DIY Doctor UK DIY Forums

I have an open vent, pumped HW system. CH and HW are switched on/off together by single-channel Danfoss 103 electro-mechanical dial timeswitch. There are no zone valves or thermostats in the system apart from the one on the return feed, integrated in the boiler. Also have an immersion heater on a digital programmer.

I would like to replace the Danfoss 103 with a digital programmer because it has failed (stopped turning). Ideally, I'd like to add a room thermostat too. I'm not intending on altering the plumbing.

I'm looking for advice:
1) Is adding a room stat a bad idea on this system?
2) Recommendations for replacement programmer, and room stat

The Danfoss 103e5 looks like a simple like-for-like solution because it should fit straight onto the existing backplate, without need for re-wiring, but doesn't seem to come with a room stat. Would it be easy enough to add a separate stat?, or would it be better to go for a digital programmer/stat combined? Are there any that would fit onto the existing backplate, perhaps? I'm happy to do some rewiring, if necessary. A wireless room stat would be ideal, but I might be able feed a wire from boiler out into hall, if necessary.

Main factors are:
- Simplicity of installation - I'm not a trained electrician or plumber.
- Cost - we are hoping to move in about a year

Some pics:


Any advice gratefully received :)

The old programmers had option 10 or 16 programs, I thought the count was a cheat as it included off, but there was a mechanical and electrical switch so when used with gravity (or thermo syphon) it could not turn off DHW when using CH.

Many of the newer models do not have this option, they will still work, but you can think the DHW is off, but really still on.

It seems Hive has a software option to work with the old system, and those with volt free contacts like Nest will also work, but Nest does not publish how to do it.

The Nest will use this [attachment=0]C-Plan_basic_Nest.jpg[/attachment] wiring diagram, Hive it is published, however it is likely you will need some changes, the question is have you got the skill.

Thank you ericmark. I have done various wiring tasks previously - I'm ok if I can work out exactly what should go where! :) I have tools. You think hive or nest would be ok? I haven't looked into those yet, TBH. Since I posted this, I have been receiving advice from elsewhere that it's not a good idea to add a roomstat because it would control the temp of the HW as well as the CH, because there are no zone valves i.e. it's just one circuit for CH and coil in the HW cylinder. No thermostats apart from integrated on the boiler return and the separate immersion heater.

Will Nest or Hive be ok with this arrangement?

Likely Hive better for you, however as to DHW and CH you only run the pump with central heating, theory is pump starves the supply to domestic hot water hot coil, but not seen this happen.

I found if pump started even for very short time once the water start to flow, upstairs rooms will continue to be heated with thermosyphon. But as long as the pump remains static, then only the DHW will heat.

Hive has two sets of contacts, one controls the pump, the other the boiler, and you set the software.

With Nest the com of DHW feeds the boiler, reverse to normal. The N/O goes to line feed. With the CH contacts line to com and N/O to Pump and N/C contacts of the DHW contacts.

The time for DHW time needs to be where it will not be during or soon after the time for CH.

It works with Nest Gen 3 that's what I have, it likely also works with Hive, but never tried it.