Radiators only heat when water cylinder needs heating

[kipper99]

I wonder if anybody could help diagnose this problem?

I have a Thorn Apollo boiler - I think it's about 30 years old and is very basic. There's a gravity-fed hot water cylinder upstairs, and 6 radiators (4 upstairs and 2 downstairs).

This winter the system has developed a fault whereby the radiators will only heat up when the water in the hot water tank needs heating.

e.g when the heating first comes on in the morning, the hot water in the water tank needs heating, so the system fires up and the radiators all heat up too. But once the hot water in the tank reaches its proper temperature the boiler shuts off, so the radiators cool down. I can make it fire up again (and therefore heat the radiators) by running hot water from a tap until the hot water in the tank has cooled sufficiently to fire the boiler up again.

One other piece of information - since I've lived here (several years), the room thermostat has never worked as a control for the radiators - they would heat up regardless of the setting on the thermostat so I presume that it's not wired in, or at least isn't properly wired in. Like I say though, it has always been like that, and I have not had this more recent problem before.

Many thanks for any ideas you might have,
Bob.

 

[Hugh Jaleak]

Your system is pretty simple, the hot water tank is heated using gravity circulation, radiators are pumped. You'll get some heat in the rads where gravity circulation takes place but it would seem the pump might not be working.

Roomstat should act as a 'switch', energising the pump when heat is called for. Pump will then move hot water from boiler round rads. Need to check the pump first, if you are confident to do so! Check pump has power before condemning it.

 

[kipper99]

Your system is pretty simple, the hot water tank is heated using gravity circulation, radiators are pumped. You'll get some heat in the rads where gravity circulation takes place but it would seem the pump might not be working.

Roomstat should act as a 'switch', energising the pump when heat is called for. Pump will then move hot water from boiler round rads. Need to check the pump first, if you are confident to do so! Check pump has power before condemning it.

Ok, I'll check it out - many thanks for the help Hugh.

 

[D_Hailsham]

I have a Thorn Apollo boiler - I think it's about 30 years old and is very basic. There's a gravity-fed hot water cylinder upstairs, and 6 radiators (4 upstairs and 2 downstairs).

Do you mean gravity circulation or just that the HW cylinder is fed from the cold water tank in the loft?

If you have a valve like this (normally near the HW cylinder), you have pumped circulation not gravity circulation.

View media item 5946

 

[FiremanT]

Were all Applo's not low water content? If so it MUST be fully pumped.

Model no would help.

 

[HH1]

Or does the system have two, 2-port valves?
instead of the one, 3-port valve near the HW cylinder.