Megaflo temperature/pressure valve won't open

Good morning! I have a dripping tundish, so decided it's time to replenish the air gap in my Megaflo - something I've done from time to time in the past. So I've turned off the water, opened a hot tap, and then tried to open the pressure relief valve on the Megaflo. But it's stuck: the plastic cap is clicking round when I turn it anticlockwise, so I presume the metal bits have seized up. Is there a way to remove the cap and use grips to turn the valve, or is that unwise?

The valve has a sort of ratchet mechanism so if you twist the cap it you should experience increasing resistance for about 1/6th (approx, from memory) of a turn as it lifts the seal and if you go too far it will suddenly clunk shut.

Anyway, it's a safety critical part so you must not attempt a DIY repair. Contact Heatrae Sadia customer support:


Who will assist you with getting the valve serviced/replaced by a properly trained and qualified technician. Megaflos have quite decent warranties on them so you might be covered.

I think PRVs are designed to be shut all the time, I think Megaflo actually say to keep it opened, a bit of a pain if you have to wait for 50/60 litres to drain out the hot tap but I find with the PRV on my firebird which I use to vent air after a refill will stay open if I turn it very slowly anticlockwise.

Thank you for the replies: the issue is that it isn't opening the valve when I turn it anticlockwise. It's very stiff and as it turns the plastic cap rises off the seat and then clicks without opening the valve. I can keep turning it, with difficulty, and it just rises a bit and clicks, but no valve opens. So I'm guessing the metal parts of the valve have seized and may need some encouragement.

If you turn it with the mains left opened, do you get it flowing into the tundish?
Also, If its still dribbling under normal conditions with the mains left opened and
after running off 4/5 litres of HW then it requires renewal.

OK - my mistake was in the setup. I turned off the cold water and then opened the nearest tap (in the nearby bathroom), according to the Megaflo instructions. Nothing came out, and then when I turned the red valve nothing happened - not a peep - so I assumed the valve was stuck, and posted my question. Then, as suggested by @John.g with the water back on I tried the valve again - this time whooshing, so the valve is OK. So I repeated the process but opened a downstairs hot tap and let it drain for a while, and this time the procedure went as expected.
Sorry for the false alarm - not sure why there was a problem this time and not in times before, but there we are. Thank you to @John.g and @Chuck for your input.

To fully recharge, the lowest hot tap should be left open until all the water has drained, this will only happen if the T&PRV is held open to allow air into the cylinder, its not draining the whole cylinder, only the volume above the dip/drop tube, so ~ 50 liters or so.