Posted by: @cg360

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Hello, after an incredibly frustrating 18 months with a Vaillant heat pump and facing our second winter without reliable heating or hot water I stumbled across this forum!

My problem is I have a Vaillant heat pump which will not function as it should. It reports low water pressure when the pressure seems fine, continually blocks the compressor and throws up the same five faults on every occasion. The first is low water pressure, then sensor input, output etc. But the thing that puzzles me is that when the compressor is blocked (which I understand is not a 'failure'), the pump reports an anticycle time off between 65,000 and 85,000 minutes - getting on for six months! The faults are:

F703 - low pressure

F585 - sensor fault capac. outlet temp

F520 - sensor fault building circuit flow temp

F519 - sensor fault building circuit return temp

F73 - building circuit water pressure

The fault goes away if I turn off the house mains and switch it back on. But how long it will work for is another question. I've reset the mains seven times today just to heat the hot water.

I want the heat pump to work, but if even Vaillant can't get it to function properly... It's currently more work than keeping my wood stove going and far more inconvenient!

It's a Vaillant Aerotherm, feeding radiators (sized by the installers and appropriately upsized), with a solar power system feeding in during the day. That's on its own distribution board, not the same one as the pump. We have good water pressure here and there are two similar pumps running in the village using the same water supply.

I have spoken to Vaillant more times than I count. Engineers from the installers and valiant themselves have visited on more than a dozen occasions, replacing parts, fixing wiring - the same faults always recurs. The engineers are stumped. They've never seen these faults occurring. 

The regional manager is coming out on Monday and I would just like to have some idea if anyone has ever seen anything similar, for whatever reason. I feel like I'm being fobbed off. 

 

Obviously it is difficult to give a definitive answer without carrying out detailed testing, but here are a few possible ideas.

The water pressure mentioned will be the water within the heating system, which is a sealed system that should have a pressure of around 1 bar to 2 bar. It is not the mains water supply pressure.

The fact that you are getting several sensor faults together would make me suspect a circuit board problem or a data communication problem, or even something as simple as the incorrect version of firmware.

First thing to check. Do any of the data/communication cables run next to, or near to power cables? This could lead to corruption of the data signal being passed between the different units.

An 'dry joint' or bad connection on one of the circuit boards could also cause the system to fail to operate correctly. Have any of the circuit boards been swapped for known good ones to see if this cures the problem.

Has anyone checked the installed firmware version to confirm that it is correct?

@derek-m Thank you for the answer, appreciate your time. 

At various times, faults such as you describe have been suggested by the engineers. All discrete circuit boards have actually been replaced due to faults, in some cases more than once, the wiring has been replaced to eliminate the interference issue as a possibility, and several loose components have been reseated/resoldered. All sensors have been replaced too.

I think it might just be a bad unit, a lemon if you like. I haven't checked the firmware version but will make sure that is up to date. Had to restart the unit 14 times yesterday, 8 times today! 

Chief engineer is here on Monday at last, so will see what he has to say. I'm intrigued by the anticycle time as I can't for the life of me think why that value would ever need to be so high. Obviously it has no upper limit imposed in the software, but some kind of input or calculation must be going awry for it to reach that number. 

I'll update when the engineer has been, can't hurt to post the info in case someone else experiences a similar situation.

Thanks again!