Project Overview
| Location | Marylebone, central London (NW1) |
|---|---|
| Call-out | Emergency — hot water cylinder thermostat not working |
| System | EHC Neptune unvented hot water cylinder in a flat |
| Fault found | Water getting into the cylinder’s dry sensor pocket, so no thermostat can read correctly |
| Work completed | New ESI electronic dual cylinder thermostat fitted and tested; cylinder replacement advised with a quote to follow |
The Challenge
We were called out to a flat in Marylebone because the thermostat on the hot water cylinder had stopped working. On most systems that is a straightforward swap: take out the old cylinder thermostat, fit a new one, set the temperature and test.
This one was different. When the faulty thermostat was removed, water came out of the dry pocket on the unvented cylinder — the sealed tube the thermostat’s sensor sits in. That pocket should never hold water. The cylinder manufacturer’s technical support could not be reached on the day, so the diagnosis had to be made on site.
The Solution: Prove the Thermostat, Then Diagnose the Cylinder
To rule out the thermostat itself, we fitted a new ESI electronic dual cylinder thermostat. It did not work correctly either — the water getting into the dry pocket stops the sensor reading the cylinder temperature properly. This was the second thermostat fitted to the cylinder with the same result, which points squarely at the cylinder, not the controls.
We explained to the customer that the fault is inside the cylinder and that fitting more thermostats will not fix it. The right repair is to replace the unvented cylinder, and a quotation is being prepared.
Why Water in the Dry Pocket Means a New Cylinder
The dry pocket is built into the cylinder. It lets the thermostat sensor measure the stored water temperature without touching the water. Once water is getting into it, the problem cannot be put right from the outside:
- The sensor cannot read properly: A probe sitting in water no longer gives the thermostat a reliable cylinder temperature, which is why two new thermostats behaved the same way.
- The fault is part of the cylinder: The pocket is not a separate part that can simply be swapped, so the leak path is inside the cylinder itself.
- Temperature control matters on unvented cylinders: Mains-pressure cylinders rely on their thermostats working correctly alongside the safety devices. Running one with unreliable temperature control is not a long-term fix.
- Replacement settles it: A new unvented cylinder brings new thermostats, pocket and safety valves, instead of a run of call-outs chasing the same fault.
The Repair Process
- Thermostat check: The cylinder thermostat was confirmed not working and removed.
- Dry pocket inspection: Water came out of the dry pocket, showing water was getting into the sensor pocket on the unvented cylinder.
- New thermostat fitted: A new ESI electronic dual cylinder thermostat was fitted to rule out the controls. It also failed to work correctly because of the water ingress.
- Advice given: With two thermostats showing the same fault, we advised the customer that the cylinder needs replacing and that a quote would follow.
The Results
The customer has a clear answer instead of another thermostat that will not last. The fault has been traced to water getting into the cylinder’s dry pocket, the controls have been ruled out with a brand-new thermostat, and a quotation for a replacement unvented cylinder is on its way.
Recommended Follow-Up
- Replace the unvented cylinder: As soon as possible, so the flat has reliable, properly controlled hot water again. A quotation will follow.
Cylinder Thermostat Questions
Why does my new cylinder thermostat still not work?
If a brand-new thermostat behaves the same as the old one, the fault is usually not the thermostat. On this Marylebone job, water was getting into the cylinder’s dry pocket, so no thermostat could read the temperature correctly.
What is a dry pocket on a hot water cylinder?
It is a sealed tube built into the cylinder that the thermostat sensor slides into. It keeps the sensor dry while letting it measure the stored water temperature. Water inside it means the cylinder itself has a fault.
Do you cover Marylebone?
Yes. Talis covers Marylebone and nearby St John’s Wood, Primrose Hill, Bloomsbury and Notting Hill for plumbing and hot water repairs. For another cylinder fault, see our leaking cylinder expansion vessel in Ashford.