Hot water issue

[NiceCupOfTea]

A little diagnostic help required!

Standard hot water/central heating system, probably fitted and upgraded in stages between the 70s and early 2000s. Gas fired boiler, pump, two zone valves, hot water cylinder in airing cupboard. Controlled by a Nest that runs a simple schedule on the heating and HW always on with a tank stat.

Recently have noticed that the hot water is not always that hot, or it is hot but doesn't last as long. I'm trying to work out what the issue is. Boiler is running fine, no problems with the central heating.

Tank stat was replaced 4 or 5 years ago, but I am suspecting this or possibly the HW zone valve - it's been a bit sticky in the past. Sadly the head isnt replaceable without draining down.

Any thoughts on anything else? I'm going to clean up/lube the zone valve head and check the tank stat is fixed to the cylinder properly...

Edited by NiceCupOfTea on Thursday 15th January 13:35

[OutInTheShed]

Be methodical.
Check the Hive is doing what you think it should.
Check the stat generates the signal to drive the zone valve
Check the zone valve moves when given the signal.


A couple of houses ago, similar issues were driving me mad, I put a neon indicator on the stat, which showed it was intermittent.
After I'd changed the zone valve of course!

[B'stard Child]

NiceCupOfTea said:

A little diagnostic help required!

Standard hot water/central heating system, probably fitted and upgraded in stages between the 70s and early 2000s. Gas fired boiler, pump, two zone valves, hot water cylinder in airing cupboard. Controlled by a Nest that runs a simple schedule on the heating and HW always on with a tank stat.

Recently have noticed that the hot water is not always that hot, or it is hot but doesn't last as long. I'm trying to work out what the issue is. Boiler is running find, no problems with the central heating.

Tank stat was replaced 4 or 5 years ago, but I am suspecting this or possibly the HW zone valve - it's been a bit sticky in the past. Sadly the head isnt replaceable without draining down.

Any thoughts on anything else? I'm going to clean up/lube the zone valve head and check the tank stat is fixed to the cylinder properly...

Hard water area?

[WrekinCrew]

NiceCupOfTea said:

... Sadly the head isnt replaceable without draining down...

You should be able to replace the synchronous motor in the head without removing it. They are pretty cheap and (AFAIK) all the same.

eg https://www.toolstation.com/corgi-synchronous-moto...

But just cleaning the contacts may help.

[GasEngineer]

With the Hot Water permanently on; have you tried turning the cylinder stat up and down to check that it is firing / switching off?

[NiceCupOfTea]

WrekinCrew said:

NiceCupOfTea said:

... Sadly the head isnt replaceable without draining down...

You should be able to replace the synchronous motor in the head without removing it. They are pretty cheap and (AFAIK) all the same.

eg https://www.toolstation.com/corgi-synchronous-moto...

But just cleaning the contacts may help.

Annoyingly it isn't - I think there are two types of the Honeywell ZV we have - I identified it as one where you could change the motor a few years back. As I took the 4th screw out it started spraying hot water at me In the past I've lubed it up and switched it back and forth a few times, so I suspect it needs changing.

Unfortunately the newest bit of the system (apart from the Nest which I fitted) is the boiler which is a 25 yo Potterton Suprima. Although it's been checked over fairly regularly, apart from a main board or two in the boiler nothing has been replaced in the 18 years we've been here.

[NiceCupOfTea]

GasEngineer said:

With the Hot Water permanently on; have you tried turning the cylinder stat up and down to check that it is firing / switching off?

Difficult to tell. Will turn the CH off later and see. Should it be fairly instant?

[GasEngineer]

NiceCupOfTea said:

GasEngineer said:

With the Hot Water permanently on; have you tried turning the cylinder stat up and down to check that it is firing / switching off?

Difficult to tell. Will turn the CH off later and see. Should it be fairly instant?

Yes. You should hear the thermostat click and the pump should switch on or off depending which way you are turning the stat.

[B'stard Child]

NiceCupOfTea said:

check the tank stat is fixed to the cylinder properly...

