Might this be steam heat?
why does thou' not replaceth the valve'eth? Not surly, for the heck of it, they are inexpensive enough.
Also,did thou, paint whilst decorating?
Because there may well'eth be a chance some Paint (or something) may have clogged it up, either on the valve'eth or elsewhere'th,
Someone painted for me and this is exactly what Had happened, a tiny speck of paint was clogging up the little steam release hole, on the valve'eth, and once it was opened up again'eth (the hole)
Thine warmth did once againe return to thine'st humble abode!
G'Day me lords and me Ladies.