Unvented HW & CH System Questions

[Kevin Walton]

Hi

We have a modern house (4 years old) with an unvented HW & CH system. A Vaillant boiler with integral circulation pump, a pressurised hot water cylinder, a small expansion vessel above it and lots of pressure valves!

Three seperate questions:

1) We get a lot of air in the Hot Water, in comes out the tap looking white because it is full of tiny air bubbles. Not a major probblem in it's self, but is it telling us something else needs fixing? Can it be fixed?

2) Upstairs Water Pressure Issues. The upstairs mains cold water seems to be also regulated by a pressure valve - this seems to be a 3 way preasure valve, taking mains water in and feeding (max 6 bar?) cold water to both the hotwater cylinder and the upstairs cold taps. The issue is that when filling the bath, if you turn both Hot & cold taps on, the resulting pressure is substantially less than only having hot on or only having cold on, what is going on? Why does the cold water need regulating in the first place (just to balance it?) and is something not working right?

3) Rads in the loft. We are having a shell loft conversion completed and intend to extend the current CH system into the loft. Are there any issues with having rads higher than the expansion vessel in the closed 2.5bar ish CH system? If this does cause issues can I just raise the pressure vessel?

Thanks in advance. Kev

[Ed Sirett]

Possibly by reducing the water temperature of the HW cylinder. Otherwise probably not as it is dissolved air coming out of solution when the temperature rises and the pressure is reduced (as it flows to the taps). But fixing (2) would probably help a bit.

Yes it is to balance it. Describe how water gets from the road to this valve. For an unvented cylinder to work really well you need: The effect is also probably downstairs as well but is less noticable when running a sink rather than a bath.

Minimum 25mm Plastic water main underground. Minimum 22mm pipework to the valve. Also full bore lever (for main cut off).

None at all. See FAQ. 2.5 bar is too high, run the system at the right pressure as shown in the boiler instructions.

HTH

[adder]

I've got an unvented hot water cylinder with the pressure reducing valve at 1 bar and it works fine. When one tap is on and another is opened there's sufficient flow to feed both taps.

[IMM]

A pressure reducer acts a regulator when set so low in relation to your incoming pressure. If your mains pressure is 2.25 bar and you set the pressure reducer to 2 bar then with a few or more taps on one may affect the other.

[IMM]

Many unvented cylinder have a pressure balancing valve after the pressure reducer. I assume this has one. The cold supplied leaves this and are the same pressure as the hot. When a kitchen tap is turned on the pressure balancing valve ensure equal pressure on both hot and cold and prevents scalding, or discomfort, in the shower.