Author Topic: Relative Humidity Problems - AC System  (Read 5182 times)

Offline kayjh

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Relative Humidity Problems - AC System
« on: August 31, 2012, 09:22:04 AM »
I'll try to keep this as short as possible.

I have a 3,000 sq ft ranch style home in western Canada. We had new AC system installed 5 years ago, including ductwork in the attic of our home. 2 AC units were installed, (both 3 ton). One provides cooling for the LR/DR/Hallways and Den and the second does the bedroom wing. From day one we got extremely cold air and excessive cycling. In addition, the RH was never lower than 62%. After some discussion with the contractor, it was discovered that the manual J called for 4.2 tons (Supplier of the equipment did the manual J but didn't provide it to the contractor, just the recommendation for 6 tons) . The contractor discovered that the air handler for the LR unit was sized for a 4 Ton unit as was the duct work. The bedroom air handler was for a 3 ton and the ductwork was also sized for a 3 ton.

After a meeting last month the contractor said the following was the fix:

1. Remove the bedroom 3 ton unit and replace with a 1.5 ton unit and matching 1.5 ton air handler;
2. Remove the 4 Ton air handler from the LR unit and discard. Use the 3 Ton air handler (from the bedroom unit) connecting to the 3 ton outdoor unit - existing (this set up will give 4.5 ton of cooling capacity);
3. Balance air flow and adjust ECM air handlers to optimum speed.

With this new setup, cooling comfort has improved. Airflow has been reduced significantly and we no longer feel blasts of cool air (the "meat locker" effect). There was some discussion during the recent installation that we might not have enough "static pressure" with the new setup given that the ductwork was 3 ton for a now 1.5 unit compressor (bedrooms) and 4 ton for 3 ton compressor (LR/DR/Den). We had no further discussion on this.

I had asked the contractor to make sure they sized the new units correctly and asked that they upgrade me to 2 stage units. I reasoned that in off peak running times, the Rheem units running in stage 1 would dehumidify without lowering the temperature much below the set point on the thermostat. They said the cost would be excessive as their remediation plan involved using some of the same equipment, wiring, setup, etc.

I don't know where to turn at this point for a solution. I feel the RH of 63% when the outdoor is 60%RH (+28C) is too high. Crackers in our cupboards are soft and it just feels "wet" in the house.

Anyone have ideas as to what is wrong with out setup and what it will take to correct it?

Thanks,

kayjh 

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Re: Relative Humidity Problems - AC System
« Reply #1 on: August 31, 2012, 10:15:57 AM »
So now you have a 1.5 ton 2 stage ac and air handler and a 3 ton 2 stage ac and air handler?

Did they upgrade the thermostats with 2 stage cooling?  Are the air handlers in fact compatible with 2 stage cooling?

Did they adjust the air handler dip switches for the proper fan speed?  The manufacturer ships the air handler with high speed cooling to prevent ice ups.  It's up to the installer to adjust the airflow to 400 cfm per ton.  Lowering the fan speed will increase dehumidification.  This will also raise your temperature drop.

There's no reason you shouln't be able to achieve 40% RH.

You could install a ERV air exchanger that would help reduce humidity, but really the ac's you have should do that.

At 3000 sq ft I wonder if you would have been better using a 2 ton ac and a 1.5 ton ac for a total of 3.5 tons.  However if you do have 2 stage ac's wired correctly to a 2 stage thermostat, you should be geeting longer run times and better dehumidification.  There's usually a menu in the thermostat that would need to be adjusted to 2 stage cooling as well.


Offline kayjh

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Re: Relative Humidity Problems - AC System
« Reply #2 on: August 31, 2012, 11:09:15 AM »
Both systems are single stage. The switch to 2 stage was quoted at $6,500.00. We are using a Tekmar control system for our boiler and AC systems (Rheem equipment). They have adjusted the handlers to reduce air speed. I believe the bedroom unit is set at around 600 cfm, I'm not sure about the 3T LR unit., but I believe it is below 1,200 cfm now.

One thing I found curious was that the contractor said he did was increase the number of cycles per hour to 6 for the bedroom unit, to improve dehumidification. I didn't understand this. In addition, to keep some air moving, the air handler was set to slowly wind down when the compressor shut off. From what I've read elsewhere, this would increase humidity as the air would pick up the moisture on the coil (?)

It seems to me that a properly sized and operating AC system should keep indoor relative humidity at 50% or less. The question is what is wrong? Do we still have too much cooling? Is the larger duct size forcing us to use higher fan speeds to create the necessary static pressure, thus reducing run times?

What do we need to do to correct this? We had 6 ton and 62% RH. Now we have 4.5 ton and the same RH (give or take). Do you think it is time to call in an engineer?

Thanks for your help.  :)

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Re: Relative Humidity Problems - AC System
« Reply #3 on: August 31, 2012, 02:07:45 PM »
Which Tekmar control do you have?  Is it model 512?

I believe adjusting the cooling cycle rate to 6 would prevent the AC from running long enough to dehumidify.  There are settings in the menu that will let you adjust the on and off times.  I would try using the auto cycle rate option.

Does your house have an air exchanger, HRV?  You would want to turn it off during the hot summer days to prevent it from bringing in humidity.

You're right, the AC coil is saturated with humidity which can be recirculated into the house, but I don't think this is causing your problem.  An ECM blower running on a low continuous speed will help circulate the air and help eliminate hot and cold spots.

