Basement air conditioning vs roof air conditioning.

When looking over the remarks about my neighbors inquiry on the Alfa Gold, BernieD mentioned the poor performance of basement air conditioning.

The "heat rises and cool falls" is absolutely true.. Theoretically, with proper placement and sizing of the supply air ducts and the returns air ducts, the basement air/conditioning should win every time. The "heat rises" is the key.

Funny how reality slam dunks theory everytime.

Has anybody else had this unsatisfactory result with basement air?

It seems to me that besides the outside temperature, with basement a/c you are also fighting the extreme heat the road or ground has absorbed.  Another thing to consider is that while heat does rise, that puts your head closer to the warmer air, and you are constantly surrounded by that warmer air.  With roof air heat is still rising, but you can catch the cool air on it's way down.
JMO

Over the years several MH manufacturers have used basement air and then gone back to roof air.  Some of the possible reasons they did this might be basement air uses valuable basement bay storage space and it's even worse with tag axles because the tag axle uses basement storage space too.  It's more costly because of all the unique ducting.  Also, the roof air storage-space gained cannot be utilized for storage because most people will not climb up on the roof to store things.  Finally all the major advancements in MH A/C have been made in the field of roof air. 

We have two 15,000 BTU roof airs with a flat ducted ceiling and we do the following which could be done with basement air but not as easily as with roof air.   When watching TV we sometimes close ALL the rear ceiling vents up to the kitchen and run only the rear A/C.  This cools the front without much A/C noise.  When sleeping on hot nights we do the reverse closing ALL the front vents back to the bedroom and run only the front A/C so it's quiet in the bedroom.  One thing that makes this work well is the 15,000 BTU capacity of each A/C and the ease of closing and opening the ceiling vents.

BTW, if you ever have to replace a roof air get the higher capacity 15,000 BTU as opposed to the more standard 13,500 BTU.  They will fit the same hole in your roof.  When running they use only 1 more amp.  I suppose on start up they also have a greater surge of amps.

JerryF

I'm trying hard to remember the comments, but some made by the Prevost owner I talked to were; more noise in the bedroom above the HVAC unit, more heat transferred into the coach from the unit, loss of storage space and harder to service. The increased heat transfer can be brutal in high temperatures.

Some people including me think the Alfa products are very poor investment.  JMO

We just sold our beautiful Dolphin LX because of the basement air.  Not because it did'nt do a good job of cooling, but because of the noise.  I am a light sleeper.  The noise from the unit wasn't all that bad, but the air flow noise was very loud coming up from the unit.  When it turned on it sounded like a tornado in the bedroom.  Just my 2 cents worth. 

DH replaced 1 roof air 2 weeks ago.  It was a PILL to get it up there! 

When we were shopping one dealer was just bent on selling us a coach with basement air and of course he was telling us all of the reported advantages.  Believing a salesman no more than I do now I went back in his service department and asked just for an estimate for servicing the air conditioner according to his estimates you could replace a roof air for what it costs to service a basement air.  Talking a little further he indicated that basement air required more service calls than roof air.  I have heard pretty much the same comments from other service folks.

And how many houses have roof air???  With a forced air system, the "heat rises" argument is irrelevant.  I don't like basement air because of the basement space it consumes, but cooling efficiency depends on the quality of the compressor and heat exchanger, not whether the ducts are above or below the compressor.

Gary

>>And how many houses have roof air???<<


Almost all houses in Yuma, AZ....Ours included.<G>

Terry A. Brewer said:

Gary

>>And how many houses have roof air???<<


Almost all houses in Yuma, AZ....Ours included.<G>

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And Texas.  The house we had had the AC in the attic as does our daughters house.

Most likely in Texas, the AC compressor is outside the house on the ground, with the rest of the AC in the attic.

caltex said:

Most likely in Texas, the AC compressor is outside the house on the ground, with the rest of the AC in the attic.

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Doesn't make much differenc where the compressor is at the air handler is located in the attic.  In a MH a basement air system the air handler and compressor are both in the basement while roof air they are on the roof.  The home system compressor is much easier accessible for service than the AC in a basement of a MH.

caltex said:

Most likely in Texas, the AC compressor is outside the house on the ground, with the rest of the AC in the attic.

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Exactly.   

When we were in CA we noticed a lot of a/c's mounted to roofs.  We were told they did it that way so they didn't use up their precious tiny yards on the a/c.  Most of those a/c's got no shade at all, but it wasn't nearly as hot there as it is in Texas.  We're Texans and we were freezing at night in mid-June in CA.

It is realized that most of us live in "hot" territory.

The A/C unit are on the roof mostly for maintenance purposes. AND we use the A/C much more often than  heat. When I lived in SW Florida the heat was turned on just to make sure it was working.

