22 Dec 2020
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United Kingdom
My flat has an old pumped central heating and gravity hot water system controlled by a Danfoss 103 electric-mechanical timeswitch. [Correction from ianmcd, below: It is not gravity HW. It is an open vent system with pumped HW].
No room thermostat. Hot water and heating cannot be switched on/off independently i.e. I think this is called 'single channel'? Boiler is a Potterton Suprima 60L conventional gas boiler.

Also have an immersion heater with a digital programmable timeswitch that we use for hot water when heating is not required.

The boiler's Danfoss timeswitch recently failed - It still ticks, but doesn't turn around any more. I was wondering if it can be replaced by a digital programmable timeswitch?

Ideally I'd like a room thermostat too.

What would you recommend?
Thanks in advance for advice.
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if you want to go fully pumped, I.E. seperate times for heating and hot water, you will have to add zone valves and probably some plumbing, if you are happy sticking with gravity HW and pumped CH ther are many programmers you can do that with
Thanks for your quick reply, Ian :) Happy to stay single zone/channel, as I spoke to a plumber a few years ago about separating the HW/CH and it looks like a big job in my flat, with lots of floorboards/carpet coming up. We're not planning to stay very long, so looking for a relatively easy (and inexpensive) replacement for the Danifoss 103 that hopefully, I can fit myself. As you've probably realised, I'm not a trained plumber or electrician, but I have some tools and always keen to learn. I've done various other plumbing / electrical jobs around the house.

I understand that if I add a room stat, the HW will not switch on when room is above temp set on stat, but I was thinking that we would manage with this, as we have the timed immersion heater as backup - we usually only use the boiler in the winter and the immersion in the summer, anyway.

Do people think it is a bad idea to add a room stat to this arrangement?

I'd be interested in any recommendations for replacement programmer. I read that Danifoss do a couple of digital replacements for the 103, which would fit straight on to the backplate without having to re-wire, but AFAIK, they don't come with a room stat.
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Thanks Pete. Yes I have been considering the 103e7 and 103e5 (looks like the single channel equivalent of the 103e7). Looks nice and easy to fit, but is there an option to add a room stat?

Assuming it's advisable to add a room stat, I was thinking a wireless stat would be easier, but I might be able to feed a wire from boiler/programmer around my small kitchen and drill a hole out into the hall, without tooooo much hassle, but I'd like to avoid this, if possible.
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The way your system works , when HW selected , power is sent to the boiler to come on, the water from the boiler circulates through gravity to the coile in the HW cylinder, when you select CH the pump is then powered and circulates the water around the rads, a room stat will simply interrupt the power to the pump so the HW will stay on but not the CH , very simple to do
Thanks again Ian, but I am not able to switch HW and CH on/off independently. My existing Danifoss 103 just has on/off which lights the boiler, starts the pump and heats both radiators and HW cylinder. There are no zone valves, as far as I know. We manage with this arrangement by having boiler off in summer and using timed immersion instead. As mentioned above, I'm not intending to change the plumbing at the moment.
Oh ok. Thanks Ian. I thought it was known as a "gravity fed" HW system, because it is fed from a large water storage tank in my loft. Apologies if it's not the correct term. What is the correct name for this type of hot water system?

There is also a small "expansion" tank up there (if that is the correct term) for my CH. That one is only about the size of a toilet cistern. From what I understand, that means my CH system is "open vented"?
Oh ok. Thanks Ian. I thought it was known as a "gravity fed" HW system, because it is fed from a large water storage tank in my loft. Apologies if it's not the correct term. What is the correct name for this type of hot water system?

There is also a small "expansion" tank up there (if that is the correct term) for my CH. That one is only about the size of a toilet cistern. From what I understand, that means my CH system is "open vented"?
exactly you have an open vent system gravity HW is a totally different thing, you can have an open vent sytem that is fully pumped, y plan or S plan, the open vent just means the system water is topped up by the smaller tank in the loft, Gravity HW means that the coil heating the HW cyl is supplied by gravity HW the HW rises and enters the cyl coil, transfers its heat to the water that comes out of the taps and the cooler water is returned to the boiler to be reheated, a constant process, open vent and gravity HW are two completely different things
Aha! Thank you for the correction and explanation, Ian. That makes sense. However, I've just been reading about fully pumped, Y plan and S plan systems and none of them seem to match my system. It looks like they all have valves which allow HW and CH to be switched on/off independently. My system does not have that and is simply on/off for both CH and HW at the same time. Plus the only thermostat that I know of in the entire system is one that is integrated in the boiler. My understanding is that it switches boiler and pump off when the return temperature reaches the threshold set on a dial on the boiler. See pic attached.
20201222_191422.jpg 20201222_191437.jpg
We virtually always have the temperature set to minimum, unless it is a VERY cold day.

I guess I need to fully identify what my system is before I consider what change to make. Any further help that in doing that would be much appreciated.
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post a pic of your hw cylinder and its pipe work and anything else near it, the pics you have posted just tell us what boiler you have , nothing else, but doesnt look like gravity hw as the pipe from the top of the boiler goes down so gravity hw wouldnt work in that scenario
20201222_193503.jpg 20201222_193529.jpg 20201222_193542.jpg
Nothing else around it. Just radiators, which just have simple valves on them, not thermostatic ones. Flat is on a single floor. Top floor with loft directly above.
Ok, thx Ian.

To summarise:

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.

Pics above.

I would like to replace the Danfoss 103 with a digital programmer. 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

Danfoss 103e5 looks like a simple solution because it should fit straight onto the existing backplate, like-for-like, without 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

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