# Direct Vent Exhaust Cap - Too Hot?



## Peter B. (Dec 28, 2010)

After 25 years burning wood without a reliable backup heat system, this year I had a direct vent propane wall furnace installed.

Never having used a similar appliance, I didn't know an awful lot about what to expect.  For the most part it works 'acceptably', but I have a question.

The through-the-wall horizontal exhaust vent is quite short... likely less than a foot... not much length for any significant cooling.  All the same, I was somewhat surprised that the vent cap outside was too hot to touch during operation.  Is this normal?

In my ignorance, I would have thought more of that heat would be effectively captured and used to warm the indoors... rather than all outdoors.

If it's normal for this sort of unit, obviously I'll have to live with it.  If it doesn't sound right to people here, I may ask the installer to come have a look.

Thanks for any/all replies.

Peter B.

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## North of 60 (Dec 28, 2010)

Sounds like a non-condensing mid range efficiency furnace.  That will be pretty much what you can expect Peter.

On install was your LP pressure noted at the burner manifold?


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## Peter B. (Dec 28, 2010)

north of 60 said:
			
		

> Sounds like a non-condensing mid range efficiency furnace.  That will be pretty much what you can expect Peter.
> 
> On install was your LP pressure noted at the burner manifold?




north of 60:

Thanks for your reply...

I should have given some detail about the furnace.  It's a Williams model 3003621... rated at 73% efficiency, 68% AFUE.

(Expense was a major consideration here.  The living space now in use may not even be inhabited next year.)

I don't think the LP pressure was measured _directly_ from the burner, but the LP man made a point of putting the regulator very close to the unit outdoors... seemed knowledgeable and conscientious... and made a variety of other checks before signing off.

So, given the nature of the beast, the hot vent cap is pretty much a given, eh?

Again, thanks for your time.

Peter B.

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## North of 60 (Dec 28, 2010)

Peter, there is one thing that you can do .... In the owners manual you should be able to find the max and min temp rise across the heat exchanger as a spec. That would be to record the return temp into the unit and supply temp out of the unit. Subtract the two and that will give the temp rise through the unit/ across the heat exchanger. That will tell you if it falls in the area of its design. RE: output of the unit to confirm it is not being overfired. With natural gas there is usually a meter you can clock to find the BTU input that the unit is consuming. With the propane you are probably running off of your own tank. This is the procedure I would preform if I had no other instruments to use. I am sure all is normal, but would be a fun and useful thing to do for piece of mind. The manufacture should list the temp rise expected.
Cheers


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## Peter B. (Dec 29, 2010)

north of 60 said:
			
		

> Peter, there is one thing that you can do .... In the owners manual you should be able to find the max and min temp rise across the heat exchanger as a spec. That would be to record the return temp into the unit and supply temp out of the unit. Subtract the two and that will give the temp rise through the unit/ across the heat exchanger. That will tell you if it falls in the area of its design. RE: output of the unit to confirm it is not being overfired. With natural gas there is usually a meter you can clock to find the BTU input that the unit is consuming. With the propane you are probably running off of your own tank. This is the procedure I would preform if I had no other instruments to use. I am sure all is normal, but would be a fun and useful thing to do for piece of mind. The manufacture should list the temp rise expected.
> Cheers




north of 60:

They may be hiding somewhere in the manual, but I couldn't find the temp rise specs when I looked.  I have a feeling even if I can find them, I wouldn't be able to conduct the test with the accuracy needed to 'prove' anything.

I may give the installer a call anyway... I picked the furnace myself, and it appears - at 30K BTU input - it's undersized.  It takes quite a while for it to 'gain' (from e.g. 50*) even in mild (20+*) weather.  I'm inclined to think there's nothing wrong with it, but it might be worth an opinion from a pro.

Nice to hold 50* overnight though... quite the luxury.

Thanks Again.

Peter B.

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## North of 60 (Dec 29, 2010)

Hello Pete, I found your unit on the internet but could not download the installers manual/start up. I did notice it was a natural convection unit with an optional blower. For sure without the blower it will be quite a bit hotter at the vent. Did you get the blower as an option or just keep it as a non-power back-up? 
http://www.gasheaterstore.com/Williams-3003621-Direct-Vent-Furnace--30000-btu--Propane_p_91.html


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## Peter B. (Dec 29, 2010)

north of 60:

Here are links to the product description page and the owner's/installation manual:

http://www.comfortgurus.com/product_info.php/cPath/200_386_425/products_id/4592

http://www.comfortgurus.com/specs/williams/direct_vent_furnaces_brochure.pdf

Yes, I got the blower, but haven't been using it a lot...hate the noise.  (However, I think my current 'policy' with respect to the blower will change immediately when we get a true cold snap again.)

While I appreciate your help and interest, I'm betting at this point there's nothing amiss.

On the other hand, this was a brand new 'from scratch' installation... the installer came first when there was no gas on hand yet.  The LP man followed, but couldn't get the main burner to light.  I 'discovered' (and corrected) the faulty thermostat wiring the installer left behind... and got the burner going.  No one but me has seen it running.  The flame color looks fine... 

And I have no complaints about the unit... other than those already stated.

Thanks for your time.

Peter B.

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## North of 60 (Dec 30, 2010)

http://www.comfortgurus.com/specs/williams/direct_vent_furnaces_install_manual.pdf

I guess page 10 is as technical as its gonna get. Enjoy!
Cheers.


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## jtp10181 (Dec 31, 2010)

Long story short, even on 80% + efficient gas direct vent fireplace with a short vent run the cap can get very hot.

Sounds normal to me.


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## Peter B. (Jan 2, 2011)

north of 60 & jtp10181:

Once again - thanks to both of you for your insights... and Happy New Year, besides.

It's only been (a few days short) of two weeks since the furnace has been up and running.  I'm slowly learning how to dovetail its use with the old wood stove... but feel like I'm making progress... and think (overall) I'm more comfortable as a result... which (of course) was one of the primary objectives.

And (once again) a uaranteed_ 50* in the morning is something I haven't enjoyed for a long time.

Now comes figuring out what I'm actually paying for all this luxury... I haven't yet looked at the propane tanks to see if there are any gauges or meters.

But... maybe 'knowing better' is something I can put off til next spring.

Peter B.

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