New homeowner (UK based) and getting to grips with all that comes with that. I previously only lived in a flat with a standard combi boiler, my new home seems to have an older system in place consisting of boiler + cylinder, but I would really like to understand how all of this works.

Now, I believe this system was upgraded throughout the years, as it includes a thermostat and a continuous flow of hot water (hot water is on demand, even when the timers are off, pictures to follow and questions at the end).

Boiler in the kitchen: enter image description here

Thermostat, downstairs and central in the property: enter image description here

Airing cupboard upstairs: enter image description here enter image description here enter image description here enter image description here

enter image description here

Behind the above I found this scary thing: enter image description here What the heck is that and should I be worried?

Based on the above I suspect I have either system 2 or 3 of the video provided in the comments, but I'd like to confirm this.

Some things to note:

  • I seem to have hot water on demand, I thought I would depend on the timers, or setting it up via the "switchmaster", but we have hot water at any time of the day!

  • Once heating comes on, it will shut off automatically once temperature on the thermostat as been reached.

Considering this, I seem to have all the benefits of a combi boiler, just with a lot more stuff taking up space and more energy?

I also noticed the shower could use a bit more water pressure, a new shower hose and head helped a bit, but not quite there yet, is there anything on this system that I could do to increase water pressure in my shower? Would switching to a modern combi system help at all with this?

  • When you moved, did you take your old shower head with you? Oct 15, 2019 at 14:28
  • No I didn’t, my old one was in a bad shape. Did buy a new hose and head today which helped quite a bit (previous owners also had some old kit)
    – joao
    Oct 15, 2019 at 14:31
  • Yeah, what you did there was reduce flow by using a shower head optimized for lower flow. When there are constrictions upstream, flow and pressure are different things, but tightly related. You could improve pressure further by reducing flow further. Oct 15, 2019 at 14:55
  • I don't understand this - the new shower head we acquired today has helped considerably, whereas before I could not properly shower, now I can, albeit not in optimal conditions - it got way better, but still missing an "oomph".
    – joao
    Oct 15, 2019 at 15:02
  • Pressure is the blast force (e.g. what a shower-massager needs to work). Flow is the sheer volume of water (e.g. how long it takes to fill a bucket). Those are two different things. If you ever have a water hose end that is dribbling high flow but no pressure, and put your thumb over it and it starts squirting violently, you created high pressure and lower flow. Vaguely speaking, you trade one for the other. Oct 15, 2019 at 15:12

3 Answers 3


The small water tank (header per Owain) looks like it has no water in it. If that is the case, then it's likely your central heating system will have air in it, and so not work effectively. Try wiggling the ball valve to see if you can get some water in the tank.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Oct 18, 2019 at 11:50

You most likely have a regular conventional boiler, with a cold water cistern (bigger one) and F&E cistern (smaller one), likely non condensing. It's got an indirect hot water cylinder. We can see that the whole heating system is gravity fed/vented/unsealed. The red thing is a pump, so it's either fully pumped or semi gravity (with heating pumped, but the water coil left to gravity and not pumped). It is unlikely to be full gravity (the original unpumped system with thick vertical pipes that encourage convectional circulation). The hot water system is also gravity fed – a centrifugal twin impeller pump can be used to boost flow rate for shower, toilet and bathroom hot water from the cold water cistern through the cylinder and cold water from the cold water cistern; single impeller pump does either hot or cold, and usually goes between the mixer and the shower head and not before the mixer, so that it does both hot and cold. A pump is not absolutely necessary, because water always flows towards the outlet, unlike in a heating system where there is no outlet or pressure drop.

If it is not fully pumped then it is a semi gravity which usually uses a C thermostat plan. Find the motorised valves and their locations and appearances, this will tell you whether it's a C, Y, W or S plan.

