ASHP

I am trying to resolve an issue with a underfloor heating system running from an ASHP. The controller comes up with fault code U1 which Mitsibushi tell me indicates a high pressure fault.

Has anyone any experience of these systems and can they advise me how the diagnose the problem, I assume its a problem with a valve. Thanks. Ray H

I would have thought Mitsubishi would have been more helpful.

The only time I have encountered a "high pressure fault" was with a ground source heat pump, and it had not ben bled properly so the brine in the ground loop was not circulating and the compressor overheated.

So firstly check the water side IS circulating properly to take heat away from the heat exchanger.  If not you need a regrigeration engineer.

high pressure is usually caused by a lack of flow from unit, you'll need to check that no valves have been closed, partially or otherwise, then you'll need to look at whether there's a leak or any air that's got into the system.

if this problem has only occurred during the cold snap that we are having then you have to look at whether there are any exposed pipes that have frozen. 

Hi Sharpend. Thanks for your reply. I have checked the fault out a bit further. It seems the controller is fine if the hot water system is turned off and it's working the underfloor  heating, It only throws up the fault code when the hot water system is selected.

This sounds as though there is a settings issue, I'm not familiar with the mitsibishi system specifically, do you have a setting for the installation set up? I.e. Hot water and UFH, do you have a large buffer vessel? 

Most heat pumps run at a low flow temperature when running just the heating to get maximum COP. When DHW is selected they will switch to a very much higher flow temperature at the expense of a lower COP.

Does it trip straight away or some time after the DHW is turned on?  Does the HW tank start to heat up? check that water is actually flowing round the DHW loop and there is not s stuck motorised valve etc.

Hi ProDave. The controller did not go into fault mode imediatley, it took arounf 15 minutes. I have manually operated the motorised valves and touch wood it run for the last 24 hours as normal. Thanks to you and all the other guys for your help.

I would like to take out a maintenance contract on my heating system. It comprises of a Mistibushi ASHP with underfloor heating downstairs and radiators upstairs. It has run perfectly for the last 4 years until this recent issue with what appears to be caused by a motorised valve. I live on the South Lincs/Norfolk border and Mitshibushi gave me the name of two companies that install their systems. One said he would not take it on as he didnt install it. The other said he was far too busy to take on more work. Do you know any heating engineers in my area who are conversant with these systems who my be willing to take it on board. Thanks

Ra y H said:

Hi ProDave. The controller did not go into fault mode imediatley, it took arounf 15 minutes. I have manually operated the motorised valves and touch wood it run for the last 24 hours as normal. Thanks to you and all the other guys for your help.

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It sounds to me like the HW motorised valve was stuck, so the water was not flowing and it overheated.

If the problem persists, replace that motorised valve.

Unlike a boiler, there is no regular "maintenance" needed so I am not sure what a maintenance contract would do. It sounds like you need a breakdown service, i.e. just a local engineer that has seen the system once and checked it out and is prepared to come when you call him to say it's not working.