Issues with hot water from cylinder.

housebuyer143

I hope someone can advise - this is the first year I've noticed an issue. 
We have a 3 year old mega flow unvented cylinder 170ltrs. We have it heat 40 mins in the morning and 30 in the afternoon and we normally have enough hot water for 25 mins in the shower. 
What I have been noticing is that the water is only luke warm - this morning it's no hotter than it was last night even though it's been on 40 mins...

When we do get hot water, it's going cold in 10 mins when having a shower- or if someone uses it in the day, even a little it doesn't seem to be keeping it hot enough - it's like it mixes with the cold and it is all luke warm again??

If I put the boiler on for an extra hour, it seems to be really hot again.

*I have only noticed this the last 2 weeks, which could correspond with the sub zero temps we have had over night. 

Is this likely an issue with the tank, or does it sound like it's struggling to heat the cold water coming into the tank fully and I need to put it on more??

Eldi_Dos

What is the location of the cylinder and would it benefit from some extra insulation to counteract the colder temperature's?

WIAWSNB

It's hard to think of a fault or situation where additional cold is somehow mixing with the hot already in the cylinder - as I suspect you know, these are 'sealed' (unvented, pressurised) tanks, and the only cold water that goes in is to displace the same amount of hot being taken off. 
So that does suggest it's more likely your other scenario, and - yes - incoming water will be significantly colder at the moment by a number of degrees - I dunno, around 5 cooler?
Just how much colder will depend on things like where your nearest reservoir tank is, whether it's located above ground, and things like that.
What to do? Personally I'd just retime the tank heating to compensate, and leave it for a few months until ambient air temps are consistently in, say, the low teens again. Then revert to your previous timings - see if it's back to normal.
And if you haven't had a G3 service in three years, I'd suggest you book one in - no rush, I'd leave that until winter is over too. Say at your next boiler service.

plumb1_2

Your incoming mains water supply will have dropped by maybe 8/10 degrees [its winter) So will take longer to heat up.

housebuyer143

WIAWSNB said:

It's hard to think of a fault or situation where additional cold is somehow mixing with the hot already in the cylinder - as I suspect you know, these are 'sealed' (unvented, pressurised) tanks, and the only cold water that goes in is to displace the same amount of hot being taken off. 
So that does suggest it's more likely your other scenario, and - yes - incoming water will be significantly colder at the moment by a number of degrees - I dunno, around 5 cooler?
Just how much colder will depend on things like where your nearest reservoir tank is, whether it's located above ground, and things like that.
What to do? Personally I'd just retime the tank heating to compensate, and leave it for a few months until ambient air temps are consistently in, say, the low teens again. Then revert to your previous timings - see if it's back to normal.
And if you haven't had a G3 service in three years, I'd suggest you book one in - no rush, I'd leave that until winter is over too. Say at your next boiler service.

Thanks - all our reservoirs are outside so the water coming from the taps is really cold - great for drinking water but not so great for everything else.
I will put it on an hour twice a day and see if that helps with the issue.
I have not had a service on it, but I do refill the airgap frequently myself because it depletes quite fast owing to our very high water pressure (7.5bar). We used to get quite bad back back filling which we resolved with a pressure reducer on the mains, but it still goes down faster than advised.  When we had an engineer out to look at that issue, he kind of suggested that as it is sealed, there is almost nothing they can do on the service apart from refill the airgap?

plumb1_2

There’s a bit more to servicing a megaflow,  checking pressure, operation of safety valves, testing electrical items.
7.5 bar is far to high, ideally should be about 3 bar, no wonder your loosing the air gap.

mr insurance we’ve had a fire/flood from our megaflow and caused £10k worth of damage. oh were sorry to hear that, have you been having it serviced by a G3 qualified engineer?  Ehhh sorry No, ok then claim closed.

WIAWSNB

As Plumb says, that's a scarily high pressure - the shock on every valve, toilet, and tap must be significant.
Has no plumber recommended a Pressure Reducing Valve?

bjorn_toby_wilde

Surprised a PRV wasn’t added to the system when the Megaflo itself was fitted. They do need a regular safety check. We combine it with the annual boiler service.

Albermarle

WIAWSNB said:

As Plumb says, that's a scarily high pressure - the shock on every valve, toilet, and tap must be significant.
Has no plumber recommended a Pressure Reducing Valve?

The OP said 'We used to get quite bad back back filling which we resolved with a pressure reducer on the mains, but it still goes down faster than advised. 

Albermarle

plumb1_2 said:

There’s a bit more to servicing a megaflow,  checking pressure, operation of safety valves, testing electrical items.
7.5 bar is far to high, ideally should be about 3 bar, no wonder your loosing the air gap.

mr insurance we’ve had a fire/flood from our megaflow and caused £10k worth of damage. oh were sorry to hear that, have you been having it serviced by a G3 qualified engineer?  Ehhh sorry No, ok then claim closed.

I asked our plumber ( G3)  to service our Megaflow at the same time as the gas service.
When he arrived he said pretty much the same as the one the OP mentioned. There is nothing really that can be easily serviced.
He said something like if we check the pressure valves, we will have to replace them.

Servicing unvented cylinders has been mentioned before in other threads, but you always seem to get a variety of answers.

Albermarle

housebuyer143 said:

I hope someone can advise - this is the first year I've noticed an issue. 
We have a 3 year old mega flow unvented cylinder 170ltrs. We have it heat 40 mins in the morning and 30 in the afternoon and we normally have enough hot water for 25 mins in the shower. 
What I have been noticing is that the water is only luke warm - this morning it's no hotter than it was last night even though it's been on 40 mins...

When we do get hot water, it's going cold in 10 mins when having a shower- or if someone uses it in the day, even a little it doesn't seem to be keeping it hot enough - it's like it mixes with the cold and it is all luke warm again??

If I put the boiler on for an extra hour, it seems to be really hot again.

*I have only noticed this the last 2 weeks, which could correspond with the sub zero temps we have had over night. 

Is this likely an issue with the tank, or does it sound like it's struggling to heat the cold water coming into the tank fully and I need to put it on more??

We have a 210L unvented tank, heated with a gas system boiler.
In warmer weather it is on 30 mins in the morning and 15 mins in the afternoon.
This is OK for on average 1.5 showers a day and 4 baths a week.
In this cold weather I have bumped it up to 35mins and 20 mins. Plus the boiler flow temperature has been increased from about 58 degrees to 63 degrees.
Everything nice and hot, although you have to turn the shower stat up a bit to account for the cold feed being colder.
Although I have to say we kind of stick to the max 4 minute rule for showers,and it sounds like yours are longer.

Do you know what your boiler flow temperature is, maybe its a bit low ?