Comments on: Mr. Bean Proposes a Radical Change to Energy Codes

By: Morgan Audetat

Morgan Audetat — Sun, 18 May 2014 16:01:53 +0000

This whole discussion makes a
This whole discussion makes a huge assumption, i.e. that anyone has done a heat load of any kind before starting construction of a new house.

Unfortunately modulation has not improved the situation as it gives the average contractor, 99% or so, more "perceived" latitude in over-sizing HVAC equipment.

While we consider MRT standards (brilliant Mr. Bean), the industry slogs along in the 19th century.

Though the installing HVAC contractor is responsible for installation, and sometimes maintenance, the builder rarely hires the forward thinking HVAC contractor. Ultimately we have to advocate for the homeowner to seek out and insist on contracting for mechanical systems directly with a qualified HVAC contractor.

Of course we could get Architects to consider HVAC systems and creature comfort as the first order of business but you would have to start with taking their window away...good luck with that!

By: James Jackson

James Jackson — Tue, 01 Jan 2013 07:04:59 +0000

David Butler wrote: ''You
David Butler wrote: ''You forgot one - icing the evap coil. All all these outcomes are indeed possible, but only if the person designing the system doesn't know what they're doing."

I didn't forget icing the coil, Its just unlikely the coil would actually ice in the 7 min low speed delay. Frost maybe but the other issues would be present with exception of the duct sweating, that takes a little more time. I completely agree with your comment about only the person designing the system doesn't know what they're doing.

I am of the opinion that the only way to properly control humidity is to slow or remove the source, then design the system according to manual J then select the proper unit using Manual S for the sensible/latent load.

My point was mainly to know the equipment you are altering. If you changed the air speed permanently or temporarily with a time delay on a system with a TXV and a accumulator its not that big of a deal. The bigger problems are present when you drastically lower the air speed on a unit with a fixed metering device.

I get calls quite often from contractors that have installed a new HVAC system with new sealed and R8 insulated ducts that are sweating like crazy. The majority of the time I find the unit is a single stage system in Dehu mode allowing the air to spend too much time on the coil, dropping the temp in the ducts well below DP. The fix is simple increase the air speed to close to 400 CFM per ton durring the regular cooling cycle then set it to drop to around 350 per ton for temporary dehumidification.

If you are going to install a relay to slow the air speed across the coil just make sure you look at the engineering specs on what the fan can produce at certain speeds and static pressure readings and try not to lower the CFM more than 50-60 CFM per ton. High to Medium or Medium-High is probably closer to a safe range.

By: David Butler

David Butler — Tue, 01 Jan 2013 06:00:58 +0000

James Jackson wrote: &
James Jackson wrote:
"By reducing the air speed you can cause system performance issues, distribution problems, sweating duct due to lower temps, flash point of the refrigerant (fixed metering device) and discomfort due to lower output temps of the system."

You forgot one - icing the evap coil. All all these outcomes are indeed possible, but only if the person designing the system doesn't know what they're doing.

Commercial (and residential dehumidification systems operate every day in this manner. To the extent that high RH is coincident with a sensible load, it's a lot more efficient to use a 13 SEER (or higher) air conditioner to manage RH than one of those stand-alone dehumidifiers with toy compressor.

By: James Jackson

James Jackson — Tue, 01 Jan 2013 00:05:50 +0000

There are a lot more things
There are a lot more things to consider than simply reducing the air speed of the blower. By reducing the air speed you can cause system performance issues,distribution problems,sweating duct due to lower temps, flash point of the refrigerant (fixed metering device)and discomfort due to lower output temps of the system.
I am very cynical of the dehu controls that simply lower the air speed to drop the coil temp.

P.S. Great article/discussion Allison !

By: Bob

Bob — Sun, 22 Apr 2012 15:12:31 +0000

I wasn’t the first to come up
I wasn’t the first to come up with the idea, this webpage explains it a bit better:

An alternative to changing the thermostat or adding a dehumidistat is to use a Delay-On-Make timer. Since we know the majority of cooling operating hours are at less than outdoor design conditions, we can operate the blower at a reduced cfm for the first 5 to 10 minutes of thermostat demand in anticipation of humidity complaints.

Install a Delay-On-Make timer in series with “Y” and the dehumidification relay coil. So a “Y” output from the thermostat will energize the outdoor unit contactor and the added timer, which will energize the added dehumidification relay after the selected time delay period.
Connect the normally open blower relay contact to the dehumidification relay SPDT common terminal.
Connect the slower blower motor speed tap to the dehumidification relay normally closed contact.
Connect the faster blower motor speed tap to the dehumidification relay normally open contact.
On each cooling demand, the blower operates at reduced cfm until the timer period has ended, at which time the blower will operate at higher cfm to finish out the cooling cycle.

http://www.fluke.com/Fluke/usen/community/fluke-news-plus/ArticleCategories/HVAC/Battling+High+Humidity.htm

By: David Butler

David Butler — Sun, 22 Apr 2012 06:53:40 +0000

re: "redneck variable
re: "redneck variable speed control"

I'm impressed, from one redneck to another.

By: Bob

Bob — Sun, 22 Apr 2012 06:19:27 +0000

I just know when humidity is
I just know when humidity is high in my own home 74 feels warm and I'll bump it to 72. Agreed, above 55% is when it has the most effect, I normally won't bump the stat if RH is already below 50%.

I do have my own "redneck variable speed control" on my furnace that runs the blower on low for the first 7 minutes then switches to high speed. It's a simple SPDT relay controlled by a delay on make timer set to 7 minutes. Red (low) blower speed goes on NC relay contacts, Black (high) goes on NO contacts. Common of relay goes to "cool speed" on furnace control board. On the 24V side one terminal of relay goes to common, the other goes to delay on make timer. Delay on make is wired between "Y" and relay coil.