The first upgrade would be to get sealed system equipment or a system boiler (which integrates the equipment into the boiler). This equipment is a filling loop to connect mains to the central heating to allow for manual repressurisation (if there is a leak) to replace the automatic F&E tank repressurisation supply, an expansion vessel to replace the expansion vent in the F&E tank, and a pressure gauge. A system boiler has this equipment inside the boiler, and also integrates a pump for the heating circuit, which is is now W, S or Y plan; W uses a diverter valve meaning only the radiators or the cylinder can be heated, but not both. Y plan uses a mid position valve allowing both radiators and cylinder to be heated at the same time, but there are only 3 positions so there's no real temperature control at the valve, instead it's time based (the valve is open more or less frequency) and not like a mixer valve. S plan has 2 separate pipes that branch off the CH flow pipe, and there are 2 separate motorised valves which are also either open or closed.

The water coil and heating are now fully pumped and unvented/pressurised/sealed, but the hot water cylinder is still gravity fed via the cold water cistern. To do away with this, you need an unvented cylinder that can withstand mains pressure, often made of steel, and the hot water circuit needs to be pressurised by connecting the mains directly to the cylinder inlet and fitting an expansion vessel in between, as well as another pressure gauge, backflow preventer and pressure reducing valve. The cylinder is now direct fed, hence pressurised, and doesn't need the cold water cistern or vent. This is now a system boiler with an unvented water cylinder. The cylinder could be upgraded to have an immersion heater as the main heater which heats the water without gas or the boiler workings. Some cylinders, particularly vented ones, have a backup immersion heater for the summer.

In cylinder systems, the hot water is not on demand. Instead, the thermostat on the cylinder sends a signal to the programmer panel when the water in the cylinder goes below a certain temperature, and if the hot water is on, it sends a signal to the motorised valve and then the boiler.

You could instead upgrade one step further. Get a combi boiler and get rid of the cold water cylinder. The combi boiler contains the expansion vessel and the heating pump like the system boiler, but the secondary exchanger is a plate that exchanges heat between the CH and hot water pipes. The programmer panel is usually incorporated into the boiler and the thermostat connects directly. There are usually 5 pipes. Gas in the middle. Central heating flow and return are the 2 outer pipes and then cold mains on the inner right and hot water on the inner left. The hot water circuit is pressurised because it comes from the mains, and the central heating is pressurised using a filling loop from the main to the central heating return. There is a further copper PRV pipe, and a plastic condensate pipe, and a flue pipe. The burner gets activated by the thermostat for central heating demand, and the combi boiler probably contains a dynamic pressure flow detector to detect hot water demand, and the burner starts up, and a motorised valve inside the boiler turns to direct the CH flow to hot water (or mid position between hot water and radiators), which passes the heated CH water through the plate heat exchanger which conductively transfers heat to the hot water pipe. There's no expansion vessel or pressure gauge for the hot water because it's just hot mains in a pipe without a cylinder, also its only being heated when an outlet is open, meaning there's not enough volume of water to expand and cause problems. I know potterton gold combi valve either has hot water or hot+CH mid position. This way, the plate heat exchange path is always open, and the the condensing gasses on the secondary heat are always inputting useful heat.

In your current set up, the shower needs a shower pump, or instead you just need to upgrade to a system boiler with an unvented cylinder, or a combi boiler, to get full pressure in the shower, which typically provides a higher flow rate than a pump does up that elevation.

  1. Please check that the immersion heater (the "Backer" on top of the cylinder) is switched off, as if that's left on it will cost a fortune in electricity.

  2. The cylinder stores hot water until it's needed, and it will keep warm for several days. As far as I can see you have a Potterton Suprima and it is not a combi boiler.

  3. The "scary thing" is a header tank in the loft holding cold water - you should have two, one for cold water to taps (and for the hot water) and one for the water round the boiler and radiators. It is normal for the radiator header to be 'scummy' but if that one tank is used for your tap water I'd be looking to get it cleaned out.

  • Thanks for the tip on the “backer”, that explains the electricity bill on the smart meter.
    – joao
    Oct 16, 2019 at 20:05

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