If you do need to replace the tank stat I have a drayton one that works fine (less than 6 mths old) that I replaced with a danfoss one that allowed a lower set point - ie 45 deg rather than 50 deg

Happy to post it out to you

[NiceCupOfTea]

Thanks for the offer B'stard Child, fairly confident the stat is OK, it's the only bit of the system that I know has been replaced in the last 5 years

Just had a fiddle - the stat is clicking away as I turn it each way past the trigger point. The zone valve activated and moved as it went each way as well, but only on about the 3rd or so go did the boiler and pump fire - so I'm guessing it's probably a tired zone valve. Whether a bit of WD40 will sort it, or it's internally corroded I'm not sure, but it is old enough to be one of the 4 screw ones where you can't change the head. There's clearly been a bit of seepage over the years as there is some corrosion on the metal cover and it has actually rotted through the bottom of it!

I think the next time we get it serviced I'll just ask them to replace both zone valves. Just a bit worried what other things will then start to fail!

[B'stard Child]

NiceCupOfTea said:

Thanks for the offer B'stard Child, fairly confident the stat is OK, it's the only bit of the system that I know has been replaced in the last 5 years

No problem

NiceCupOfTea said:

Just had a fiddle - the stat is clicking away as I turn it each way past the trigger point. The zone valve activated and moved as it went each way as well, but only on about the 3rd or so go did the boiler and pump fire - so I'm guessing it's probably a tired zone valve. Whether a bit of WD40 will sort it, or it's internally corroded I'm not sure, but it is old enough to be one of the 4 screw ones where you can't change the head. There's clearly been a bit of seepage over the years as there is some corrosion on the metal cover and it has actually rotted through the bottom of it!

I think the next time we get it serviced I'll just ask them to replace both zone valves. Just a bit worried what other things will then start to fail!

Oh that looks well past it's best

At least with a modern zone valve you can remove the actuator and free off a sticking valve or even work it manually with a spanner in emergency

Hopefully you don't have a wiring centre like this



I found out to my cost that modern zone valves aren't compatible with old Sunvic relay based wiring centres..........

[NiceCupOfTea]

My concern is that it may have a knock on effect on the rest of the system. At some stage in the next 5 years I want to get it all overhauled - at the moment we have very hot rooms and very cold rooms, so I'd like to get TRVs fitted throughout, more efficient modern rads, and a new boiler. Whether we go for another condensing one with a bigger hot water cylinder or a combi I don't know. My experience with combis isn't great but that goes back to a rented flat 20 years ago. It was either no hot water or scalding hot. I am sure technology has moved on.

I've made sure it's oiled up as much as I can but you can see the amount of corrosion on it. It seems to be working (switching as required) - but don't know if that was the problem before.

Interestingly the flow from the shower doesn't seem to be as good as it was before - it's a power shower, the cold seems fine but a bit weak from the hot, but I'm not sure what that suggests... Thoughts?

As somebody asked earlier, we are in a hard water area so I'm sure there is an awful lot of fur in pipes.

[B'stard Child]

NiceCupOfTea said:

As somebody asked earlier, we are in a hard water area so I'm sure there is an awful lot of fur in pipes.

Pipes probably OK but the tank is what I was concerned about.

I asked because like you we are in a hard water area and have vented system, CW tank in loft along with F&E tank, and HW tank in an airing cupboard.

Back in 2012 I noticed that the water just wasn't getting as hot and we seemed to be running out when Mrs BC took a bath or we took 4 showers in a day which had never been an issue before.

We turned up boiler flow temp it made little difference and in the end we started using the immersion to boost the temp whenever a bath was needed. Then the immersion failed and I tried to remove it and the tank top started to buckle around the immersion boss. The tank was original for the house build so 30 years old and was a plain copper one with a puffa jacket type coat to help with heat loss. I decided best thing to to was replace it.

Bought a new tank (same size but pre-insulated with spray foam)
Drained down and disconnected it - I underestimated how damn heavy it was empty
Got it down stairs and fitted the new one (so much lighter)
Filled up the system - checked for leaks - fired up the boiler and the water heated really quickly (like 30 mins when we normally had heat HW for 60 mins)

Local scrap metal place would take the tank but only if the top was cut off - so I did - bottom third of the tank was filled with limescale must have been a good 1cm of scale on the heater coil and the immersion element was one big lump.

Emptied out the scale and got £40 back on the tank to offset the cost of a new one.

Shortly after this I fitted a water softener...............