Downstairs Rad Cold When Hot Water On

[JasonC]

I have a small standard CH system - 1 rad downstairs, 2 rads upstairs, Boiler + programmer + room thermostat downstairs, Pump + Hot Water Cylinder upstairs in airing cupboard.

My problem is this: if I turn only the Heating on all 3 radiators get hot but if I turn the Heating AND Hot Water on the Water gets hot, the two upstairs Rads get hot but the downstairs Rad remains stone cold (even the supply pipe is cold).

I was informed by someone "in the know" that this would be caused by a faulty pump so I've just fitted a shiny, brand new Grandfos pump but it has not made any improvement - the downstairs Rad is still freezing cold when the hot water is on.

My next thought is that maybe there is a problem with the Mid-Position Valve but I would have thought that it would switch off all of the Rads if it was faulty not just one of them.

I don't really want to blindly throw any more replacements parts at the system until I understand what is going on so any tips would be greatly appreciated.

 

[fitz1]

could do with some system history.has it always been this way or has it just started.has any work been done.

 

[JasonC]

I've been in the house 5 years and in that time no work has been done on the central heating at all and it has always worked perfectly. Recently noticed that some days it would take a lot longer for the downstairs to heat up than the upstairs (normally the hot water and heating would always be on at the same time under timer operation) but it would always heat up eventually. Then about a fortnight ago the downstairs radiator stopped heating up at all. After some investigation discovered that if I turned off the hot water the radiator would heat up. Have left hot water and heating on for 24 hours straight and the downstairs rad never heats up, turn off the hot water and its hot within 5 minutes.

 

[fitz1]

try turning upstairs rads off at rad valve.see if it gets hot then.with heating and hot water on.hot water is priority.

 

[JasonC]

I've tried turning the upstairs rads off completely - the downstairs rad still doesn't heat up. With the upstairs rads off and the hot water up to temp the pump shuts down altogether - as soon as I open up one of the upstairs rads the pump fires up again.

 

[fitz1]

do heating only on downstairs rad.systems not zoned up and down with another valve by any chance

 

[fitz1]

also check that bypass valve is not open too much

 

[JasonC]

The system is definitely not zoned - there is a single room thermostat downstairs that controls the whole system and a single mid-position valve in the airing cupboard to switch between heating and water.

 

[fitz1]

wot happens when hot water only is on

 

[JasonC]

When just hot water is selected the water heats up and none of the rads get hot.

Bypass valve - I'll have to admit to ignorance here, I'm fairly certain I don't have an automatic bypass valve, it is a 20 year old heating system. Where would I find a bypass valve? (even if I have one it has definitely never been adjusted so I don't see it giving a problem)

 

[fitz1]

just below 3 port,look for a tee with a pipe running to the return pipe with a gate valve inbetween.

 

[JasonC]

If this is right:

then the valve is currently fully open. I've never touched this valve - is it not supposed to be open?

 

[Agile]

"""I was informed by someone "in the know". """

Do you still really think that he was "in the know" ?

Why not ask him to pay for your unneccessary new pump in view of the wrong information he gave you?

Have you tried closing the lockshield valves to just one turn open on the two upstairs rads?

Tony

 

[JasonC]

I've tried all sorts of changes with the lockshield valves but nothing seems to make any difference - currently I have the lockshield valve on the downstairs radiator fully open and the lockshield valves on the 2 upstairs radiators just open about half a turn each and still the upstairs rads are hot and the downstairs rad cold when the hot water is on.

 

[chrishutt]

Sounds like you need to take the rad off the wall and flush it out, also checking the flow out of each pipe connection. You must have some sort of blockage, usually sludge.

Alternatively, if you have a TRV on the rad this may be stuck shut. Take the head off and check for free movement of the pin. See FAQ topic for more details.