Air source heat pump heating issue

[Themorganator4]

My study of the ASHP wiring diagram (difficult because it is so small) indicates that the NO output of the DEFROST module, the one you have disconnected, is required to energise two relays R6 and R9. The CH signal to R5, which controls the start of the compressor, passes through the normally open contacts of R6 and R9; which means for a CH signal demand path to R5 these relays need to be energised by the NO output of the DEFROST module going live to 230V; at the moment this cannot happen because you have disconnected the NO output to R6 and R9.

The ASHP DEFROST mode is is actually the CH mode but with the radiators bypassed by the 3 port diverter valve - see page 21. What is meant to happen is that on entering DEF mode, the 2PMV(CH) opens, the water pumps starts, the diverter valve is energised and water circulates through the Buffer Tank and ASHP heat exchanger. At this stage there is no demand from the Buffer Tank stat to start the ASHP. When the water in the Buffer Tank has cooled below the set temp of its stat because it has been circulating as just described, the BT stat closes generating a start demand to run the compressor to produce hot air which warms the circulating water going through the heat exchanger which at the same time defrosts the ASHP because there is no cooling from the fan which is off in DEF mode- you will see steam being produced from the ASHP for about 5 minutes.

The problem I believe you have is that the CH mode at the moment derives its 230V live from the output of the HW stat and the ASHP is set to give priority to the HW sub-system. What this means is that once the HW demand (ie: HW stat closed) is satisfied 'cos water is above its set point (60C), the HW stat opens removing 230V from the relay logic of the CH sub-system which needs it to generate an ASHP start demand via the Buffer Tank stat. So the ASHP in DEFRO mode remains stuck 'waiting' for the Buffer Tank tank stat to produce a 230V live output to create a start demand which it cannot do even though its contacts have closed because the HW stat is open.

This is the wiring fault I believe exists on page 22 which my test was a simulation of correcting. But I had forgotton ( :-( ) that you had disconnected the DEFRO NO output.

I am almost dead certain (98% now) if you reconnected the DEFRO module and did the last test again the ASHP would start.

Thank you for that explanation. I will reconnect defrost mode tomorrow and attempt the test again, i will then report back.

So are you saying that this connection between 9 and 15 could solve all the issues? Trouble is I don't know how you would test the defrost mode as it is not yet cold enough to go into the mode

 

[Wilko]

Trouble is I don't know how you would test the defrost mode as it is not yet cold enough to go into the mode

Hi - this might be done with a light spray of pipe freeze in the general direction of the sensor, I’m thinking. Or if the sensor is suitably located and IP rated to perhaps just slowly pour a bucket of ice water over it, which should get it close to zero.

 

[Jim_e_Jib]

Give him his fuel money and bung Him a bit more and a bottle of his favourite plonk if it makes you feel better.

Marconi top bloke.

Full marks to Marconi for the work he's doing on this one. Especially if the wiring diagram does indeed have that error!

I'll second (or third) this - one of the finest threads on the forum.

 

[marconi]

I aim to arrive between 1000 and 1030 Sunday 13 Oct. Could you send me a Private Message(PM) with your address and mobile number please?

 

[Themorganator4]

I aim to arrive between 1000 and 1030 Sunday 13 Oct. Could you send me a Private Message(PM) with your address and mobile number please?

Pm just sent

 

[Themorganator4]

Top marks to Marconi and Wilko for coming out today! Identified the problem (a broken wire) and fixed it. Wait to see if the defrost mode is now solved but we have CH! Hurahh

 

[DPG]

Top marks guys, respect to you both.

 

[Wilko]

Hi - ever hopeful that the central heating is still working when asked, here’s a quick update on what was seen and done .

Apartment block, PME with recent good installation of fuses, meters and isolation switches in remote locked cabinet. In the apartment there’s a plastic (Good Brand) twin RCD CU. Zdb 0.15 Ohms. The heating is all run from a B40 via 6mm which runs to a second internal plastic CU with a contactor on B20 for switching the Air Source Heat Pump and a B16 for the backup immersion heater.

Opening the main heating system junction box revealed - to quote Marconi - “ a snakes wedding of conductors”. Noting that some heating control conductors may remain live unless the system is fully isolated we began to chase down the signal to the ASHP to supply central heating only. L was sent from Programmer, made it through the Room Thermostat and back to the Wedding. But no further. We isolated the system and used Long Lead to confirm there was no continuity on the conductor that had been used for CH control of the ASHP. As the control cable had 12 conductors and only 10 were required for full functioning, we found a spare, confirmed its conductivity and tried it. Immediate success. L now appeared at the correct terminal and the ASHP sprang into life.

IR of the control cable was good. So my guess is that conductor was damaged during installation, or perhaps it was simply that the wrong conductor was used at the remote end during installation. You will have to forgive me not tracking this down, as it was raining. Weather wuss, I know.

Next, the test of installation’s RCDs revealed one would not trip. No response to the trip button and no amount of leakage could trip it. Testing at the RCD with L and N removed from output side didn’t change things, so that RCD (unusually) does seem dead and is to be replaced.

 

[Themorganator4]

Yes, heaters blaring out heat as we speak. Now I can get back to turning the thermostat down and telling my SO to put a jumper on!

Thanks again!