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MG Midget and Sprite Technical - Elusive Heater Lever

Having just put back my dashboard & gauges, in particular the temperature gauge sensor, I topped up the coolant. I've read threads about omitting air from the system & some suggested opening the heater valve. For the life of me, I can't see one, it's a 1500 midget, 1977. I've never had the heater running. It does get warm around the legs & in the cockpit if the roof is up, so never been concerned about the heating.
Nigel Axtell

Nigel

The heater valve should just to the left of the battery as you look from the front, it's a short lever about 1 1/2 inches long. If your heater is working correctly and is turned on, it should almost set your leg on fire with the roof up.

Bob
R.A Davis

Just a thought . . . .

I found the under-bonnet heater valve to be worse than useless on my 1976 1500, so I removed it and rerouted the pipework accordingly (ie hot coolant now always flows through the heater matrix). Whilst I do understand that the valve was designed to stop the flow of coolant into the heater matrix in summer, it didn't seem to be an efficient way to control cabin temperature. Pulling the (dashboard) heater knob out shuts the heater box butterfly flap which prevents hot air entering the cabin (by blocking the fresh air inlet into the heater matrix). Also, the small oblong metal vent/flaps inside the cabin (in the footwell) can also be closed to prevent hot air entering the cabin footwells. There is so little airflow to the windscreen 'demisters' that it makes no difference.

The under-bonnet valve just seemed to be another component to fail/seize and leak coolant, hence I removed it. Maybe the previous owner of Nigel Axtell's 1500 has done the same.
Andy Hock

Nigel -

When I flushed and refilled my 1500 cooling system recently, I undid the top heater hose when topping up, the intent being to provide an air vent. I think it worked...

-:G:-
Gryf Ketcherside

Nigel,
you've hit on one of my examples for owners needing to have a copy of the original Driver's Handbook, it gives loads more information that you'll want and need too - Ref:0058 - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

Andy,
I'm just going to pick you up there, it's turning off the heater valve (tap) not the the pulling the 'H' knob that stops the heat

the 'H' knob which adjust the large flap on the heater box attachment controls the amount of fresh air entering the cabin from the ram effect of the car moving forward, this can at times be boosted by switching on the fan

you are quite right the heater valve (tap) can leak or seize but it doesn't have to, fitting a well made valve (tap) correctly and operating or testinging it fairly regularly will keep it working - I agree the system is antiquated though

if the whole coolant and heater systems are cleaned and flushed thoroughly and all components in good condition the heater is too hot rather than too cool I've found but I will admit my heater tap doesn't give my control over the amount of heat

again I think if you had a Driver's Handbook to refer to things would have been clearer
Nigel Atkins

Nigel, if the H knob is fully out then the flap is fully closed and there will be no air - fresh or otherwise - entering the heater unit and hence the cockpit. Therefore the only way heat from the heater can enter the cockpit in this state is by radiation which given there is a steel bulkhead in the way will be minimal. The cockpit does get heat from the engine and gearbox and exhaust system by other means of course.
David Smith

David,
unless I'm missunderstanding, in addition to the radiated heat, the heater box is not a sealed box, the large trunk fresh air flap seals the fresh air from coming in but the box is totally open in the other direction to the chamber below that feeds the footwell and windscreen apetures, the footwell flaps can be closed leaving the heat to go to the windscreen

try shutting off the the fresh air flap entirely, closing the two footwell flaps and turning on the fan motor - it will not explode, the earth will still turn and air will come out of the windscreen vents, not as much as when the fresh air flap is open but still there

as I keep saying the heaters on Spridgets are antiquated and not intuative to use which is just one minor example and reason for most owners to have a copy of the Driver's Handbook
Nigel Atkins

Nigel

How do you close the fresh air flap and have the fan motor turned on? mine won't allow that.

As you say though, the air will still find its way past the closed "fresh air flap" and still allow an appreciable amount of heat into the car.

Bob
R.A Davis

Very good point there Bob

I forgot

like many Spridgets the switch part of my flap control has been rewired to a non-standard additional switch on the dash so I and many others can do this but those lucky enough still to have a working original combined cable control/switch can’t

I was being humorous about using the fan motor with the flap closed as some think the motor will be damaged and some think the heater is easy enough to work out that you shouldn’t need to look at instructions – but we wont start all that again other than to say the gang of four against is probably a gang of five :)

I think you can now get an Ashley Hinton new cable/switch for about £50
Nigel Atkins

David Smith - thank you; you succinctly said what I was trying to express in my first post. With the heater box flap closed, there will be no (or virtually no) airflow -hot or otherwise travelling through the heater matrix into the cabin.

Assuming the matrix/heater box is in reasonable condition (ie the internal foam seals are in good nick), then no air will flow through the heater box/matrix. Or into the cabin.

As RA Davis says, with the heater box flap closed (ie dashboard knob pulled out), then it should be impossible for the blower motor to be switched on. The dashboard knob is designed that way (blower motor cannot be switched on with the know pulled out) so that the blower motor doesn't burn out by operating under extreme load (ie no air flow).

There is so much radiant/conducted heat from the engine/exhaust into the footwell irresepctive of the under-bonnet water valve on/off position.

In summary - remove the under-bonnet water valve and rejig the pipework accordingly. Its inclusion is tantamount to a BL design flaw.
Andy Hock

Andy,
it's a simple antiquated design of heater operation also used in other models but it's not a flaw

with the fresh air flap closed the fan motor will be under more stress but not immediate danger, the heater box is not sealed so air can get in as well as out

what you do with your car is up to you and I'm not criticising you but you have removed an element of control of your heater by removing the valve

in the summer with the roof up I personally don't want any more heat in the cabin even a small amount coming through the windscreen vents or radiated by a heater matrix containing hot coolant

in winter with the roof down then the heat from the enging, transmission and exhaust are not enough so having the heater on is handy as is being able to restrict its output (marginally I agree) by turning the vavle (tap) down and/or directing (some.most) of the heat by the use of the footwell flaps
Nigel Atkins

"the heater box is not sealed so air can get in as well as out"
- what are you on about? If the inlet flap is closed then the only other access is the main vent into the cockpit - do you really believe that air flows in both directions at once?
David Smith

thank you for that warning David I'll never enter a room or building with only one door again :)

there are two holes above the hardly sealed footwell flaps these open holes are trunked to the windscreen vents

I don't know if it's your style of writing or if you do mean to be rude and/or aggressive if so please don't hold back on my account please spit it exactly what you really mean I can take it :)
Nigel Atkins

On my car, - and I only speak from personal experience, in summer I turn off the tap (1275, not 1500 so its a tap not a lever). If I don't warmed air continues to leak in through the system even with the inlet flap shut. The reason is I think simple enough, air leaks past the "door" as it is under a significant ram effect when travelling at any sort of speed. The door seals off most, but not all of the air. This in fact makes it worse as the small volume of air going through heats up rather more than when there is a larger volume coming through when the flap is opened. Indeed, in Winter the way of fine tuning the heat coming into the cockpit is to open the air flap wider, which moderates the temperature of the incoming air.

So, usually around Easter time I open the bonnet and close down the heater valve. For me, it is not sufficient to simply close off the combination of flaps and air valves.
Guy

This thread was discussed on 13/03/2012

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