carb heater union

[Mark Harwood]

The hinge on my carb heater union flapper is marginal. It is also difficult to get to for repair. It works but a crack can be seen in the hinge on close inspection. I would hate to have it come off and get sucked into carb.I am having difficulty understanding a scenario where the flapper would be in the closed position since air would always be going overboard or to the carb on the leg of the union that has the flapper. What would happen if the flapper was removed? Seems to me it would function normally unless there is a scenario that I am missing in which the flapper in the union is shut. May just add another bolt to that leg of the union on carb side off flapper to catch it if it did come off. Any suggestions?

[Bruce Fenstermacher]

Are you talking about the 3 way Y between the heat scat and the carb heat box which can be found on early 170s and has a flapper in it.

Or are you talking about the carb heat box itself which is attached to the bottom of the carb and has a flap controllled by your carb heat control in the cabin?

[GAHorn]

Well, I should say that I do not have first-hand experience with this union, so I may need some education on it. Anyone with another explanation of this system is invited to straighten me out on this if I'm wrong.

But it appears to me that you already have a bolt that should restrict a large object from getting to the carb airbox in the bolt that attaches the union to the airbox.
It is also worth mentioning that while a bolt may prevent a large flapper from being ingested... any fractured hinge is likely to also produce a few smaller bits that will blow/suck right past a bolt. So I do not think that is a worthwhile endeavor (and is also an undocumented/unapproved alteration.)

The flapper to which you are referring...is that possibly the "cap" illustrated in Fig. 38, item 78 of the 170A IPC? (Fig 34, item 86 in 170 IPC.)

Or are you discssing a check-valve/flapper upstream of that in the cabin-heat side of the union?

My understanding of the purpose of a "flapper" in the carb-heat inlet to the airbox is to prevent gasoline fumes/backfire from entering the cabin air supply, yet allow heated air from the muffs to reach the carb airbox should the pilot select carb heat. In normal heater operations both muffs produce heated air to the cabin heat distribution, but the flapper prevents any "vacuuming" of fuel/air or reduction of inlet manifold pressure by the cabin heater. Then, when carb heat is selected, the heated air produced by the muffs is allowed into the carb airbox.

Why can't you remove the union and make a weld-repair to the hinge and then return the union to it's installation? Dropping the lower cowl should provide easy access.

.

[Bruce Fenstermacher]

George your description of the Y part and it's purpose and function is right on. I would have been more descriptive but it's been a while since I've looked at the ones I have.

If we are in fact talking about the part Mark intended to discuss I like you wouldn't be worried about the flapper itself but the little parts.

BTW this Y part apparently was used on later 150/152s and may be very easy to come by. I found one several years ago in a junk box at Sun N Fun I bought for $2.

[Mark Harwood]

Sorry for not being more descriptive. The carb heater union (I think this means carb and heater union) attaches to the carb air box and has three other openings. Two for hoses from mufflers and one to cabin heat. The "flapper" is on the leg and that attaches to carb air box. The bolt that holds this "union" to carb air box may not stop the flapper from possible ingestion if the flapper were to exit the hole perpendicular to the opening. This part is made of two halves welded together with the flapper mechanism out of reach of repair. We have it off and in our hands and see no practical way to repair it. Keeping fumes out of the cabin in the event of a backfire seems to be a logical explanation for the flapper. The hinge is thin and has a tear in it. If the tear continues the flapper could come loose. If this "union" is easy to come by, that would be the easy solution. If it is the same as something used on late 150's or 152's I would expect my mechanic would have seen them, but maybe not. I have a 1977 150 and it uses one muffler for cabin heat and one for carb heat.
I either have to find another "union",use the one I have with or with out extra bolt or use another configuration for cabin and carb heat. I read some threads on different heat confiurations. If I can use this "union" I get the advantage of both mufflers providing cabin heat and it solves the problem of keep air flowing when cabin and carb heat are not being used.
I am so new to this and just trying to figure out best avenue. You guys are such a wealth of info! All input is appreciiated!! Markng

[GAHorn]

I can't imagine a band-saw not being capable of cutting the thing in half on one of the seams, so it can be spread and the flapper rebuilt/replaced... or alternatively a section cut out of the union so the flapper and hinge can be removed for repair, then reinserted and welded back together making new seams. But it's likely an expensive way to repair something that can be purchased far cheaper (provided the used unit is not just as worn out.)

