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MG MGB Technical - Condensation on inside windows

Merry Christmas all, I've had my MG on the road for over a month now and something is starting to bug me now. When it is cold or freezing weather the car tends to have condensation on the front and rear screens. This shouldnt be a problem with heaters but mine doesn't seem to produce any heat and the blower on the driver side isn't blowing air at all.

What is causing the condensation? How can I sort it and also the issue with the heater? I've looked through my MGB handbook and cannot see anything and tried lots of variations to get heat out of the heaters, I think maybe the matrix is gone.

Thanks in advance.
MD Rice


to start with is it a roadster or a Gt?
Iwan Jones

most probably an air lock. when the engine is hot and the heater open, have you tried running a thermometer or finger along the in/out pipes to the heater to see if there is a big drop (most likely on the bottom pipe). that could be the heat problem but the air is directed by pipes which can come apart when fiddling behind the console. the heater in the MGB is known to be underpowered but when connected up and filled properly is adequate and will clean the front screen.
for the rear, my demister has never worked despite trying repairs and I use these (Thomar)

in the boot which work.
Graham Moore

Sorry it is a GT.

Thanks Graham will have a go at that and see how I get on,
MD Rice

I have a feeling that there is a leak in the matrix. This usually causes the screen to.have condensation. Do you get a smell of antifreeze inside the car as this would confirm the diagnosis. Check that you get a flow of water from the heater valve when it is open and also from the return connection at the end of the pipe on the rocker cover. If you get water at these points then almost certainly the matrix is choked as well. Fit an uprated one and good luck because it's a pig of a job.
Iain MacKintosh

It's winter, simply putting a breathing organism in the car will cause condensation. Working correctly the heater should be more than adequate at keeping *misting* at bay, although droplets in cold wet weather where you are stuck in traffic will defeat it, in those case opening the quarter-light a fraction is usually enough in my experience.

But it sounds like your heater is the major problem, as indicated elsewhere:

"I used my V8 for several years in all weathers including long periods of below freezing weather day and night and was warm enough, and the roadster heater can burn our feet, outputting 60C/140F at an ambient of 8C/46F.

There at least ten things that can contribute to poor heat output, see here: Get those right first before considering any 'enhancements, as some of the faults can prevent even those being effective."
Paul Hunt

I had something similar once, which had nothing to do with the heater as such. Leaves and debris had blocked the 'knob' below the air intake grille creating a very damp environment. The air coming into the heater was very damp.

Also, seemingly more with a gt than in a roadster, a leak into the car; collecting in the footwell area can cause huge amounts of condensation.

Do you mean one side to the windscreen ducting doesn't work (usually separated pipes/ducts as mentioned by graham); or, to the footwells (problems with the mechanism at the knobs and or flaps)? The blower is a 'constant' if it is on on one side then the problem lies with the other side of the system ..... if that makes sense.
Iwan Jones

Hi Ricey,
as well as advice here see the thread titled 'A cold MGB GT' for other details and suggestions

a damp interior or leaks to the interior will also cause condensation

some other ideas -

excess condensation can be just from the location, direction and position the car is parked

use a chamois to clear the windows on the inside and open the quaterlights or door windows on the start of your journey to help clear the condensation

angle the windscreen visors to deflect the air from the vents back on to the screen

use and keep in the cabin a couple of small battery powered dehumidifiers or something like the Pingi bags

check the donkey’s/Tom’s dick isn't blocked and causing a damp or wet carpet and possibly same for driver's and passenger's legs

if you've not already got them or turned them down before you can have my (updated) simple basic notes relating to UK Bs just email me
Nigel Atkins

This thread was discussed between 26/12/2014 and 27/12/2014

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