I've just moved into a new flat which has a combi-boiler taking cold water in at mains pressure, and I'm trying to understand how it works. Specifically, I'm trying to understand how I can get maximum water pressure to my shower.

My questions are:

  1. There are effectively two temperature controls for the water supply to my shower: the shower's control, and the boiler's control. Now, I imagine that the shower's cold water comes straight from the mains at mains pressure, whereas the shower's hot water comes from the boiler at a lower pressure. So to get the highest shower pressure, I would want the most amount of the cold water, and the least amount of the hot water. To achieve this, I should set the boiler to its maximum temperature, and the shower's temperature control to only let in a small amount of hot water. Does this make me sense? Or will the shower pressure be the same for any combination of boiler / shower controls?

  2. The combi-boiler has a pressure dial, with instructions on how to increase the pressure. Is this representing the water pressure for the central heating, or for the hot water supply to my shower, or for both? And will increasing this water pressure then result in higher pressure to my shower?


2 Answers 2


The boiler should not significantly affect your water pressure. Tankless boilers do put water through a somewhat more twisty path than a traditional water heater, but if the unit is properly sized for your installation and properly piped, the impact of this on water pressure should not be noticeable. If your hot water pressure, ideally tested from multiple faucets within your home, really is lower than your cold water pressure, your system may be incorrectly installed or require maintenance.

If your boiler has a pressure reducing valve, it is most likely the valve that regulates the pressure of cold water supply into your hydronic heating system. A picture and/or model number would help confirm.

Though you haven't said it explicitly, I'm guessing you are getting less water pressure from your shower than you'd like. If you aren't seeing lower hot water pressure at other fixtures, then it's also possible you have an issue with the mixing valve in your shower faucet. These can clog due to mineral buildup or perform incorrectly due to wear. You may also have a shower head that doesn't let much water through.


The pressure dial is typically the pressure in the radiators (central heating system) and isn't related to the domestic hot water supply.

There appear to be companies selling low flow shower heads that claim to reduce the amount of water required. Some examples here.

Your reasoning about the boiler reducing pressure on the hot water makes sense and I've noticed in my own house that the cold water is higher pressure than the hot, however I have no idea whether your method would make a noticeable difference. It'll be dependent on many factors: your cold water pressure, boiler specs, pipework diameter, type of shower, and preferred shower temperature at the very least.

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