Megaflo - hot water outlet pipe always hot?

[ndoubt]

Sounds like you're losing heat via the cylinder coil, possibly through gravity circulation.

At a guess DHW zone valve permanently open - seized or wrongly wired.

I'd start with feeling the primary pipework for warmth half an hour after switching off rads and hot water.

Primary pipe work meaning the pipe work after the DHW valve?

I’ve felt the pipework after the DHW valve while the heating has been on and it’s cool suggests the valve is working ok?

 

[ndoubt]

OK, so next task would certainly be to check the hot water kit. Though presuming that was ok was a given, as really that would be the first thing to check but no matter. Your cylinder certainly shouldn't be dropping 23deg over 8 odd hours. I'd run is heat up the HW, then temp test at an outlet, then do the same in the morn.

The only way the coil would draw heat out of the cylinder to any real degree I would think is if cold water was being circulated through it.

Yeh the kit was literally fitted a few days ago .. can’t imagine there’s an issue with it.. but point taken..

One thing I will say is although the reading is 30 degrees, I am able to have a hot shower even on that reading.. which tells me the evohome may not be giving a fair representation of the entire tank..

 

[Johntheo5]

Check for any circ pump, either boiler internal or external stopped with boiler off.

 

[ndoubt]

Check for any circ pump, either boiler internal or external stopped with boiler off.

Yup that was what I thought was the leading issue to begin with - we had a secondary circ pump - but put that on a timer so ruled that one out..

 

[Madrab]

Run the hot water at an outlet and see what it's temp is and compare it to the evo. That being said if it's @ 55deg at night then it suggests it is ok

I'd also use a laser thermometer to check the cylinder lower down at the thermostat point, can't remember if you can get to the actual cylinder pocket in the thermostat box tho.

 

[Johntheo5]

Yup that was what I thought was the leading issue to begin with - we had a secondary circ pump - but put that on a timer so ruled that one out..

It could possibly still be thermo circulating through that loop even with the pump off, feel the pipework.
Might require a NRV.

 

[ndoubt]

It could possibly still be thermo circulating through that loop even with the pump off, feel the pipework.
Might require a NRV.

Pipework does feel cold…esp the pipes leading to heating coil - certainly below room temp. Wld have thought when hot water is off the heating coil pipes should be at least room temp..?

 

[Johntheo5]

The secondary circulation is around the cylinder not the coil.

 

[ndoubt]

Run the hot water at an outlet and see what it's temp is and compare it to the evo. That being said if it's @ 55deg at night then it suggests it is ok

I'd also use a laser thermometer to check the cylinder lower down at the thermostat point, can't remember if you can get to the actual cylinder pocket in the thermostat box tho.

Yeh that’s the thing evo says it’s 30 degrees but there’s clearly hotter water in there ..

 

[ndoubt]

The secondary circulation is around the cylinder not the coil.

Yeh sorry I conflated 2 things in that comment - secondary circ seems fine - I was going along with the theory that there is cold water circulating thru the primary flow and return when the HW is off which is cooling the water.. and the primary flow def feels very cold

 

[Johntheo5]

Yeh the kit was literally fitted a few days ago .. can’t imagine there’s an issue with it.. but point taken..

One thing I will say is although the reading is 30 degrees, I am able to have a hot shower even on that reading.. which tells me the evohome may not be giving a fair representation of the entire tank..

You have ruled out thermo syphoning and hot water leak, doesn't leave much, the stat/temperature probe is normally ~ 1/4 way up the tank so if say 80% of the tank is cooling down from 55C to 35C overnight it means loosing 2.33kwh/100L or ~ 4.7kwh (80%) if a 250L HW cylinder in say 9 hours or 12.5kwh/24 hours, very unlikely?.

 

[ndoubt]

You have ruled out thermo syphoning and hot water leak, doesn't leave much, the stat/temperature probe is normally ~ 1/4 way up the tank so if say 80% of the tank is cooling down from 55C to 35C overnight it means loosing 2.33kwh/100L or ~ 4.7kwh (80%) if a 250L HW cylinder in say 9 hours or 12.5kwh/24 hours, very unlikely?.

If the probe is 1/4 way up conceivable that’s it’s just measuring temp of the bottom 25-30% of the water .. still is high right. Does feel like there could be an issue where cold water is somehow flowing thru the primary flow and cooling that bottom section over night ..

On that note, there is a pipe that spurs onto the primary return that feels like it’s always cold.. could that be thermosyphoning?! Pic attached (excuse mess lots work going on) red is return pipe, blue is this cold pipe I can’t tell where comes from

 

[Johntheo5]

The reason for having a cylinderstat/sensor low down is to start reheating as soon as 20/30 Litres of hot water is drawn off. If you have a household thermometer, draw off a few litres of hot water and measure the temperature and ditto in the morning.