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residential humidity control through reheat

Posted by ionized (My Page) on
Tue, Oct 16, 12 at 16:26

We have a good number of posts here that have to do with residential humidity control. We have discussions about blower speed control as a mechanism for that. We have occasional posts about whole house dehumidifiers. So far, I have not seen anything to do with desiccant wheels, but was advised by a manufacturer several years ago that they are not practical for residential scale use.

In commercial and manufacturing situations, overcooling and reheating is done all the time as some of you know. I daresay that it is being done in the building I am sitting in right now. The question is, in residential situations, why can't very conventional existing mechanical cooling systems coupled to existing gas furnaces be used to overcool and then reheat to provide comfortable temperature and humidity? Is because there will be too much heating capacity that can not be effectively throttled, or some other engineering aspect. Is it just that there has not been the demand to develop proper controls?

It may not be the most efficient way to do it in a lot of areas where demand for this kind of conditioning is high. In other areas, it might be useful for the limited times where low cooling loads are concurrent with very humid conditions.

Of course, a second "condenser" could be added to the indoor air handler for reheat and that would be more energy efficient at the cost of added equipment.


Follow-Up Postings:

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RE: residential humidity control through reheat

You can do that but other concerns such as proper sub cooling temps remain at coils, compressor slugging, cold weather operation and others.
Since you have to generate heat to offset the cooling effects of dehumidification, your much better off using a central ducted dehumidifier..

I have one like this mounted in the attic connected with 14" ducts with an intake and exhaust on open second floor. Mines a desertaire, but same type.

http://www.aprilaire.com/index.php?znfAction=ProductsCat&category=dehumid

Here is a link that might be useful: Central dehumidifier


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RE: residential humidity control through reheat

"In commercial and manufacturing situations, overcooling and reheating is done all the time as some of you know. "

At a large increase in operating costs and maintenance complexity.

Large commercial installations have a full time building engineer available to keep things running.

Can you really afford that?


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RE: residential humidity control through reheat

"You can do that but other concerns such as proper sub cooling temps remain at coils, compressor slugging, cold weather operation and others. "

Would that be so hard to avoid with the correct electronic controls?

"At a large increase in operating costs and maintenance complexity."

Already covered, really. How would it be more expensive than a new, separate refrigeration system with both hot and cold coils in the air stream? Cost of operation would be extra gas and electricity on humid days rather than just electricity on a dedicated dehumidifier. We all know that gas is cheap right now.

The question is, what is so technically difficult about running both heat and cooling at the same time for short periods?


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RE: residential humidity control through reheat

"Would that be so hard to avoid with the correct electronic controls? "

The typical controls do not measure or take into account the problems you will encounter.
Ever seen a residential system with a pressure sender?

"How would it be more expensive than a new, separate refrigeration system with both hot and cold coils in the air stream?"

Typical residential cooling systems are not designed to operate below about 50F outside temperature.

If liquid coolant returns to the compressor it can be instantly destroyed.
Liquids do not compress well, and if you are lucky all that happens is the compressor locks up, and then the electrical thermal overload shuts it down.
Many times they simply have mechanical failures of rods, seals, and motor shafts.

An often significant difference between these larger systems is the presence of an accumulator tank for liquid refrigerant, and a separate control loop to keep that tank filled.
Variable metering is then used to allow only the required amount of refrigerant into the evaporator based on humidity.
Temperature is also sensed to prevent inadvertent freeze up.

Industrial air systems are far more complicated than just cascading two separate systems together.


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RE: residential humidity control through reheat

Thanks for looking.

"If liquid coolant returns to the compressor it can be instantly destroyed.
"
How are daisy-chained heating and cooling process going to cause liquid to return to the compressor?

"The typical controls do not measure or take into account the problems you will encounter. 
Ever seen a residential system with a pressure sender?"

What problems? Why would I need a pressure sender?

"Typical residential cooling systems are not designed to operate below about 50F outside temperature."

Then it can't do what I want below 50F, or maybe 55 so shut it off/lock it out. Won't likely need dehumidification at that temp anyway because that is heating weather.

"An often significant ... freeze up.

Industrial air systems are far more complicated than just cascading two separate systems together."

I'd like to try again. Lets ignore the complexities of industrial and commercial HVAC systems which is irrelevant to the question. I can ask this in two ways.

Why can't I run a residential HVAC system, that is conventional beyond some minor control changes, with both the mechanical cooling and the gas furnace running at the same time? I obviously can not do that with a conventional thermostat because they won't heat and cool at the same time.

To make the question even simpler, why shouldn't I "hot wire" my furnace to run while the thermostat is in the cooling mode and the cooling part of the system is operating normally?


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RE: residential humidity control through reheat

""If liquid coolant returns to the compressor it can be instantly destroyed.
"
How are daisy-chained heating and cooling process going to cause liquid to return to the compressor? "

If the entering air is not warm enough to evaporate the refrigerant in the evaporator you can 'slug' the compressor.


You can do whatever you want.

It is your money and your time.

It is NOT done the way you describe for the reasons given and plenty more.

But you are obviously better than all the HVAC engineers that create these systems.

Let us know how it works out after a compete set of seasons.


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RE: residential humidity control through reheat

LOL!
@ brickeyee

ionized might be that deadly combination
of an engineer with a lot of curiosity.

the heater is physically in the wrong place.
you reheating the air after the cooling coil.
presently heaters are installed
before the cooling coils so they
don't rust out.

this makes my head hurt just to
think about it.

keep pondering ionized!


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RE: residential humidity control through reheat

Brickeye, I know that there are a lot of people that know more about HVAC design than I do so I ask for help in understanding. Up to now, I have only seen hand-waving and no reasons why it can not be done. Why not explain why it can not or should not be done?

"If the entering air is not warm enough to evaporate the refrigerant in the evaporator you can 'slug' the compressor."

One reason that I can think of that would make it not practical with existing systems is that I think the air flows through the furnace first and then the cool coils. Although that would preclude slugging, it would make for very inefficient dehumidification.

If the system were turned around so the air were cooled first, it should work. Can the return air in a residence be too cool and cause slugging? If so, a simple control addition would prevent that by locking out the compressor if the temp a the return was below desired temperature.


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