View Full Version : rooftop question
01-14-2006, 10:54 PM
Many rooftop units have heat exchanger located down stream of evp. coil, doesn't this cause condensation on heat exchanger?
I cannt see the point why they are designed this way...
Thanks in advance
01-14-2006, 11:04 PM
No it does not.It causes condensation inside the tubes rotting them much faster lol.
01-15-2006, 01:45 AM
The evap coil removes a lot of the moisture before it hits the heat echanger. Not to be a smart ass, but that's how air conditioning works.
01-15-2006, 02:26 AM
I will admit it's a bit weird. After all on a furnace the coil always goes after the HX. But on rooftops I believe it's almost always the other way around.
But a RTU most likely runs more and longer, so maybe that helps them justify it?
01-15-2006, 07:54 AM
Nobody has ever explained that one to me. RTU heat exchangers are lucky to hit 10 years before they are history. We service a hardware store with 7 5 ton Carriers from late 80s. All are on their 2nd heat exchanger and most are about ready to go again. In a residential installation, coil in the return is prohibited to protect the heat exchanger. So why do it on a gas pack?
I think you'll find that the RH% of air coming off an evap is extremely high. Also the 55 degree air cools the heat exchanger and warmer air inside the heat exchanger condenses, especially on a RTU where outside air can get in the heat exchanger.
01-15-2006, 09:04 AM
Designed to fail, just like everything else these days. In my experience, the heat exchanger failure rate is directly proportionate to how much outside air it is exposed to. Just taking 5-10ton units now.
All brand bashing BS aside;
Carriers seem to fail more in my area because the HEX is generally mounted under the blower but the burner openings are in the compressor compartment. Trane's voyager least because only the flue openings really are exposed to the outside air.
01-15-2006, 11:09 AM
Thanks all for your replies
It is not something difficult to arrange heat exchange upstream the coil, but manufacturers still do the other way, so it must be for a purpose. The only explanation I can think of is for dehumidification reheat operation,where return air is cooled dehumidified through coil and reheat through heat exchanger to maintain temperature level. But this function is only available on a few highend models, it doesn't explain everything.
Any more thougts...
[Edited by felixking on 01-15-2006 at 02:53 PM]
01-15-2006, 06:38 PM
It depends on the unit manufacturer and also seems to depend on the size of the unit. Could be a design issue?
York units seem to have evap in return before heat exchanger......
Lennox smaller GCS units have evap after heat exchanger although the L series and larger GCS units are before.......
Carriers all seem to have evaps in return before heat xchanger......
Older YORK units SA, evap after heat xchanger but SS units before....
01-15-2006, 07:00 PM
Most low end RTU'S can be used for dehumidification IE running both heat & cool @ the same time, but most high end units electronics will not allow this to happen. In these units you have to order a hot gas dehumid coil & the gas heat isn't used. Look into info on rooftop life expectancy usually 15 years, although alot of heat xchangers only last 10 years.
01-15-2006, 07:26 PM
in order for the water vapor in the air downstream of the coil to condence on the heat exchanger the heat exchanger would have to be at dew point temp of the air that would be colder than the coil it just left. if this condition were true they you would be better off not running mechanical cooling anyway. if proper ducting is attached to the unit the chance of the condesation occuring is very limited
01-15-2006, 08:07 PM
The problem is the opposite. The 50 something air cools the heat exchanger. Hot humid outside air can enter through the burner compartment or through the inducer and it condenses inside the heat exchanger.
Below is a Rheem we changed out, under 10 years old. We've been selling Rheem furnaces with tubular heat exchangers since 1992 with not 1 failure. Our supplier says he's never had to order one either. But the techs who cleaned this every year said this 3 ton gas pack exchanger was badly rusted after just a few years.
01-15-2006, 08:23 PM
another thing that prematurely rusts out ht xchangers is too much outside air. 5,10,& maybe 15% is ok anything more than that should be stainless, but no one is willing to pay the higher upfront costs. I have seen lots of rooftops running @ 25-30% outside air & heat exchangers start rusting after 1 to 2 years.
01-16-2006, 10:17 AM
Too much outside air and being oversized.
01-16-2006, 10:54 AM
it probably accepted for minimal moisture,never saw one with the filters...Heat-X then the DX and only saw real condensation and water running out with that humid day and pushing the circuits to run on a start-up.a simple thing like a condensate trap on the DX coil not draining with those big Trane boxcar units.that water in the condensate pan will be picked up and dropped onto the Heat-X during the summer run.......you want to see rotting out,always check those drain off and gas fired units.
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