Hot water from a Megaflo or other unvented cylinder that fades after a minute or two is two separate problems, and the tap tells you which one you have. If the stream thins while the water stays hot, the cold mains side is the place to look: supply pressure, the strainer in the inlet group, the expansion volume. If the stream holds its force while the water turns cool, the stored volume or the reheat is behind it, and pressure is not the issue at all.
Flow drops but water stays hot, or flow holds but water turns cool: how the two patterns differ
Hot water from a Megaflo or other unvented cylinder fades in two different ways, and each one leads to a different part of the system. Watch what the tap does over the first two minutes and compare the two columns.
| Criterion | Flow drops, water stays hot | Flow holds, water turns cool |
|---|---|---|
| What changes at the tap | The stream thins while the temperature stays where it was. | The stream keeps its force while the temperature slides down. |
| Cold water at the same mixer | Often weak too: the balanced cold take-off feeds mixers from the same inlet group as the hot. | Cold is unchanged; only the hot side loses temperature. |
| Parts an engineer looks at | Cold mains supply, the pressure reducing valve and its strainer, and the expansion volume inside the cylinder. | Stored volume in use, the heat source and its controls, the blending valve and air in the reheat coil. |
| Supply figures that matter | Megaflo Eco quotes 1.5 bar and 20 litres per minute as the minimum cold supply; at those values flow can be poor with two outlets open. | Supply figures do not explain it; cylinder size against the amount of hot water drawn does. |
| A clue the manufacturer names | Intermittent discharge at the expansion valve is listed by the maker as a sign the air volume has been lost. | Maker fault lists point to the mixer setting, the heat source, the controls and reversed hot and cold connections. |
The balanced cold take-off is the cold feed drawn off inside the cylinder's inlet group, between the 3.5 bar pressure reducing valve and the 8 bar expansion relief valve. It serves showers and mixer taps so hot and cold arrive at a matched pressure. A restriction before the cylinder can therefore weaken both sides of a shower at the same time.
When the flow drops while the water stays hot on an unvented cylinder
A stream that thins within a minute or two while the water stays hot points to the cold mains side of your unvented cylinder, not to the stored water. The cylinder still has hot water in it. What has changed is how much water can get through to the tap.
- A supply close to the minimum. Heatrae Sadia gives 1.5 bar working pressure and 20 litres per minute as the minimum cold supply for domestic Megaflo Eco sizes. The manual adds that outlet flow rates may be poor at those values if several outlets run at once.
- A second outlet opening. The shower is full for the first minute, then someone runs the kitchen tap and the shower thins.
- A blocked strainer. Manufacturer fault tables list a blocked strainer in the pressure reducing valve behind poor or absent hot water. Where the mains carries suspended solids, the maker requires adequate filtration because the strainer can silt up.
- A lost air charge. The expansion volume loses air over the years, and intermittent discharge at the expansion valve is the sign the maker names. More on that in our guide to an unvented cylinder expansion vessel losing its charge.
One mechanical explanation is that stored pressure carries the first seconds, after which the flow settles back to what the incoming main really delivers. Treat that as a possible explanation, not a diagnosis. Only a test of the supply and the inlet group settles it.
When the flow holds steady but the water from the cylinder turns cool
Water that cools while the stream keeps its force means the hot store or the reheat has run short, not the pressure. The tap is still delivering the same number of litres per minute. What drops is temperature.
- Two draws close together. A bath followed by a shower, or two showers back to back, can empty a store sized for a smaller demand. Our guide on what size hot water cylinder you need sets out how volume is matched to a household.
- The heat source is not reheating. Manufacturer fault lists name a heat source that fails to reheat and a faulty external control among the causes of a low blended temperature.
- The blending valve is set low. A mixer thermostat set too low gives lukewarm water at the tap even with a full cylinder.
- Air in the reheat coil, or reversed connections. Hot and cold connections swapped at the cylinder appear in the same fault lists, which matters after recent work on the system.
Manufacturers are explicit that the occupant should not alter the blending valve setting. Where the temperature at the tap is too high or too low, they say to call an engineer qualified on unvented hot water systems.
What you can check yourself before calling out an engineer to the cylinder
Four checks are safe for a householder: the boiler pressure gauge, the thermostat and programmer settings, the reset button, and bleeding radiators. None of them restores flow at an unvented cylinder, so be honest with yourself about what they can show.
The programmer is still worth a look in the cooling case. A hot water period that ends mid-morning leaves the store with nothing to reheat it before the evening. The cylinder thermostat setting is worth reading off at the same time.
Everything on the cylinder itself is engineer work: the strainer, the pressure reducing valve cartridge, the expansion volume, the safety valves and the blending valve. Recharging the air volume is done only with the system depressurised, and the maker warns against overcharging. Ask for an engineer qualified on unvented hot water systems, and where a gas boiler heats the coil, a Gas Safe registered one for anything on the boiler, the flue or inside the casing.
What the rules in England require for an unvented hot water cylinder
Unvented hot water systems in England sit under Part G3 of Schedule 1 to the Building Regulations 2010. G3 requires the system to withstand the temperatures and pressures of normal use and of reasonably anticipated malfunction. Stored water must be prevented from exceeding 100 degrees Celsius, any discharge must be visible and safely carried away, and hot water at a fixed bath must not exceed 48 degrees Celsius.
Approved Document G, in the version with the 2024 amendments and marked for use in England, adds the detail. Unvented systems up to 500 litres and under 45 kW should be a proprietary unit or package. They should carry at least two independent safety devices in addition to the thermostat.
Notification follows the installer. If the installer is not registered on a competent person scheme for unvented hot water, the building control body must be told before the work. A registered installer self-certifies, the owner receives a Building Regulations compliance certificate, and completion is notified within 30 days.
Manufacturers also set an annual service by a competent person, covering the safety valves, the strainer element and the air volume, to avoid faults in operation. Our guide on servicing unvented cylinders and Megaflo systems goes through what that visit covers.
Which of the two to report: flow drop, if the stream thins; cooling water, if only the heat fades
Report a flow drop if the stream thins within a minute or two while the water stays hot, and particularly if the cold side of the same mixer weakens with it or the expansion valve has been discharging on and off. That version sends the engineer to the supply, the inlet group strainer and the expansion volume.
Report cooling water if the stream never changes and the water only goes lukewarm, typically after a bath or a second shower. That version sends the engineer to the stored volume, the heat source, its controls and the blending valve.
If both happen together, say so rather than choosing one. A thinning stream and cooling water can sit side by side, and the engineer can then look at the supply and the store in the same visit. Note the time of day and which outlets were running, since that detail narrows it down.
Where the cylinder itself is at the end of its life and a replacement is on the table, a free home survey comes first and a written fixed price follows: see Megaflo and unvented systems.
Sources
- Heatrae Sadia — MEGAFLO Eco SystemFit product guide
- Heatrae Sadia — Megaflo Compact installation manual
- Heatrae Sadia — Megaflo Eco installation manual
- Heatrae Sadia — Megaflo Eco Plus 250/300 installation manual
- Baxi — Assure SystemReady installation and service manual
- Baxi — ASHP cylinder installation manual
- Glow-worm — Hydracyl unvented cylinder instructions for use
- The Building Regulations 2010, Schedule 1 (Part G3)
- GOV.UK — Approved Document G: sanitation, hot water safety and water efficiency
- The Gas Safety (Installation and Use) Regulations 1998
- HSE — Gas Safe Register