Radiators get hot from domestic hot water

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Staffordshire
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I know very little about plumbing or central heating so I'd be very grateful if someone could exlain whats up with my mums central heating. She has an Ideal boiler which was installed about 5 years ago when she moved into the house which at the time had no working central heating. For a long time now the radiators have been getting hot whenever the hot water burner comes on and because it comes on regularly to preheat the water the radiators stay permanently hot unless they are all turned off individually. I don't think it has always done this but I'm not exactly sure when it started. She puts up with it because the house stays warm without the cental heating ever having to be on and the radiators can be turned off if it gets too hot.

I'm now wondering if it needs to be sorted out as her last gas bill was quite high but maybe thats due to the cold weather and rising energy bills. I had assumed that losing hot water to the radiators was not wasting gas because the boiler never has to burn for central heating. I've read the boiler manual and it says the hot water pre-heat should come on for a few seconds on average every 90 mins or so but I've monitored it for a couple of hours and it comes on about every 15 mins and burns for about 3 mins at a time, which meand that the gas is burning for about 12 mins per hour, although when the radiators are individually turned off it burns for significantly less time. To save on gas I thought about switching the boiler off when no heating or hot water was required but are there any negative implications to doing this? The manual says to shut down the boiler for a short period just turn the power switch off but for longer periods the electic supply needs to be off and the system drained but it doesn't specify what constitutes a short or long period. Since the boiler broke down a while ago and had it's PCB replaced my mum has taken out breakdown cover so maybe she could use that to get someone to look at it but if the boiler itself is not the problem presumably this wont cover it.
 
If your mums system has a 3way valve, it may be stuck in the mid position thus when the hot water is called for, it also heats the rads.
Have a look at FAQ 3 or 4
 
My guess would be an Isar or a Mini. Both prone to the fault mentioned above.

OP, it would really help if you had told us exactly what boiler/system your mother has :roll:
 
My guess would be an Isar or a Mini. Both prone to the fault mentioned above.

OP, it would really help if you had told us exactly what boiler/system your mother has :roll:

Thanks for your comments guys, It is an Isar. The manaul is for HE24, HE30, HE35 not sure which it is. If it is the valve fault you mentioned should that be covered by the breakdown insurance?
 
Onewould imagine its covered, but diferent contracts cover diferent things. For example we do a cover where only labour is covered but parts have to be paid for.
 

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