Danfoss heating control system (2)

5 Nov 2021
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United Kingdom
Hi all i have a danfoss heating controller if that’s the correct term. It controls my oil heating and thermostats for each zone, it also should engage the pump for my back boiler when it’s thermostat gets to temp but it doesn’t seem to be getting any power. I’ve disconnected central heating pump power and connected to back boiler pump and the pump fires when u turn the heating on so I know the pump works. As I said doesn’t seem to be getting power on its own. I’ve attached photos of the danfoss controller to see if anyone has any quick fix for it! The back boiler and stat lives are into the bottom of number 3.Thanks in advance


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I have two zone valves and two pumps, and it needs relays to make it work correctly, under floor heating controllers often have relays built in upload_2021-11-20_9-5-17.png but with set up you have likely bespoke, where some we hope cleaver heating engineer who installed it has designed a system.

So first question, has it ever worked?

So in my case the zone valve has a micro switch, so as it opens it can switch some thing else on, for example boiler and pump, or boiler, or pump, but if boiler and pump, then that means both boiler and pump are connected together, as only one micro switch.

So in my case the micro switch turns on a relay, this relay has three sets of contracts, however only need two sets, so it switches on both pump and boiler, but when not activated boiler and pump not connected, so another relay can turn on same boiler but a different pump.

I can't see any device which would allow the two boilers to work without some sort of relay.

I am lazy, and if I had to do that job likely I would use a PLC to control it, as I know how to program PLC's so for me easy, but how a plumber or heating engineer would do it not sure.

I would start with a truth table, I would guess.
1) If no fire lit, water must not circulate, as hard to light a fire if water cooling the fire.
2) Once fire lit, water must circulate.
3) Likely a three port valve like used in W Plan, so if rooms warm enough water goes to sink radiator to simply get rid of heat, and other wise circulates through house radiators, but could also be a by-pass valve.
4) Some method to ensure flow even if pump fails like a power cut, sons system used 12 volt battery backed pump, but could also be thermo syphon to heat domestic hot water (DHW).
5) Relay on pump to stop oil boiler firing if back boiler heating water.

My brother-in-laws old house had similar system, he had twin storage tanks something like this Torrent pipe example.PNG and a very complex control system to ensure always spare capacity for hot water from solid fuel water heater.

Going back in history mother house would boil the water, many times I would be sent to run off hot water, as it was boiling. Noise told us what was going on, how this happened killing Rhianna Hardie I don't know? may be insulation stopped the noise normally associated with water boiling.

The main point is it should fail safe, be it sink radiators to get rid of heat, 12 volt battery backed pumps, thermo syphon, or other method, we see adverts for very cleaver stoves wallnoefer.PNG Hughes Condensing Stove 2 small.jpg but very little on how one ensures the pumps don't fail or methods to put fire out should pumps fail.

Gas and oil is easy, remove power to oil or gas valve and boiler stops. Solid fuel is not so easy, I remember my mother raking out fire onto a shovel because the water had been turned off in the street. Which is why quarry tiled floor, hot coals would not damage it.

Same with sons narrow boat, tiled floor under stove so fire can be raked out without worry of setting floor on fire. Clearly raked out onto shovel, but can spill coals.

Personally I would want some one trained how to control solid fuel fires, too easy to miss some safety feature.
Hi no it has never worked the thermostat on the pipe coming from the back boiler will click on and off but the pump never fires, even if u turn it all the way off and then on (I’m told
It should fire on and off) it does nothing when I lit the fire last week the hot water tank was getting hotter so I presume the flow of water is there, there are two circuit breakers in the conduit above the danfoss controller
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My dad connected one radiator to back boiler, heated a cool bedroom, single brick in bay window, but the back boiler was not really enough at that time to do much more.

The problem is our homes have changed, cavity wall insulation, loft insulation, double glazing so the size of boiler required to maintain the temperature has reduced, but to get it there to start with has not changed.

My parents when central heating was fitted used it to pre-heat house in the morning, but once up used coke fire, being a steel works town, coke was cheap.

This house when I moved in, realised the central heating would have never really worked, had to go outside down a set of steps and into flat below main house and plug in pump. It had clearly never really worked since garage turned into a flat.

Since it seems you have thermo syphon to heat DHW the safety aspect is not such a problem, but how to design an automatic system not so sure.

Looking at that system, it uses a heat store to combine, but some one at some point designed yours, and it may be very different. You want DHW hot enough, so it would make sense to have the pump only activated once the DHW is hot enough, but what I am doing is trying the read the mind of the designer.

The whole idea of multi-fuel is relativity new for domestic, and often also can be very costly, so we see loads of compromises, so there is no C, S, W, or Y plan, it is what the installed has decided.

I really could not believe when we bought this house that there was no thermostat for main house, and not only different supplies for central heating, but actually different consumer units. It was hard enough working out what had been done when here, unlikely anyone would work it our remotely.

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