Works well considering it costs less than $20 in parts. Helps with coil getting colder quicker and starting the dehumidification process. During low load conditions, the cycle is rarely over the 7 minutes required to engage the relay. Under high load conditions the A/C spends most of its time on high speed.

By: David Butler

David Butler — Sun, 22 Apr 2012 00:05:50 +0000

Bob wrote: "Most people
Bob wrote: "Most people simply hit the "cooler" button on their thermostat when it "feels muggy" in the house."

Agreed. Not good. But your enthalpy thermostat would simply automate what we agree is bad behavior! The fact that the stat does it more precisely than the HO does not make it good.

"The stat could also raise the setting 1 degree for every 4% under the humidity setpoint"

It doesn't work like that. Reducing the RH below about 55% will have little if any benefit in terms of dry-bulb comfort. This will just lead to comfort complaints.

In my previous home in Charlotte, RH generally stayed below 55% with ocassional excursions into the high 50's (even low 60's when there was no sensible load). I moved to SE Arizona about 5 years ago and we've only been able to raise the setpoint by 1 degree, even with indoor RH at 30%. During monsoon when RH ocassionally climbs into the low 50's, we find we don't need to reduce the temeprature to maintain comfort. So I would say that there's about 1 degree of impact below 60%.

Above 60%, the impact of RH on comfort is greater, but maintaining comfort under these circumstances isn't necessarily accomplished by reducing dry bulb temperature. For example, many areas of the country can have extended periods of damp weather in the fall and spring when there's no sensible load (as you pointed out). In that case, the indoor temperature may be in the low 70's, so the body senses cool and wet. That's different from warm and wet, which is what your enthalpy stat is attempting to address. In particular, it makes no sense to reduce the temperature further when it's already at the low end of the comfort range. This will just lead to comfort issues AND higher energy bills. When there's a latent load and no sensible load, there are but two choices: dehumidify or stop the source.

A properly sized AC will prevent "warm and wet" in most cases, and as we agree, fan control will improve a system's latent capacity. But if that's not enough, there may be a narrow range of conditions in which lowering the stat one degree might (eventually) help. However, using your example of 74F/60%, lowering the temperature to 73 would initially *raise* RH to 62% (humidity ratio hasn't changed yet). Depending on how quickly the AC satisfies the new setting, there's likely to be no reduction in RH during first several cycles. Keep in mind it typically takes the evap coil at least 10 minutes to begin condensing moisture. More likely, it would take several days to see a significant reduction in RH, depending on sensible loads and whether the system is properly sized. Moreoever, because of the long lag times involved, again, this strategy would expend energy while creating even more comfort issues. One more thing, if your enthalpy stat were installed on a home with a grossly oversized AC (e.g., zero latent capacity), you'd end up with very cold occupants and very high energy bills.

By: Bob

Bob — Sat, 21 Apr 2012 16:15:02 +0000

Agreed, eliminating the
Agreed, eliminating the source(s) of moisture is the best option. Running the blower on lower speed to reduce humidity is also a great option IF there is enough of a sensible load to trigger A/C operation.

Hot gas reheat is an expensive retrofit for most installations, not practical for residential unless a new system is being installed in a high humidity climate.

Here in Oklahoma there are times when it's wet outside but not warm enough to trigger to operation of a conventional thermostat. Current humidity sensing thermostats won't trigger the system since it's not warm enough (at least the White Rodgers line doesn't).

Most people simply hit the "cooler" button on their thermostat when it "feels muggy" in the house. Having a thermostat that would lower the temperature about 1 degree for every 4% the space was over the humidity setpoint would help a lot IMHO. The stat could also raise the setting 1 degree for every 4% under the humidity setpoint, saving energy on days when the system runs a lot and humidity is low.

For example if the stat was set at 75 degrees @ 50% humidity and the space was 74 @ 60% humidity the stat would run the A/C until the house got to 73 @ 58% (humidity would go down because system would be running). Another example is if the house was 75 @ 40% the thermostat could raise the temperature to 77.

This thermostat is something that would be easy for HVAC contractor to install, even handy homeowners could do it. The stat could even have a "dehumid" terminal for use with Variable Speed furnaces or a Hi/Low relay on PSC blowers. This would help to get to the actual setpoint of 75/50% instead of overshooting for comfort. Installation of the dehumid feature would be optional, few homeowners would know how to wire up a PSC blower relay. PSC systems are 90% of what is in homes, although VS is slowly catching on.

White Rodgers has a "cool savings" feature that takes raises the temperature when the system runs a lot, and while fairly effective doesn't take into account the actual humidity in the space. "Cool savings" also does not lower the temperature in cases of high humidity.

By: David Butler

David Butler — Sat, 21 Apr 2012 08:46:54 +0000

Bob wrote:
Bob wrote:
> I'd be happy if somebody would come up with some sort of "empathy control" ... Automatically lower setpoint when humidity levels are high.

That would be a bad idea. What is needed is already available -- humidity control. This can be accomplished with a humidity sensing thermostat (or separate humidistat) and one of several methods to increase an AC's latent capacity (reduce fan speed, add hot gas re-heat for low cost off-cycle dehumidification, or a dehumidifier as last resort.

Simply reducing the setpoint would actually increase the RH in the short term, and perhaps even in the long run depending on the particulars of the hvac system, thus imposing a significant energy penalty. I can provide a short lesson in psychrometrics if required.

In many cases, high RH can be traced to one or more sources that can then be mitigated. Always best to attack moisture at the the source whenever possible.