Hot water seems under higher pressure when DHW production active - is it a concern?

[robinbanks]

I've an ASHP that heats my DHW and the setup hs been the same for many years now. The tank typically runs to cooo before being heated
A recent observation I've made, that I have no recollection of occurring in the past, is that when the DHW has been on for a while if I open the kitchen hot tap, it seems the hot water is under slightly higher pressure - the "hiss" from the aerator spout is slightly louder and the water stream slightly wider. It settles down to what I recognise as "normal" (roughly equivalent to the hiss from the cold flow) after a few seconds

I ask in case there is some pressure relief or expansion vessel type affair associated with the potable side that may have a problem, and warming the water generates excess pressure that has nowhere to go. The CH side has an external EV but I don't recall seeing anything similar for the HW potable side (before the whole affair was buried behind a refrigerator)

 

[MeldrewsMate]

Some UV cylinders have internal expansion volumes, one example is Megaflo, which need to be regenerated from time to time.
The pressure does indeed rise once the cold water in the cylinder is heated and expands.
Let's assume you have 100 litre of cold water and 20 litre of expansion volume (air) at 3 bar.
Let's assume the water expands by 5% when heated to 60C. This adds 5 litres to the water volume, so the 20 litre of air at 3 bar becomes squeezed into 15 litre, and it's pressure rises (Boyle's Law?) P1 x V1 = P2 x V2, so P2 = P1xV1/V2 = 3 x 20/15 = 4 bar. Because there is 5 litre of hot water draw-off before the pressure drops to the original 3 bar (at this point the cold water rushing in at 3 bar and stabilises the flowrate) you may not notice it; however if the expansion volume drops to (say) 10 litre then the same 5 litre of water expansion will cause 3 x 10/5 = 6 bar...about the most you'll get without activating the safety relief valve. Furthermore the pressure will drop by not one, but by 3 bar over those 5 litres of hot water draw-off, you are more likely to notice the more rapid change in pressure.

Check and re-charge your expansion vessel, or re-generate your internal expansion volume.

 

[Rodders53]

Make model and a picture of the hot water cylinder may help? (to identify if it's an air bubble or should have an expansion vessel somewhere).

My Oso unvented has an air bubble in it and needs regular replenishment. They early warning sign is the hot water flow rate change you describe.

Unvented cylinder should need an annual service - so time to pull out that fridge methinks.

 

[Johntheo5]

Some UV cylinders have internal expansion volumes, one example is Megaflo, which need to be regenerated from time to time.
The pressure does indeed rise once the cold water in the cylinder is heated and expands.
Let's assume you have 100 litre of cold water and 20 litre of expansion volume (air) at 3 bar.
Let's assume the water expands by 5% when heated to 60C. This adds 5 litres to the water volume, so the 20 litre of air at 3 bar becomes squeezed into 15 litre, and it's pressure rises (Boyle's Law?) P1 x V1 = P2 x V2, so P2 = P1xV1/V2 = 3 x 20/15 = 4 bar. Because there is 5 litre of hot water draw-off before the pressure drops to the original 3 bar (at this point the cold water rushing in at 3 bar and stabilises the flowrate) you may not notice it; however if the expansion volume drops to (say) 10 litre then the same 5 litre of water expansion will cause 3 x 10/5 = 6 bar...about the most you'll get without activating the safety relief valve. Furthermore the pressure will drop by not one, but by 3 bar over those 5 litres of hot water draw-off, you are more likely to notice the more rapid change in pressure.

Check and re-charge your expansion vessel, or re-generate your internal expansion volume.

Just in case you are a G3 or even if not then remember you are dealing with absolute pressures on the air side so the two calcs above equal 4.33bar and 7.0bar.

 

[Madrab]

Get your unvented cylinder serviced by an experienced and qualified UV engineer - chances are it is an expansion issue but that should be dealt with by the engineer during the service.

Check and re-charge your expansion vessel, or re-generate your internal expansion volume.

The latter can usually be performed by the resident/owner, the former should not.

 

[Madrab]

duplicate

 

[robinbanks]

Make model and a picture of the hot water cylinder may help?

 

[Rodders53]

Says 'expansion vessel charge pressure' so it is likely the unvented cylinder now needs servicing. (Annually).
No user adjustable parts the experts here warn.

Nice that the installer hasn't filled in their details on the tank itself. Hopefully the Benchmark booklet has that info.
There will be a (usually white) potable water expansion vessel somewhere nearby on the HW outlet side.