Furnace question..........Churchmouse or anyone?

[ghitch75]

If you're going to post about fuel pressures, I'd be nice if you post CORRECT information!

Normal input pressure for Nat is 5-7" wc with a 3.5" wc burner pressure.

Normal input pressure for LP is 11-14" wc with a 10" wc burner pressure.

**Always check equipment labels for the exact specs, as all equipment is not the same.

BTW, 80% don't sweat, a low temperature rise causes the flu gases to condense. Condensation can also destroy a high efficient furnace, when the condensation happens in the primary heat exchanger. Temperature rise specs are there to protect your equipment.

i have ran these pressure for years.....and yes low pressure make low temps which will make them 80%'s sweat......seen it to many times with rotten out draft blowers and upper heat exchangers......


only time i have seen a heat exchanger rot from low temp was when a outside air damper was stuck open.........if your find them with condensation in the primary tell your customers to shut there windows!!!!!!!!!!

 

[sig-guy]

i have ran these pressure for years.....and yes low pressure make low temps which will make them 80%'s sweat......seen it to many times with rotten out draft blowers and upper heat exchangers......

If you've been doing this for YEARS, you wouldn't be using the word "sweat" to describe condensation of flu gases. And would hope you'd know how to set air flow to allow the unit to operate with-in manufactures specs. Assuming you're measuring the temperature rise. Do you take Carbon Monoxide readings in the exhaust?

only time i have seen a heat exchanger rot from low temp was when a outside air damper was stuck open...

^^ Really, did you forget what you just typed?
"low pressure make low temps which will make them 80%'s sweat......seen it to many times with rotten out draft blowers and upper heat exchangers"

So which is it, low pressures or stuck dampers leading to rotted heat exchangers? Cause if the 'only' time you've seen one rot was from a stuck damper, then low pressures are not causing rotting failures.

if your find them with condensation in the primary tell your customers to shut there windows!!!!!!!!!!

I highly doubt people are leaving their windows open long enough to lower the house temp to a level where it'd lower the return air temp enough to be a problem. Cause if they liked it that cold, they wouldn't be running the furnace now would they?

 

[churchmouse]

Lets get along her boys. This is high tech stuff you know......

I have seen low rail pressure eat up a furnace. Just saying, it can happen. Incomplete combustion seems to make a more caustic environment in the furnace.
Sig, if you set them up right from the initial start up as I am sure you do, it is likely you may never see this.

 

[sig-guy]

CM, I understand what happens when condensation happens in a primary heat exchanger. This is why secondary's are made from stainless steel and plastic. Even the epoxy coated aluminum didn't fare so well in this category.

What I have a problem with, is a (so-called) 'professional' recommending to set Nat gas fuel pressures at 5" wc for equipment spec'd for a 3.5" wc manifold pressure. That is flat out wreck-less and dangerous!

Low rail pressure doesn't exist, if the pressure is set at the manufactures specifications. Now it can happen if something fails, but you still have to set-up the equipment properly from the start.

 

[ghitch75]

CM, I understand what happens when condensation happens in a primary heat exchanger. This is why secondary's are made from stainless steel and plastic. Even the epoxy coated aluminum didn't fare so well in this category.

What I have a problem with, is a (so-called) 'professional' recommending to set Nat gas fuel pressures at 5" wc for equipment spec'd for a 3.5" wc manifold pressure. That is flat out wreck-less and dangerous!

Low rail pressure doesn't exist, if the pressure is set at the manufactures specifications. Now it can happen if something fails, but you still have to set-up the equipment properly from the start.

(so called)????....5"wc is not wreck-less.......i guess you have never seen regulators stick or fail.......i'm done with this.....

 

[tmschuller]

Good info.. one other thing if you have not checked this : check you tubing coming from the pressure switch.. the age of your furnace and symptoms and if your tubing has not been replaced recently.. trim the ends of the tubing and put back on until you can get new.

Do you have other gas appliances? Are they running ok? If so I would look other places for the issue. I have serviced and installed for years and this is common. Hope you can get it figured out soon. Tim

 

[sig-guy]

(so called)????....5"wc is not wreck-less.......i guess you have never seen regulators stick or fail.......i'm done with this.....

There's a HUGE difference between setting them up incorrectly and experiencing an equipment/part failure.

"so-called", yes. I have to go on what you post here, as I do NOT personally know you.