We all know A/C removes heat, what remains is air of a lower temperature.

Generally speaking, the air at the ceiling is warmer than air on the floor, because warm air does rise. If you went to a home, say in New York or that latitude which did not have air conditioning, they normally have a whole house fan located in the attic or crawl space. The house heats up during the day, at night doors and windows are opened and the fan is turned on. This cools the house. and it does it by removing the hottest air first, etc..

My poorly made point was: why is supply air that had just been "cooled" allowed so quickly back to the roof A/C. It would seem drawing the ceiling hot air, "cooling" it and supplying it floor level in a 400 sq  ft room while the ceiling return continues to draw hot "uncooled" air would be far more efficient. In a stick house the return vent is not located close to the supply vent.

I agree, it's too costly and noisy, hard to maintain and  breaks away from the norm. Toyota will probably have to show us how it can be done properly, probably within the next ten years.

"Heat rises, cool falls"

If the air conditioning vents are in the ceiling, you're introducing cool air into the hottest part of the room.  Since cold air is heavier than hot air it then sinks, cooling the entire room.

If the intake is likewise at the ceiling, you're drawing the hottest air back into the air conditioner.  The majority of cooled air is directed away from the intake so it has a chance to distribute throughout the room instead of being drawn right back in.

Most RV ducts are not very well insulated, so part of the coldness produced by the air conditioner is lost to the outside as the air travels through the ductwork.  A basement unit has much longer ductwork than a roof mounted unit, making it's duct losses correspondingly greater, leaving less cooling for the room.

If the basement unit uses floor ducts and registers, you're introducing cold air into the coolest part of the room.  Since cold air is heavier than hot air, it will pool at the floor instead of rising to displace the hot air near the ceiling.

Everything else being equal, you'll get better performance out of a system that discharges cooled air high, then lets it fall, than one trying to blow the heavy, cold air up towards the ceiling.

One point not yet mentioned.  Our Monaco model normally comes with a drop center section at the ceiling and a polished ceiling panel above it.  I guess this gives it the Bordello look.

We did NOT want that type of ceiling because it reduces the A/C efficiency quite a bit.  We ordered a flat ceiling like is on most all MHs, 3 extra A/C ceiling vents, and two 15,000 BTU A/C units.  This reduced efficiency is because the A/C vents that usually blow the cooled air straight down toward the floor would blow against the drop ceiling panel that is 4-6 inches below the A/C ceiling vent.  This very effectively stops the cool air and pools it above the drop ceiling.  Well, guess what, the A/Cs air return intake is close to the pool of cool air and you have a vicious cycle of cool air being almost immediately recycled back into the A/C unit without ever moving down toward the floor.

Of course this happens with the A/C vents that are fore and aft of the A/C return air intake seriously reducing the cooling effect in the coach.  For example with two A/Cs above the drop ceiling, it almost nullifies the cool air from four A/C vents.  Furthermore, it reduces the cool downward airflow on all the A/Cs vents that are above the drop ceiling, even the ones that are not next to the A/C return air vent.  The bad news is that the drop ceiling is usually in the front section of the coach and that usually is the most difficult area to keep cool.  If the bedroom has a drop ceiling then it's even worse for cooling.

Before we ordered our coach in 2003 I already knew that 40 footers with drop ceilings and three A/Cs were not being cooled as well as 40 footers with two A/Cs and a flat ceiling where the A/C vents can blow the cool air downward.

I know of people that have removed the drop ceiling, at great cost, and added a cross block (thwartships to the coach) in order to reduce this immediate recycling of cold air back into the A/C.  Then they reinstalled the drop ceiling. 

We chose function over form!

JerryF

JerArdra said:

We chose function over form!

JerryF

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Good choice too Jerry.  I have heard several folks that had someting similar installed and then complaining about loss of AC efficiency.

Jerry:

In your opinion, had you not had a flat ceiling, which would have been more efficient: basement or roof  air conditioning?

The short circuiting of already "cooled" air going back to the A/C has always irked me.  I took it as a sign of very poor HVAC engineering, not well thought out.

Kerry

Kerry,

I think that roof air would be more efficient because there is less ducting and fewer turns of the ducting to slow the air movement.  For example on air coolers (swamp coolers) if the rated air flow capacity of a specific air cooler is 6,000 cubic feet per minute that means air flow with a straight in-flow of air (no turns in the ducting).  But if there is an immediate 90 degree turn of the ducting, the air flow rated capacity is somewhat less.  Air cooler mfg'ers can tell anyone what the reduction in airflow is depending on whether the ducting turns 45 degrees or 90 degrees or whatever.

JerryF