I think you may be right, but I don’t think I’m going to win the argument to start replacing other components while the error is pointing to the pump.

So there is a risk we wait for a pump for 1 month, and then start replacing other bits until it clears the fault. Parts and labour are under guarantee but I’d rather have a warm house.

I’ll mention this to the installer though (once I’ve figured out what these parts are in French!). He is still trying to get some emergency heating going so is coming to the house regularly.

Quick update for the sake of completeness. Still no full resolution to this problem. The installer replaced the pump, and the system would still not start the compressor even though there were no error messages. He came back after a couple of weeks having ordered new circuit boards and didn't need them in the end. He did lots of restarts of the system and tightened the connections between the wall thermostat and the hydraulic controller and the compressor started.

It worked for 2 weeks, and then the buffer storage and a relief valve in the hydraulic unit dumped some water on the floor pressure went to zero. The heat pump stopped with a 'dry boiler' message. After re-adding water to normal pressure, compressor wouldn't start like before, despite no error messages.

Installer came yesterday to replace the valve at the top left of the hydraulic station, and checked the expansion tank of the buffer storage which seems to be working fine. Did lots of restarts again and the compressor is working again.

So it looks like there is a hydraulic issue somewhere, as well as some kind of electrical bug that prevents proper restarts. For the latter, the installer wondered whether the fact that the cable to the thermostat passes through the duct with all the electricity cables doing to the main electrical panel. Could that cause random bugs due to EM interference? He has grounded the shielding round the thermostat cable. We'll see what the reliability looks like over the next few weeks and months, but it's clear there is still an unresolved problem.

@bobshopsupreme alfomas don't like cold water, so if the system has been down for a few days in this time of the year, the water I reckon will be below. 18° stand me if I'm not correct. Any heat pump needs to grab some energy to get itself going in the first place. That energy normally comes from a heating system or a buffer that's been on already for a while. So there's a little bit of heat energy built up. My suggestion is you get the cylinder going on immersion and then manually move the valve when trying to start the system. So in fact you're bringing energy back from the heated hot water tank out to the heat pump and that hopefully will trick it into getting going.

Posted by: @ken-bone

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@bobshopsupreme alfomas don't like cold water, so if the system has been down for a few days in this time of the year, the water I reckon will be below. 18° stand me if I'm not correct. Any heat pump needs to grab some energy to get itself going in the first place. That energy normally comes from a heating system or a buffer that's been on already for a while. So there's a little bit of heat energy built up. My suggestion is you get the cylinder going on immersion and then manually move the valve when trying to start the system. So in fact you're bringing energy back from the heated hot water tank out to the heat pump and that hopefully will trick it into getting going.

Could you please explain why a heat pump would require a supply of heat energy to get it started? Seem very strange to me. 🙄 

Posted by: @derek-m

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Could you please explain why a heat pump would require a supply of heat energy to get it started?

afaik some Samsung heat pumps won't start the compressor if the water temperature in the pipe is less than 10 degrees.  I have not needed to do this but afaik you have to heat up your DHW tank with the immersion then when the circulation pump runs will take some heat out of the tank, once the circulating water gets to 10 degrees the compressor will start.  I will go through the manual and see if I can find this documented.

@william1066 Interesting. I don't see anything in the Vaillant manuals about this. I live in France and the EU version of the Vaillant DHW cylinder doesn't have an immersion heater so I'm out of luck if the whole system is cold. I think in my case it is an electronic bug or similar because it gets going after several hard resets and is then fine. Frustrating because otherwise it seems to heat well and be pretty efficient.

@bobshopsupreme @william1066 I don't think its in the manuals, but it is in the installer training (for samsung at least). particularly flagged in the context of a brand new system being filled and commissioned with water thats just come out of the ground in winter. @derek-m its something to do with the physics of how the compressors behave - if refrigerant pressure too low due to water too cold then can't work.... to get beyond that you'd need to find someone who REALLY knows their stuff from the refrigerant side of things. All us mere mortals need to know is that a completely cold ASHP system may potentially need an external heat input to get it going.

@iancalderbank Thanks for that.  That also probably explains the "warming cycles" the pump does on it's own using heat from the DHW when outside temperature is below 3 degC (see my picture below).  You probably don't want to mess with that.  Given that the anti freeze valves typically trigger at a water temperature around 3-4 degC the 60ish minute refresh is useful to have.  (I was keeping my heating on overnight to avoid the anti freeze valves from triggering in cold weather, it seems I don't need to do that if my DHW tank has heat in it.

This also means, that even if you have glycol, you can't avoid this (unless you want to get to a point where you compressor refuses to start).

Finally, this makes calculating the efficiency benefits of not having/having glycol more complex. 

I chose not to use glycol to simplify maintenance of the system and improve efficiency, and am now convinced that was the right choice (as long as you take great care where you put the valves relative to the heat pump).

@william1066 25C is mentioned as the RWT temp at which it will take steps to protect itself. which is exactly where yours looks to be. I think its that, rather than the outside temp, thats triggering it.  And it also says that the protection methods are to either fire up a backup heater (if one is configured / present) or if not, use the DHW as a heat source. Like you say, glycol appears to be irrelevant to this, as this isn't about pipes freezing , its about compressor temperature. I guess if you left running all the time even on a low demand this wouldn't occur.

Posted by: @iancalderbank

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25C is mentioned as the RWT temp at which it will take steps to protect itself.

It is actually mentioned in a Samsung manual [ the 3 degC trigger]  The heating is still off right now, and no triggers since outside temp went above 3 deg.  Whether there is another trigger or not at a lower temperature, I will have to wait and see.

ah yes ok good point there are two triggers then . one for outside temp of 3C , one for RWT of 25C. experiment with your system to see which one does what 😉 ?

Posted by: @iancalderbank

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ah yes ok good point there are two triggers then . one for outside temp of 3C , one for RWT of 25C

I think this is outside < 3C AND RWT < 25C - can't test today, had to put the heating on before the RWT got to 10C.  There is also a recommended min temperature for the anti freeze valves as well, which I think is above the 15 my RWT is at right now.