb]

Sounds pedantic, but when comparing controls look for one that is suitable for GRAVITY HOT WATER, common problem is when people say they have a gravity fed system, they are absolutely not the same thing

Understood, yeah the terminology seems to be very specific. Will look for gravity hot water and not fed.

 

[DP]

You are also correct CH does fire HW

Gravity circulation

 

[stem]

Understood, yeah the terminology seems to be very specific. Will look for gravity hot water and not fed.

If only it was that simple. You still need to be careful. Hive refer to 'Gravity Circulation' as 'Gravity Fed'. However, from their description of what they mean below, it shows they do actually mean 'Gravity Circulation'. Confusing init.

 

[deancwb]

If only it was that simple. You still need to be careful. Hive refer to 'Gravity Circulation' as 'Gravity Fed'. However, from their description of what they mean below, it shows they do actually mean 'Gravity Circulation'. Confusing init.

View attachment 284230

That's exactly it, i was struggling to work out what on earth it all meant hence why I ended up posting here!

My plumbing and heating isn't exactly up to scratch, so I had no idea what to look for. Being that my system seems to be a very old system too, I was clueless.

i think the hive v3 dual channel would fit the bill. nothing ordered as yet..