We recently moved into a second hand house and are having dreadful problems with our rads downstairs.
They didn't give out great heat to begin with but we thought once they were bled they'd be fine.
However, all the rads downstairs do not heat up now despite having bled them and all the rads upstairs are fine!
We are totally baffled at this stage and are going to call a plumber out to have a look but thought we'd post first in case anyone else has had this problem and there is an easy solution!
We had the same problem a few years back, you may need to balance the radiators, a plumber can do it but it will cost you and with a bit of patience you can do it yourself. I found this page sorted out our problem!
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