Megaflo Tank question

Hi everyone, I'm after some advice.

I've an problem with the hot water from my unvented 170l Megaflo tank losing pressure, it's up on the second floor of our house and has been installed for about 15 years, we've been in the property for 14 years.

The hot water pressure has recently dropped off significantly compared to how it's been for years. Now it will gradually get worse over a few days until it is barely a trickle, I can recharge the air gap to get some pressure back but it's never as good as it was and won't serve the second floor shower at all. The pressure to the ground and first floor taps will then fade again over a few days.

I got a Baxi engineer out (I think he said he was an engineer rather than a plumber) through a bit of a convoluted process of taking out a repair plan but he took one look at it and said it had not been installed properly so he couldn't do anything. He said it has no drain off valve so he couldn't inspect the tank. When I asked what to do he said it needed to be re-plumbed by a G3 qualified plumber. I asked, well how can that be done it the tank can't be drained, he said it would be a messy wet job etc etc.

So my questions in advance of getting a plumber in are:
- Is it highly likely that the fault is actually with the pressure control valve(part: 95605869)?
- Is there another way to drain the tank? ie turn off the main water inlet to the tank and run the hot taps for days
- Is there a whizzy tech way to stem the water pressure to get a temp drain off fitted to facilitate the proper re-plumbing job?

I'm asking because I like to try and be an intelligent customer (a feature of what I do in my day job) and I am very wary of less than thoughtful trades folk (sorry to the reputable ones!) even if they have the right qualifications so like to know a little bit about what's going on just to ask a few sensible questions! I will absolutely NOT be trying to do any work on this myself.

Any other advice would be welcome. I've attached a pic of the current plumbing where you can see the absence of a drain off. The company that did all of this (as part of a loft-dormer conversion) is long gone - we found this out when the roof leaked...

Thanks for looking!

That has indeed been installed by a total helmet. Draining down is simple enough to do but will likely result in the expansion relief valve needing replacing afterwards. It could otherwise just be a blocked filter or that faulty check valve.

Either way, I would budget for a new safety group and a couple of hours to reinstall the feed to the cylinder correctly that will enable easier future servicing like this mockup before installing:

Thanks Dan, much appreciated!

Its an OSO brand unvented hot water cylinder. It has an internal expansion space at the top. This is basically a buffer of air at the top of the cylinder to accommodate the expansion of the water when it is heated. Over time, this buffer can get depleted, and needs to be reinstated. The reinstatement procedure does not require a G3 qualified engineer, its a user procedure. I can't remember from memory whether the procedure is printed on the side of the cylinder, but I think it is. If not, then it can be downloaded from the OSO site.

Basically (but check the OSO instructions) it's:

1. Turn off the incoming cold water (small black handled valve at the left hand side of the combination valve in your picture).
2. Open the lowest hot tap and let it run until the water stops.
3. Open the temperature and pressure relief valve (not shown in your picture. You'll need to identify it).
4. Hold the TPRV open until the "glugging" noises stop.
5. Release TPRV, turn incoming cold back on, close hot tap.

Thanks Oldbuffer. I've done the re-instatement procedure for the air buffer and "yes" there is a sticker with instructions on the tank but it says to run the tap and hold open the TPRV at the same time, but start the tap first. I tried to summarise it in my original post and through doing this I can get some pressure back but it's never as good as it was originally. However, I have to admit that it was taking ages to drain all the hot water from the lowest hot water tap (kitchen) and I just got fed up holding the pressure relief valve (after about 40 minutes). The glugging definitely stopped at the TPRV valve after about 10 minutes though. So after 40 minutes I finished that process because I had some pressure, but then it fades again after a few days.

Do you think I should full drain the tank from the lowest hot water tap? Will that take hours?

Thanks.

oldbuffer said:

Its an OSO brand unvented hot water cylinder.

Click to expand...

No its a Megaflow

The problem has nothing to do with the air bubble - it is a restriction somewhere in the system. The first step is to eliminate and resolve the obvious problem with the safety group.

oldbuffer said:

Its an OSO brand unvented hot water cylinder. It has an internal expansion space at the top. This is basically a buffer of air at the top of the cylinder to accommodate the expansion of the water when it is heated. Over time, this buffer can get depleted, and needs to be reinstated. The reinstatement procedure does not require a G3 qualified engineer, its a user procedure. I can't remember from memory whether the procedure is printed on the side of the cylinder, but I think it is. If not, then it can be downloaded from the OSO site.

Basically (but check the OSO instructions) it's:

1. Turn off the incoming cold water (small black handled valve at the left hand side of the combination valve in your picture).
2. Open the lowest hot tap and let it run until the water stops.
3. Open the temperature and pressure relief valve (not shown in your picture. You'll need to identify it).
4. Hold the TPRV open until the "glugging" noises stop.
5. Release TPRV, turn incoming cold back on, close hot tap.

Click to expand...

I think this is in reply to a different post - this one https://www.diynot.com/diy/threads/dripping-valve-thingy.512177/#post-4252207

Thanks Dan, I'm proceeding with your suggestion anyway. The recharge of the air bubble is useful in that it just about keeps the hot water flowing on the ground and first floor until I can get the plumber in for the proper job. I was just curious as to whether I could make it last longer or get more pressure (so the second floor hot taps work) through running that recharge procedure longer.

Anyway, thanks all for the input so far.

The fact that recharging the bubble helps flow implies the problem is upstream of the tank (further implicating the safety group).

Dans right will defo be the safety group that style aren’t available anymore will need to do the job properly this time.

Thanks guys, I just hope there's room in there to do it properly. With the cold water feed coming up through the floor the space before it goes into the tank is pretty limited. Fingers crossed for this not being a huge job when I get someone out to it!

Decent plumber won't have an issue.