Danfoss FP751 CH/HW Programmer Problem

[m60vxr]

Hi folks,

I have a defunct old style Danfoss FP715 programmer and need to replace it (Call for heat signal constant from programmer even when hot water off, CH side works fine). Having looked around I see that there is a Danfoss FP715si (I assume this is the modern replacement as it is almost identical in looks/functionality but seems to do a bit more like auto change clock for BST/GMT and such)

The thing holding me back is that I noticed on Screwfix is it says its a 5 wire install however my system only has 4 wires, L/N/CH on/HW on. The CH off and HW off on my existing programmer are not connected.

Can anyone tell me what is meant by 5 wire install and would this make the newer FP715si unsuitable as a direct replacement?

Thanks

 

[DP]

First thing I would be doing is check supply fuse. Is it 13 amp? If yes, replace with 2 or 3 amp

Yes, FR715Si will work in existing.

 

[m60vxr]

First thing I would be doing is check supply fuse. Is it 13 amp? If yes, replace with 2 or 3 amp

No its 3A but why would a 13A fuse in the main supply cause the programmer to show Hot Water call to heat to be on permanently?

Yes, FR715Si will work in existing.

Excellent Thanks.

 

[DP]

No its 3A but why would a 13A fuse in the main supply cause the programmer to show Hot Water call to heat to be on permanently?

Because the programmer is not rated to carry 13 amps. If this rating fuse is fitted, and there is a short circuit, contacts can weld, foil on the pcb often evaporates due to passage of heavy current

 

[m60vxr]

For some reason I read your reply to mean you were saying a 13A fuse might be causing the problem instead of the programmer being faulty. Seems even a 3A fuse doesn't prevent that. It was a short in the Tank stat that was the original cause but it appears it took out the programmer before the 3A fuse in the mains isolator blew. I replaced the 3A fuse after fixing the short which incidentally was caused by some really bad wiring in the stat (Earth was far too long and curled up inside, had become detached, unwound and made contact with the unused sat terminal which routed the 240v to earth when the HW tank reached temperature!)