Hot water from storage cylinder leeching into radiators ?

18 Aug 2015
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United Kingdom
Hot water coming out of the hot water storage cylinder into upstairs radiators after heating/HW turns off and running out of hot water in the mornings.

I have a Glow Worm 24HX Condensing boiler on the ground floor with a pump in the same cupboard, a hot water storage cylinder in the airing cupboard upstairs (Kingspan 1500 x 450 grade 3, 162 Litre. cylinder) in the eaves area I have another pump (both pumps are Salus MP200 domestic circulating pump). Upstairs I have two Drayton 2 port Zone Valves ZA5, one on the hot water supply to the hot water cylinder and one to the central heating supply.

The hot water and central heating is timed to go on in the afternoon 4pm to 1030pm every day and has been working fine for the last 7 years, I think the whole system is around 11 years old.

One evening I found that the temperature of the water coming out of the upstairs shower was warm and I couldn't get it to go hot, no matter how much I adjusted the shower control.

It appears that when the heating and HW goes off at 10.30 pm a couple of upstairs radiators (2 in bathroom and 1 in bedroom) stay on hot for an hour or so when the others have all cooled down. I have checked the boiler and it goes off at 10.30 pm.

I'm wondering if this is a zone valve ZA5 problem either on the HW or heating where the
hot water is going through from the upstairs hot water cylinder to the radiators.

Can anyone let me know they think this is a zone valve problem, which valve is is suspect, heating or HW or anything else ?

I'm looking to change zone valve heads soon but I am trying to find out if it could be anything else causing the problem, before I make a purchase, in case it does not cure the problem.

Many thanks.
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Hi Mike,

Not sure why you have x2 pumps, is the house massive? Had any work carried out recently?
Hi, I have one pump by the boiler downstairs and two pumps upstairs due to the water underfloor heating. I have normal radiators throughout the house but just underfloor heating in the lounge downstairs. The house has 4 bedrooms and I have had no work done recently.
Is your hot water cylinder a conventional one (heated via an internal coil connected to the heating pipework) or something else? Do you have room stats, is there a stat on the cylinder, has someone nudged the cylinder stat with a broom or some washing (favourite trick that). Have any rads got TRVs on? Convection is a marvellous mechanism- have those upstairs rads been staying hot for ever or is this a recent manifestation?

Where exactly are the zone valves? Have you had a look at them to see if they are opening and closing properly (not sure if the Draytons have a manual override lever). It is possible that the layout of your pipework would allow convection from the coil in the cylinder back to the heating circuit (if the zone valve was not closing properly)- there's a type of T should be used in setups like that but I can't remember what it's called.

Things to investigate;
Is the cylinder stat actually set at your required temperature?
Is the cylinder at the temp set on the thermostat?
Are the zone valves actually physically closing/opening on command?
Pipe temperatures after 10.30pm- check the temp of the heating pipes downstairs and upstairs, if you are getting convection it may reverse (so return becomes warmer than feed)
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Hi, I think I have the answer from another user so am reassured about replacing the zone valves so I think I am all sorted now:

Yeah, it'll be the one going to the heating circuit to blame, those heads come off quite easily, you can check that the valve operates easily enough by taking the head off and seeing if the spindle turns, if it doesn't budge then new full valve, if it does move fully a new head will sort it. You can sometimes free up stuck valves but they tend to stick again as there's an underlying reason they're stuck that needs addressing.
The valve not closing means that the heat will dissipate to nearby rads, the pump won't be running so no reason for the hot water to circulate - it just bleeds heat to nearby rads. Heat rises, so you'd expect your downstairs rads to stay cold in this situation.

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