I am surprised that you ever used that warm air heating system.
I soon replaced it with wall mounted electric panels for the couple of years before we got the radiator system.
It must have played havoc with your asthma. I got severe rhinitis, was tested for allergies (negative), and the conclusion was - the sheer volume of airborne dust from the vents was irritating my nasal passages.
I had noticed that the amount of sediment in my air filtration machines had increased.
Back on thread topic -
I have realised another possible cause for my gas bill being so low
(other than a possibly faulty meter and good insulation)
My hot water use is negligible, because of my difficulties in using the bath
(as explained in my 'Hyndburn Homes shower install' thread).
Warm wet flannel wipe downs, and a shallow bath once a week seems to have saved money.