Changed the DHW sensor, now what?

I'd be really grateful if you could help me with this.

I have a Pusacoil 2000 that is now 4 years old. I live in the Chilterns and the water here is hard.

Over time the unit's performance has been decreasing. Whilst hot water pressure still remains the same it began fluctuating from hot to cold to hot to cold etc. The LED was still doing its double flash indicating everything was fine. More recently this changed and the hot water was hot for only 10-15 seconds after opening the tap before going completely cold. If I waited another 30 seconds before opening the tap again I could get another 10 second burst of hot water. This situation remained until in addition the LED started doing sets of 7 flashes indicating the DHW sensor had gone.

Being a bit hard up I thought I'd investigate if I could fix the problem myself, which is how I found this site.

I checked the top up tank above the unit and whilst I couldn't reach the bottom of it from my position I couldn't feel any water. I filled it up and then spent a while listening to lots of bubbling and gurgling from the unit as it refilled - presumably the water level in the thermal store was very low...

I then fitted the new DHW sensor and the the unit has returned to permanently fluctuating between hot and cold, and the LED is back to double flashing, which is a bit of an improvement but still not perfect. The hot/cold output from the taps is exactly in sequence with the pump on the thermal store - as I hear/feel the pump running hot water is delivered, but then it stops running for 10 seconds or so and cold is delivered, the pump then starts again I have hot water and so on. I cannot however feel any/much fluctuation in the temperature of the heat exchanger whilst this happens.

Interestingly, to change the DWH sensor I turned the stopcock off and opened all my cold and hot taps to release any mains pressure on the "house side" of the stopcock. Despite this when I took the old DWH sensor out water flowed from the hole. I didn't give it much of a chance and just stuffed the new one in it's place, but the flow looked fairly steady to me. I wasn't expecting any water pressure here at all...

I think I'll try a new PHE sensor next but have a few questions first:
1. Is this what you would suggest?
2. Could it be the heat exchanger scaled up? Should I change both just to be sure?
3. Both are fitted to the thermal store water system and so presumably I need to empty the unit of water before doing either of these?
4. Is the following correct? - To empty the unit I need to turn the mains off and using a hose open the tap (I think you plumbers call them outlets) from UNDER the thermal store pump (between the thermal store and the pump) to drain the unit. Or is it the tap/outlet ABOVE the pump, between the heat exchanger and the pump? Once it's empty close the tap, replace the parts and then fill the water tank above the unit which will in turn refill the unit. Will this create any airlocks? How do you get them out? Bearing in mind the water here is hard, will it be ok to refill with mains water?

My sincere apologies for writing an essay. I'd be extremely grateful however if you could help with this. The family have endured months of dodgy hot water and more recently almost no hot water, yet we can't really afford to pay someone to fix it. I'm fairly confident in my technical ability to do the work, it's my knowledge I'm not sure of, which is why I've asked so many questions!

Hi Matt,

This is almost certainly a scaled-up heat exchanger.

The water coming out of it and passing through the pump back into the store should be cool, but I predict yours will be hot. The PHE sensor looks for a rise in temp to indicate that the hot tap has been turned OFF after use and when it finds a rise, it turns the pump OFF. Scale also causes the return water to be hot, fooling the system into thinking hot water demand has ceased when it hasn't. If this turns out to be your problem, a new DHE will fix it.

Cheers, Mike

That's excellent, thanks Mike.

Yes, the thermal store is hot on both sides of the heat exchanger. I'll change the heat exchanger then, and as it's only £25 I'll get a new PHE sensor while I'm at it.

I need to drain the thermal store to change the exchanger don't I? To do this it's stopcock closed, and open which outlet - the one above or below the pump, or both?

As for the PHE sensor, looking at it it looks like a dry pocket sensor, so no draining to change it. Am I right? If so I need some heat induction paste?

Anything I need to know about refilling. Hard mains water ok? Anything I need to do to avoid/get rid of trapped air?

Once it's all sorted I'm going to fit a Super Imp

Yes you do need to drain the system when replacing the sensor.

Yes you can drain it from that point but isolate the pump isolator valve.

Paste is really up to you, you don't always need it but it would help.

Filling the system can take time so don't be shocked.