Self filling loop on EcoTec Valliant boiler losing pressure

draughts

Dear MoneySavingExpert members,

I have a Vallliant Ecotech boiler that was installed 5 years ago. For the paat 3 years it gets an F22 low pressure reading in the winter when there is a sudden drop in temperature. I refill it and the same drop again two weeks later. We have radiators upstairs and underfloor heating downstairs.

 I had it looked at last year by a Valliant person who gave it the all ok. This winter the same again, a heating engineer came to assess and couldn't find any faults bar an under pressured expansion vessel which he replaced. The problem is now worse and the boiler gets an F22 every two days. 
We isolated the underfloor heating but still it lost pressure - ruling out a leak there. I isolated the outflow to see if there was an issue with the safety valve but I get the F22 without an emission there. 
Our heating engineer is now suggesting an auto filling loop to keep it permanently  topped up but I worry we are missing a leak somewhere or there is another issue that we are missing. 

Of note our area has very low water pressure so we have a water pump that activates whenever a tap or water is required in the house. 

Apologies for the long post but I need the Sherlock of Valliant boilers and am not sure where to find them! 

All the best
D

tacpot12

Yes,  I think you are missing a leak somewhere. I would recheck the pressure relief valve - i had a similar problem with a Vaillant Ecomax where the PRV was letting the presure out because of  blockage. I ran Sentinel x800 around the system for four hours (never have x800 in the system for more than 20 hrs) and this fixed the problem. Add inhibitor after draining the x800. 

The robofill is a bd idea as you will be running fresh, oxygenated water around your system and this will cause corrosion. 

daveyjp

That boiler has a pressure sensor and an analogue pressure gauge inside the cover, we had a new sensor about 4 years ago due to the F22 fault.  The pressure in the boiler was OK.

tacpot12

You might also have a watch of this video: https://youtu.be/4meZgMAUecw

draughts

@tacpot12 @daveyjp
Thanks again for the responses. Just an update. Had a good hard look with an excellent heating engineer and found a tiny leak behind the automatic switch just below the boiler. This coupled with the recently changed expansion valve was making even a small leak appear much larger. Early days but it's been a week with no F22 so fingers crossed! 

Thanks for all your help

D