Cabin Heat - Weird Pressures going on

So I am early on in my phase 1 and I had to use cabin heat the other day. Funny thing is my heat won't stay on, it wants to pull itself shut. So I am sure to fix I need to tighten up the spring that ratchets on the cable. Maybe this problem is common?

But I started thinking.. about the pressures that might be causing the door to want to close and decided there are 3 pressures involved.. 1) Cabin Pressure, 2)Pressure above the engine/baffles where the heat intake is, and 3) pressure below the engine/baffles where the box overflow is.

In this case when I hold the heat on heat comes into the cabin, so clearly pressure above the engine is greater than Cabin Pressure.

But the fact that it wants to push close, means that pressure below the engine is greater than above the engine, which I don't understand.

This would mean that pressure is building at the cowl exit around my exhaust, greater than on top? My CHTs are low enough to know I am getting plenty over the engine even though I am still breaking in.

Any thoughts on this? What else could be causing this?

Chad,

If your heat select valve located on the firewall is like mine, the airflow coming to it tends to push it in the closed direction which allows the hot air to pass through back into the engine compartment.

Perhaps your cable to operate the valve is not a locking type or needs more "friction" if it is the friction type.

Bevan
RV7A

Cable

Lets make sure your cable is correct.
Does the cable have a ratcheting feel as you pull and push it, if it does not, you have the wrong cable?
Is the outside part of the cable near the heater door on the firewall secured properly? It should have enough tension so that it does not move as you push and pull on the cable knob. If it moves your door could close by itself. Your cable is setup so that if you pull it, you get heat, push it, no heat.
If all this is right, then I'm all out of suggestions. Good luck! Hope this helps!

Make sure the little spring is in place that ratchets in the cable grooves.

I have the correct cable and my heater does the same. Fooled with ratchet but no joy In fact, I get higher volume airflow when the valve is partially open vs all the way open. My intake is on the right ramp per -M1B directions

I have not done pressure measurements yet but the suspicion would be disrupted air at the inlet due to proximity to spinner and also high pressure in lower cowl.

If that is true, I have speed yet to be released via cowl work. My CHTs are happy even with the high lower cowl pressure.

Thanks Guys, Yes I have the right cable and to correct it all I have to do is make sure the spring is tigher in the grooves to make it a harder ratchet..

The issue is though that I am wondering if high lower cowl pressure pushes air in backwards through the heater box.

Like rzbill states, if more air comes in when heater is partially open, then there is higher pressure in the lower cowl pushing air that has gone through the engine into the heater box and into the cabin. Potential CO ?

Thx

Venturi effect

Chad, my guess is that as the air speeds by the flap some kind of Venturi effect is sucking it closed. Could this be possible?

Hi Chad

I had a similar problem.
A simple way of increasing cable friction:

http://www.vansairforce.com/community/showthread.php?t=59282

Jim Talbot
RV7A
New Zealand

704CH said:

Thanks Guys, Yes I have the right cable and to correct it all I have to do is make sure the spring is tigher in the grooves to make it a harder ratchet..

Click to expand...

I tried this and failed. YMMV so let us know if you are sucessful.
Jims note above may be useful.

At the moment, we are using our pulse oximeter as a "clamp" to keep the darned thing open.

Yesterday while @ SnF, I bought a locking button cable (A-700) from Spruce.
I should be an easy replacement for the stock Vans cable. I may have to enlarge the panel hole slightly, but not much. (Its not nearly the size of the verniers)