Unvented cylinder heated by combi boiler losing heat quicker than expected

[fenderjazzbass]

Hi everyone, looking for some advice.

We had a plumber/heating engineer come round and take a look, and they just sent through a quote for an entire system replacement which might be ideal but we'd rather fix what we've got if at all possible.

I've attached some images of the setup we have. It looks like one of the previous owners installed a combi and then decided that it wasn't delivering enough pressure or hot water, so hooked it up to an unvented cylinder. The combi's hot water output has been disconnected from the hot water pipe.

Hot water is scheduled between 6-7am and 6-7pm, during which we have hot water.

After those two one hour periods, the water temperature stays hot for about an hour, then becomes tepid after two hours and stays this way for a few more hours. I'd say that about 6 hours after heating up the cylinder, we then have cold water coming from the hot tap if nobody has used the hot water.

Water heats up pretty quickly if we turn it on manually on the hot water scheduler - only need to wait 5/10 minutes before it's hot enough for a shower.

I've drawn some red lines on which pipes are warm at 10am today, and blue lines on pipes that seem to have flow going through them (I can hear running water, feeling the pipes, there's a slight vibration to them), despite no water being used in the flat (the digital water meter doesn't have the little flow icon).

Central heating seems to run through the same pipes that heat the unvented cylinder, I imagine that the two silver Honeywell valves are supposed to open & close depending on what requires heat?

From what I understand, an unvented cylinder should keep water hot for a day or so.

My questions:

Does anyone have any idea what could be going wrong here?

What is this thing that I've circled in green?

Why would a pipe heading into the inlet be warm (the pipe I've drawn a red line over)?

Thanks,
Dan

Edit: worth mentioning that our water pressure is pretty low

 

[Johntheo5]

The item circled in green is a ABV (automatic bypass valve) which should open when the boiler shuts down to allow its heat exchanger to cool down, its strange though that it seems to be installed after the cylinder motorized valve, can you look for a arrow on the valve body and see which wav its ointing?
Can you also post any details from the cylinder labels etc.

 

[fenderjazzbass]

Thanks John! Yep! The valve direction seems to be out from the primary flow pipe into the primary return pipe. Images attached.

 

[Johntheo5]

If you follow that pipe back (first photo, first post) there seems to be another motorized valve on that same pipe, just confirm that it isn't.
Where is the cylinder temperature sensor or cylinder stat located?

 

[fenderjazzbass]

Are you referring to this (image attached, red circle)? Or the pipe that I drew a red line over in the first post?

If you're referring to the thing I've circled in red, that does indeed seem to be a motorised valve, and it leads to the central heating loop.

If you're referring to the pipe heading behind the cylinder, that seems to go outside and down the side of the building...

I have no idea where the cylinder tempreture sensor is located... I just looked up the product stats for this cylinder and it says that the thermostat is preset to 55-60c. https://www.wolseley.co.uk/wcsstore...std.lang.all/_t/ds/176702_CenterStore_TDS.pdf

I would imagine that the thermostat and/or sensor is behind that plastic cover below the label on the cylinder?

 

[Johntheo5]

You may be circulating water around the HW circuit which will very quickly remove the cylinder heat, the bronze pump for this secondary circulation is/should be on a timer and possibly a temperature sensor somewhere near the furthest away HW draw offs, can you post a photo of that Grundfos circ pump and see if its connected to any of the pipework near the cylinder.
See attached, trying to see if the Grundfos pump is a secondary circulator.

 

[fenderjazzbass]

I think you're on to something John... When I close that isolator valve the pump does change pitch. That pump does run constantly, which I had thought is a bit strange.

It's connected to a pipe that heads outside (see first post), and then back into the inlet with the isolator valve.

The power cable attached to the Grundfos pump heads off in the direction of the hot water scheduler control panel (a Siemens RWB29), but I can't see exactly what it's attached to without pulling it all off the wall and taking a look behind.

When you say the "bronze pump", are you talking about the Grundfos pump? If so, what should be controlling this timer do you reckon? It's definitely never been turned off while we've been living here.

It would make a lot of sense that this would draw heat out of the water that's stored in the cylinder.

Any idea why this kind of setup would exist or what it's trying to solve for?

Thanks so much!

 

[fenderjazzbass]

I just read up on why you'd want a secondary circulation and it makes sense. When hot, our water does come out hot almost instantly.

However, it's not been well insulated at all and it even goes outside (!?), so I can imagine this is drawing off a LOT of heat.

 

[fenderjazzbass]

Traced the cable back and it's wired up directly to mains inside a backbox with a 13A switch screwed on top (no wire is connected to the 13A switch)... I guess someone really didn't want this pump being turned off.

What are the risks involved with disconnecting this pump and turning off that valve? I imagine that since it goes outside it could potentially cause problems if the temperature goes below freezing.

I imagine that it would be fine temporarily to test if this keeps the water hot in the cylinder for longer, but if we decide we don't need the secondary circulation, would it be quite an involved process?

Worth noting this is in Victorian 2 bed 1 bath top floor flat...

 

[Johntheo5]

Traced the cable back and it's wired up directly to mains inside a backbox with a 13A switch screwed on top (no wire is connected to the 13A switch)... I guess someone really didn't want this pump being turned off.

What are the risks involved with disconnecting this pump and turning off that valve? I imagine that since it goes outside it could potentially cause problems if the temperature goes below freezing.

I imagine that it would be fine temporarily to test if this keeps the water hot in the cylinder for longer, but if we decide we don't need the secondary circulation, would it be quite an involved process?

Worth noting this is in Victorian 2 bed 1 bath top floor flat...

You can certainly turn it off temporarily for now and shut that blue isolating valve, is there much of the pipe outside and is it underground?
That pump was installed in week 11 2012 (PC1112) so must have wasted a lot of energy running continuously for the past 12 years, there may be some smart way of scheduling it on with a few temperature probes and a timer, the only downside withhaving it off is it will take some time for HW to arrive at the endest user say in the morning but see how it goes for a few days.

 

[fenderjazzbass]

Good spot! There's none of the pipe underground. This is in our attic, and we occupy the top two floors of a 4 storey Victorian building.

It heads out the back of our attic, down one story to the bathroom (I imagine), maybe 2-3m of it outside against the wall?

Yep, this has definitely wasted a massive amount of energy just running continuously. We were using about 12kWh worth of gas just to heat up water every day for a total of two hours.

Quite annoyed that the plumber who came for a callout didn't identify this as a potential issue.

Thank you so much for all your insight, I really really appreciate it. I'll let you know how we get on. Have a great weekend!