Does your boiler heat the domestic hot water as well?  Could hot water be migrating into the heating coil?  Maybe it could be throwing heat and humidity into the air?  If there's a heating coil and domestic hot water there's probably some type of recirculation mode built into the air handler.  This would force the system to heat for 5 or 10 minutes to prevent water from stagnating in the heating coil during the summer months.  Or I've seen check valves on circulation pumps that stick open and let the hot water migrate into the heating coil.

You could get someone else to complete a heat gain calculation.  I think they size AC's in Manitoba using the 1 ton per 1000sq ft rule. (just as a basic rule of thumb) In your case 3 tons may be all you need.  How many sq ft does the 1.5 ton and 3 ton AC cool seperately?

Offline kayjh

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Re: Relative Humidity Problems - AC System
« Reply #4 on: August 31, 2012, 09:04:41 PM »
Which Tekmar control do you have?  Is it model 512?

I believe adjusting the cooling cycle rate to 6 would prevent the AC from running long enough to dehumidify.  There are settings in the menu that will let you adjust the on and off times.  I would try using the auto cycle rate option.

Does your house have an air exchanger, HRV?  You would want to turn it off during the hot summer days to prevent it from bringing in humidity.

You're right, the AC coil is saturated with humidity which can be recirculated into the house, but I don't think this is causing your problem.  An ECM blower running on a low continuous speed will help circulate the air and help eliminate hot and cold spots.

Does your boiler heat the domestic hot water as well?  Could hot water be migrating into the heating coil?  Maybe it could be throwing heat and humidity into the air?  If there's a heating coil and domestic hot water there's probably some type of recirculation mode built into the air handler.  This would force the system to heat for 5 or 10 minutes to prevent water from stagnating in the heating coil during the summer months.  Or I've seen check valves on circulation pumps that stick open and let the hot water migrate into the heating coil.

You could get someone else to complete a heat gain calculation.  I think they size AC's in Manitoba using the 1 ton per 1000sq ft rule. (just as a basic rule of thumb) In your case 3 tons may be all you need.  How many sq ft does the 1.5 ton and 3 ton AC cool seperately?

I'm not sure what model Tekmar we have but I believe it has customization features. The 6 cycle per hour setting for the bedroom unit was done after the 1.5 ton unit was installed. Our house has a newly installed HRV (not commissioned yet) but only operates to exchange air in our basement. We just completed a basement renovation (top to bottom) and while our radon levels are relatively low (about 400), we though we could improve that by exchanging air. The added benefit is better air quality in the winter (given the hot water heat system).

Our boiler does heat the DHW, but this system is 100% independent of the AC system. The Boiler system is a Weil McLain and is zoned to service continuous base board radiators in the house. The AC system was installed with new ductwork in the attic area (2 separate sets of duct work) and the compressor units are, of course outside. The coolant lines run up the side of the house through the soffitt and into the attic space. We have a 5 KW heater in each duct system. The Tekmar control runs the air movers in the winter at a low speed to circulate air. When the outside temp is below 55F, if the exit air temp in the duct is lower than the thermostat set point in the house, the duct heaters come on to avoid the potential for condensation in the ducts. This system isn't active in the summer.

The 1.5T bedroom unit cools about 1,500 sq. ft. of the total but the bedrooms are on the north side of the house. The LR/DR/Den are on the south side and have floor to ceiling windows on most walls. The thinking was the heat gain in these areas dictated more cooling.

On the whole, we have very good balance of cooling with the thermostats in most rooms reading within 1.5C of each other. It is the high humidity that puzzles me now.

Thanks in advance for your comments.  :)

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Re: Relative Humidity Problems - AC System
« Reply #5 on: September 02, 2012, 09:32:11 AM »
The attic is probably an unconditioned space and very hot and humid in the summer.  Have you been up there to see if there are any breaches in the return air ducting?

Offline kayjh

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Re: Relative Humidity Problems - AC System
« Reply #6 on: September 04, 2012, 04:04:38 PM »
The attic is probably an unconditioned space and very hot and humid in the summer.  Have you been up there to see if there are any breaches in the return air ducting?

The duct work was a new fabrication and install in 2006. Every joint has mastic applied, followed by a layer of aluminium tape and 2 layers of insulation all taped at the seams. the stale air return is a 24" x 24" ceiling vent (one for each unit).

I wonder whether the ductwork size and the speed the units are running at is at play here. Remember that the units were initially oversized for the required equipment. The HVAC company sent their duct guy out to the house to start building the duct work and on site were 2 air movers ( a 4 T unit and a 3 T unit). He built the duct work for those capacities. I believe the main plenums were 16" round. I'm not sure if the round duct work was a different diameter for the 4T unit and the 3T unit. The ceiling supply diffusers are 4" x 10" supplied by 6" round solid (some flexible sections in some locations to get around structure).

Now that the ductwork is handling 1.5T (running through the 3T sized ductwork) and 3T (running through the 4T sized ductwork), I wonder if the HVAC company had to make ECM speeds match what was necessary to have the proper static pressure to distribute the air efficiently? If so, does this have an effect on the ability to dehumidify?

I guess I'm trying to understand what factors (other than defects such as air infiltration, or leaking intakes, etc.) are at play in determining how well an AC system dehumidifies a space. As I write this, I am sitting in my air conditioned office and have a hygrometer sitting on my desk. It reads 23.7C and 36% RH. The same hygrometer was reading 22.5C and 59%RH in my home this morning. I feel "wet" at home and perfectly comfortable at my office. Same goes for my car on the way home; 23C and 49%RH. If they can do it in my office (same HVAc company) and my car is comfortable, what could be wrong in my home?

Thanks for listening.  :)