I thought I’d fixed the hot water as per my previous thread, and it seems to be mostly working although it’s not very hot for some reason.

So in poking around the airing cupboard I noticed something weird. I can’t figure out what all these pipes are.

airing cupboard by molgrips, on Flickr

So the green arrows seem to be the hot water feed from the boiler. It goes through the pump then down through either CH or HW circuits depending on which valve is open. These seem to open and shut as expected. In the case of hot water, the green arrows show the path of water to the heating coil in the tank, as far as I can tell. All fine.

However, after the pump and the CH valve junction there’s another T junction, and some of my hot water (maybe the majority) is going down through these two pipes into the floor, and I have no idea why. It would seem to be bypassing the hot water tank altogether. Plenty of hot water goes down there, because it got hot quickly possibly quicker than the tank heating circuit. I do have towel rails but they are on the CH circuit – everything seemed to be stone cold when I checked.

Anyone shed any light? This comes after the hot water got stuck on and got really really hot several times. There is something that might be a safety valve at the first elbow where that first red arrow turns downwards (you can just see it) – could this be a one-time safety diverter valve or something?

Bloody hell, and folk wonder why people prefer combis!

Have you identified the return(s) to the boiler?

I think one of those red pipes will be boiler return, the other might be a bypass circuit for safety

+1 Stoner

There should be a pressure relief valve in the bypass circuit. What is it at the 90deg bend in your red arrow pipe at the vertical / horizontal joint. It is the likely location for such a valve. It relieves pressure if there is not sufficient flow in the CH / DWH circuit.

If that has failed, or is set incorrectly, it would certainly explain why your hot water isn’t that hot.

Also, can’t see the return from the tank coil. You have identified that? And it goes into one of the red pipes??

The other end of the primary coil is just behind the thermostat, which is the box just to the left of the immersion heater. Given that, it does seem like the the first red elbow is a bypass circuit, and the red thing is highly likely to be a pressure release valve of some kind.

Pics of it:

Untitled by molgrips, on Flickr

With the cap off:

Untitled by molgrips, on Flickr

Aren’t all of the red bits tprv’s, as that’s what they look like.

Possibly yes. I’d thought the fact they were red was some kind of emergency related colour coding.

That valve is solely a bypass essentially a valve with a spring that you wind in to adjust the pressure at which it opens.
a pressure relief valve in plumbing terms is completely different.

Well that matches with observations. There is a scale on the red bit marked 0.1-0.5 bar which increases as you screw in. The question is, what’s changed? What should the pressure in the system be, on the gauge?

There is some dark goo around it, wondered if it was some kind of wax plug type safety feature but I guess not.

I would say that that valve is a bypass valve that should generally be closed until both the hot water and central heating switch off, but the boiler is still on overrun mode (for about 30 seconds to get rid of some heat). Due to both motorised valves bring closed the pressure will build up and open that valve so the water can still circulate till the boiler switches off, which will also switch the pump off. So wind the valve in quite a long way so that it stays closed until both motorised valves are closed, in which case it will allow water to circulate and cool the boiler a bit. Why the pipes with your red arrows split I have no idea, maybe to make a bigger bypass circuit. These pipes will join up with the return pipe to the boiler. So if the valve is too open it is much easier for the water to go that way than through your coil

Hope that makes sense.

There is a scale on the red bit marked 0.1-0.5 bar which increases as you screw in. The question is, what’s changed? What should the pressure in the system be, on the gauge?

With all due respect Molly, is it not time you just got a professional in to take a look?
I know you’re not suggesting anything on the gas side, but any work associated with an unvented cylinder needs to be done by a competent individual with the right bits of paper.

Get it sorted properly before the temp drops and you really need it.

Regarding the red arrows after the bypass valve, one of the arrows you’ve marked could actually be pointing the wrong way and could be the return from the radiators.

I missed the fact that the return loop is behind the stat, so I would agree with flux above, one of the red arrows could be the return from the central heating and the bypass valve is a quick loop from flow to return

Hmm.. The ch water would be pumped through the pump the through the valve to the rads, then back to the boiler no? Would it really come back to the cupboard?

Anyway what you have al said makes sense. The boiler does run on for a while after the burner goes off. The thing is, those red pipes got hot when only the water was on, not the CH.

Although, now the ch is on and not the water, some weird stuff is happening. The bypass loop is hot but only as far as the hot water primary coil exit! And the two red pipes are cool! Wtf? Well one pipe is lukewarm the other is warm but not hot.

Assuming your other thread is the one where you’ve replaced a rad, you may have an airlock somewhere that is responsible for the weird things. The thing on the elbow above the HW valve may be an automatic air valve; are there any others? Bits of pipe or rads that don’t get hot?

I can’t quite follow what’s connected to what on the left side. If your bypass circuit is behaving oddly, are your rads balanced using the lockshield valves?

I can’t remember the details, but my [certified] installer described a complicated fault that could occur if you leave the filling loop connected, resulting in the boiler exploding. Probably best to disconnect it, as it’s a legal requirement to do so anyway.

The filling loop has two taps on it, both of which are closed. Need to open it occasionally to top the pressure up.

I bled the rads, but they all get nice and hot – this is about the hot water rather than the heating.

Due to both motorised valves bring closed the pressure will build up and open that valve so the water can still circulate till the boiler switches off, which will also switch the pump off.

I just watched it come on then off – the pump switched off as soon as the valve STARTED to close, so no overrun.

I just watched it come on then off – the pump switched off as soon as the valve STARTED to close, so no overrun.

Often the pump is wired so that it is powered by the valves, so you can’t have the pump on with both valves shut. On boilers which need an overrun, they should have a separate power feed for the pump, so the water can circulate through the bypass when the valves have closed.