[Mark Harwood]

I'll run those repair ideas by my mechanic. Are these unions a common item that should be easy to find? Mark

[Bruce Fenstermacher]

Mark

Again I think I've seen several on EARLY meaning 1960s 150s and perhaps 152s. I'd think you could find one but who knows.

The Y is shown on page 72, fig 38, number 76 part number 0550119

I've run my heat ducted with and without the Y. I can't really say the heat is better with it. Currently it is not installed on my aircraft.

To run without it there would be an elbow that replaces the Y angling the inlet towards the back of the aircraft. The right scat tube attaches directly to the elbow from the heat muff. The left directly to the cabin heat manifold. You can see a picture of this elbow on page 86 fig 50 number 32 part number 0450314 of the 170B parts catalog. This part is available from Cessnaparts.com for $165. Preferred Airparts (preferredairparts.com) says they have 17 new surplus in stock and they sell for 50% off list. I'd call them if you go this route.

Assuming you don't have an elbow to install you could either make on or at least attach a short straight tube to the carb heat box to give more tube to clamp the scat to it. I don't think yo actually need to have the scat come in from the back of the carb but you should check clearance.

As for repair of your Y. I just looked at mine and it indeed is spot welding very generously around it's edge. Drilling out each spot weld would make a bunch of holes in the edge that you could use to bolt it back together or rivet it but that would be a lot of holes and big bolts or rivets for the job.

I'm thinking a hack saw blade run carefully through the seem may work to cut the spot welds to separate it but it might also take more material away from the two piece halve than you want. It's a dilemma which would be the best process to separate it.

[bsdunek]

In the late 50's Cessna put out a service bulletin to remove the "Y", and run heat from one muff to the carb, and the other to the cabin. This always assured heat to the carb when needed. It also resulted in poor cabin heat.
I put my "Y" back in. It allows both muffs to heat the cabin. When the carb heat is required, the lower pressure to the carb causes the flapper to close and route most/all the heat to the carb. Now you have heat from two muffs going to the carb. When not using carb heat, you have both muffs providing cabin heat; much better in both cases. Of course, if you have to run carb heat for long periods of time, the cabin gets cold.
I felt I would rather have both muffs providing carb heat when necessary.
If you can find a new "Y" fitting, I would use it. You should be able to repair your existing one, as said above. Otherwise, used to alternate set up. IMHO.

[Mark Harwood]

Bruce,
Did the service bulletin require changing the cabin heat box so the air has a place to go when not using cabin heat? I also like the idea of both muffs providing heat to cabin! It has been a few days since I held this part, but I think the flapper is on the leg that goes to the carb. even when not using carb heat there must be more draw to that leg from the "air dump" (I will call it) at the carb heat box than to the cabin. Seems like that flapper would always be open unless you had a positive pressure coming from carb heat box such as could result from a backfire. Do you think that flapper is closed when not using carb heat? That would make since for it to be for cabin heat purposes,but I can't see how it could be. It seems like having mechanical control of a valve at that flapper position could force more heat into the cabin. I have read some of the threads on cabin heat, but don't seem to find a tried and true method. We repaired my Y to the satisfaction of my mechanic and myself. I will give it a try and see how she heats. Mark

[Bruce Fenstermacher]

Mark if I recall right the heat manifold vale at the fire wall dumps the hot air overboard when the valve is closed.

[Mark Harwood]

Bruce,
Mine does not appear to. It looks like the overboard for both muffs happens through the "union" and then out carb heat box. Mark