Water temp from Megaflow initially very hot

[Pablo79]

Hi all,
I seem to have a minor problem with my hot water. I recently had a new kitchen installed and had to drain the system (this might be coincidental). Since then, the was hot was has been a little strange... when you initially get in the shower, for example, the water comes out super hot... too hot really, and then it settles to a normal temp. Previously I never adjusted the temp mixer on the shower, got in and it was consistent from start to finish, but now I have to turn it down initially and then turn it up after 30 secs to a minute.

The system is a standard potterton boiler, a megaflow tank, and two honeywell two port motorised valves. I've checked the values and they spring back into position if I move the levers, so I'm assuming these aren't stuck. They're both operational too when the hot water or heating is used. I sat there and tested them by putting on the water, heating, and both together.

Why would this be happening with the water? The water in the tank is hot. After the initial really hot water, the temp settles and the water is lovely and hot. I just can't figure out why it initially comes out so hot, as it never used to.

Many thanks

 

[Agile]

Are you sure the problem is not with your shower?

Why not measure the flow temperature coming out of the cylinder?

I suspect you will find that it is constant. Should be about 55 C to 60 C

Tony

 

[Pablo79]

Thanks for your reply. What in the shower could cause it? It's a simple wall mounted two dial mixer (Grohe) and a shower head elsewhere. One dial for flow, one for temp. If you do not touch the temp, it comes out super hot and gradually gets cooler over a minute or two, and then settles.

I'm pretty sure I had the same issue with water coming from the kitchen tap, so I believe the issue is not the shower, but I'll double check with the other shower and taps in the house.

If the shower is fine, what else could be causing this issue? I don't use the electric part of the megaflow... only the boiler to heat it.

 

[rules&rulers]

The temperature needs to be accurately measured with a thermometer at all the hot taps and the shower (hot only) then mixed /,as what Tony has advised. If you cant do that and you think the temperature is incorrect then call in the people that service your megaflow.

Maybe ask the people who drained your system if they can add any information.

Its a Process of elimination

 

[MeldrewsMate]

Draining and refilling a Megaflow will regenerate the airgap, and this may show itself as an initial increase in hot water pressure, possibly slightly higher than the cold water pressure, and possibly for the few moments you describe. If you are sure that no one has increased the cylinder temperature dial then it may be that the shower thermostat allows much more hot water flow than usual during these moments, causing the mixed temperature to rise for a period. If I am right then two things will be true. Firstly, you have a kitchen mixer tap too and, secondly, you won't be able to repeat the initial 'overheat' scenario at both the kitchen tap AND the shower within a few moments of the other 'overheating'.

The solution?
1. Consider reducing the hot water temperature to about 55C (unfortunately a Megaflow has a 1-5 scale only!)
2. Live with it, or
3. Wait for someone else to offer a better explanation/solution.

Hope this helps
MM

 

[Pablo79]

Thanks for the reply, MM. I used a different shower this morning to test, and the same thing occurred, so it's definitely not the shower that's at fault. It's only the first time the shower is used after the hot water has previously been on. If I use the shower later, the water temp is consistent.

There's no chance anyone else has touched the megaflow as it's just me living there. I have drained the system a couple of times due to installing a new kitchen, so would this lead us to believe there is an air gap somewhere?

I'd really like to resolve the issue, and I nearly scald myself each time i get in the shower as the temp setting is up from the previous use! It's worked fine for the last 3 years, so this is a little frustrating. Any ideas on how to resolve the issue?

 

[Alec_t]

It's only the first time the shower is used after the hot water has previously been on. If I use the shower later, the water temp is consistent.

Since there is a temperature gradient in the Mega and hot water is drawn off from the top of the tank, the water drawn off initially after the HW has been on will, unsurprisingly, be somewhat hotter than that drawn off later (unless by then the water has had time to get fully heated again). That doesn't, however, explain your experiences if this is something which has happened only recently. Puzzling.

 

[picasso]

The initial slug of hot water could be a passing non return valve, if the cold nrv is stuck open the cold pipe will fill with hot and this causes the initial slug of very hot water, showers are notorious for using cheap nrvs.

 

[Pablo79]

I did test both NRVs to make sure they operated when heating and hot water were used. Both levers moved and released when they should've done.

Also, rather oldly, I switched the heating off yesterday (finally! old cold house) and this morning i ran out of hot water after a shower of only 10 mins or so. I can't see how this is related, because when the heating is on and the hot water is off, the NRV stops water flowing to the tank. The pipes don't get hot.

 

[Agile]

I am going to suggest another possibility!

I suspect you are not reheating the cylinder as soon as hot water has been used but only for a short time once or twice a day.

There are two tests I would like you to do.

One is to keep the reheating of the cylinder on for 24 hrs and see if that makes any difference.

Other is to draw off 2-3 litres of hot water from a tap BEFORE you use the shower.

You can do just one of those tests.

My theory is that you are reheating the cylinder almost from cold and the expansion is causing a high pressure and the higher pressure is forcing more hot water through the shower mixing valves thus increasing the temperature initially.

It could be that the air bubble has become rather small is is not well absorbing the expansion. But the real cure is to avoid reheating the cylinder from cold.

Tony

 

[picasso]

NRV don't have levers so not sure what you have been doing.

 

[Pablo79]

NRV don't have levers so not sure what you have been doing.

My mistake. I meant the honeywell motorised valves. I'm getting my terminology mixed up.


Tony, thanks for your reply. That all makes perfect sense. However, it's just me using the shower and it's a big tank. I only have one or two showers a day and there's plenty of hot water left, so I'm definitely not running the tank empty. The HW does only come on once a day, but this is the way it's always been. Nothing has changed regarding how much hot water I use, or how often the water is heated. The only thing that changed is that the system was completely drained when the new kitchen was put in, and this problem has occurred ever since.

Could this therefore mean the air bubble needs to be restored? I actually have that on my list of things to do this weekend. It would make sense that the bubble was lost when the system was drained and refilled. I'll give it a try.

 

[Pablo79]

I'll also try the other tests too, btw.

 

[Agile]

My tests are based on an expectation that the bubble is not working well.

Even if you are not completely reheating the cylinder there will always be some expansion.

Remaking the bubble correctly will probably remove the problem.

Tony

 

[CamberleyBaker]

Pablo79, did you find a solution to this problem? I have exactly the same issue. We have 4 showers - 2 of them are fine after the initial surge of hot water and 2 cut out because the water is too hot, but they work perfectly after 6 or 7 button pushes to turn them on.