ECONOMIC CENTS: Staying Cool in Summer (and Warm in Winter)
Feeling drained by the weather? Temperature's partly to blame. But not the only factor. Bruce Saller explains.
By Bruce Saller
COLUMNIST
We are in the dog days of summer, and our air conditioners are working overtime to keep us comfortable.
There are two factors besides temperature which impact how hot we feel: humidity and wind. Our bodies get rid of heat through perspiration, which cools our skin by evaporating. Higher humidity slows the evaporation process causing it to feel hotter. Wind can increase the evaporation, causing us to feel cooler. There is a formula (at end of article) to calculate the apparent (feels-like) temperature. Here is a table of apparent temperatures for some temperatures and humidities:
So, increasing the humidity from 50% to 70% causes the apparent temperature to rise by several degrees.
Running an air conditioner will lower the humidity a little, but it is not designed to remove large amounts of moisture. You should consider installing a dehumidifier to reduce your humidity level, especially if you have a basement.
Humidity enters your house through open windows and doors, and when pulled in when using kitchen/bathroom exhaust fans and the dryer. So keep your doors and windows closed as much as possible and minimize your use of fans and the dryer when it is humid outside.
On the plus side, using fans can save you money by lowering the apparent temperature. A seven mile per hour breeze reduces the apparent temperature by 3.8 degrees, allowing you to set the temperature to 75 degrees and have it feel like 71.2 degrees.
In the winter, higher humidity will make it feel warmer, so consider installing humidifiers to raise the humidity to near 50 percent, which will make it feel several degrees warmer.
Hopefully these suggestions will save you energy and money.
For those interested, the apparent temperature formula from Metservice Blog is:
Apparent Temperature (Celcius) = Temperature(T) + .33 x (relative humidity/100 x 6.105 x exp(17.27 x T / (237.7 + T)) - .7 x air speed (meters/sec) - 4
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"Running an air conditioner will lower the humidity a little, but it is not designed to remove large amounts of moisture. You should consider installing a dehumidifier to reduce your humidity level, especially if you have a basement."
Yeah, I don't know about all that. No doubt, humidity is a factor in summertime comfort and Mr Saller touches on some of the preventable causes (open doors) but not using spot exhaust when needed is not a solution.
The coils in an air conditioner and heat pumps work essentially the same way as a dehumidifier. If they are not removing the moisture from your home, it is likely to be due to an underlying cause rather than having bad equipment.
The most common cause is a condition called oversizing of the heat pump or air conditioner.
For over 20 years, new construction hvac contractors have been expected and are now required by law to perform calculations to determine what the heating and cooling load is for a home, then match equipment to the load that is sized based upon the cooling load rather than the heating load.
With the system using additional equipment such as a backup furnace or strip electric heat to meet any additional heat load notmet by the equipment.
Why use the cooling instead of the heat?
Because if the cooling is sized substantially higher than the load, then the equipment comes on quick, cools the temperature down quick, and then cuts off quick. Two downsides to that.
One, it uses more energy, which may seem counterintuitive, but isn't. Yes it runs longer, but it runs smoother.
If you are old enough to remember merry-go-rounds - the big energy usage is getting the thing started, once it got going - you could keep it in motion with little additional energy (Newtons 1st Law of Motion). That reason is somewhat mitigated in more modern (5 years or less) models. Today's models increasingly use electrically commuted motors (ECMs) that ramp up more slowly and thus, more energy efficiently (for the same reason that drivers that cruise up to speed get better mileage who jam the gas and slam on the brakes).
Two, and this is the one that matters for this conversation, they are designed to use that longer run time to allow the coils to get nice and cool - and begin dehumidifying - removing the moisture in the air by condensation.
That water that comes out of your condensate (white) pipe when the system is running is the result of that action - humidity being changed into moisture thru condensation then removed from your air.
If they don't run long enough to perform that function, then even if the house is cool, it is uncomfortable, because the moisture never got removed.
So why do systems get oversized?
Calculating the load correctly requires both knowledge and time. Loads can change significantly based upon the home's location, materials, orientation, quality of construction.
Code officials have neither the training, time, to work environment to enforce the law. Was recently in training with a group of them, and one related that for years he was the only code official in a large city in this state, and he was responsible for approving 10000 sets of plans per year. Over 40 per day, 200 per week. How closely could he look at them, even if he did have the specialized knowledge?
And if there's ever a business group that wanted to be more regulated, rather than pushing for less, I've not heard of them.
For equipment suppliers, bigger equipment means bigger costs (profit for them), so they have a vested interest in selling the largest or most numerous systems they can.
And for installers, they too benefit from bigger systems, through equipment markups, as well as the additional labor/materials involved in installing more ductwork (I've seen supply ducts installed at rate of more than 1 per every 100sf, some serving as little as 30sf, when the industry standard is ~300sf, all due to the need to bleed off excess airflow due to oversized equipment. Someone pays for that equipment to be installed.
And most importantly for installers, they fear call backs from customers whose needs are not met.
They oversize to prevent callbacks. Warranty callbacks are lost money/time/profit.
They feel forced to use "rules of thumb" rather than running the calculations. Either because customers are unwilling to pay that upfront cost, they l(the installer) ack the knowledge or resources to do the calculation, or they are competing with another contractor who isn't as ethical.
So they play on their customer's ignorance and fear rather than running the numbers properly.
Not realizing that the most popular standard (Manuals JDS from their own organization (Air Conditioner Contractors of America (ACCA)) already has a 25% fudge factor built into the calculation.
The result?
Oversized, overpriced equipment that cools quick, but doesn't remove humidity.
I have a 13 year old, Energy Star certified home from a reputable builder, and a hvac contractor that is listed on the ENERGY Star website. With a 13 year old compressor and air handler. But it was built right and sized right. As was the house it was built to serve.
It removes the moisture through the heatpump. There is no need to pay for additional dehumidification, which I priced recently on a job - and for a permanent, good quality product cost over $1300 for a buyer before installation. Worthwhile if you need it, but it would be better not to need it. Planning can make that difference.
Finally, the idea of not using spot ventilation such as bath fans, range hoods, or dryers during high humidity times. If you want to use dryers during off times, sure - why not if your schedule allows?
If you choose to eat out rather than start the oven when it's hot - again - your choice.
But if you're creating moist conditions in your home - you should remove it, regardless of outside conditions. That moisture coming in from outside is unlikely to be as moist the air in your shower.
That range hood is not only removing moisture, it's also removing particulate matter, carbon monoxide, and a whole lot of other things you'd rather not have your children breathing.
Ventilate it. Especially in today's ever tighter homes.
A more common risk, and one that frequently leads to people buying additional dehumidification, is that system you are already paying for doesn't work as planned.
Range hoods that recirculate rather than duct to the outside.
Bath fans that are running but not exhausting. I see that happen about 1 in 10 times in new construction, and 1 in 3 times in old. You're paying for a fan that makes noise, but doesn't move air. A bad flap, blockage, improper installation. Its common.
Want to know if a fan is working?
You can do a redneck airflow test. Stick a piece of two-ply toilet paper up to your bath fan. If the fan holds it up, you're moving some air. If it can hold something thicker like a dollar bill, then it's probably adequate for a bath.
For range hoods or baths, walk outside and find the exhaust port. It likely has a flap hanging down. If you turn the fan on and the flap lifts, it's moving air. If it doesn't, you've got a problem.
But regardless, whatever the problem - please be methodical and thoughtful in how you resolve it.
There are many excellent resources out there. Few more informative than the Building America Solution Center run by the Department of Energy.
https://basc.pnnl.gov/
Take care.