Author Topic: Efficient Use of a Modern HVAC  (Read 498 times)

Nicholas Carter

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Efficient Use of a Modern HVAC
« on: June 25, 2018, 07:53:18 AM »
So I'm very familiar with the advice about frugal home environment that I should set my AC above 75, my furnace below 65, and turn the thermostat off all together when I'm not in the home.
However, I was recently talking to a friend whose field is the installation and maintenance of HVAC units. They said that, with HVAC units that are less than 10 years old, this is actually terrible advice!
According to them, newer HVACs are built to be optimized around the normal usage rate of average Americans. That is to say, the units start up and shut down so inefficiently, and idle so wastefully, that it actually costs more money to set the thermostat outside the 69-72 range. They also added that newer machines are not really designed to be turned off for long periods of time, and leaving it on and idling is better for the longevity of the unit than turning it off every day while I am gone.
Our conversation turned to other things, but I thought this forum might have more insight: Is it really more costly to set the AC to 75 than to 70? If so, why? Is this something I can change with limited know how? Is the diminishment of HVAC unit lifetime from letting it set off (when temperatures are above freezing) meaningful, in terms of how much it saves amortized?

bacchi

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Re: Efficient Use of a Modern HVAC
« Reply #1 on: June 25, 2018, 08:58:43 AM »
Somehow, according to this advice, that extra 5 degrees it has to work to get the inside temp to 70 uses less energy than cooling it to just 75? Wuuut? That violates the laws of thermodynamics.


Eta: Any HVAC that runs for 5 minutes 3 times is more inefficient than an HVAC that runs for 15 minutes straight. There might be a greater range at the 70 point -- it allows for +-4 before it starts and then brings it back to 70 -- where, when set to 75, the range is tighter (+-1) and so the HVAC runs more often.

This should be in the manual or you could experiment. Set it to 70 and watch the thermostat. Does it go to 74 before starting again? Do the same thing with it set to 75+.

My HVAC allows for changing the range on the control panel. It's currently +-3 (for all temps, not just 69-72).

I can't say anything about turning it off. I just turn the thermostat up or down when I leave for the day. I set it to 90, for example, and adjust it when I get home.
« Last Edit: June 25, 2018, 09:12:40 AM by bacchi »

MMM98

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Re: Efficient Use of a Modern HVAC
« Reply #2 on: June 25, 2018, 09:36:02 AM »
I have a five year old SEER 15 HVAC with heat pump and heat strips as a back up heat source.  I watch the electric consumption on my Sense energy monitor.

I can assure you that the advice you were given is patently false.  With 10,000 samplings of my energy consumption every second (6.4 billion per day), I think that I have a valid point of reference.

The simple truth is the greater the delta between the home's temperature and the ambient outside temp the greater the speed that the home loses cooling or heating.  This explains it well: https://www.energyvanguard.com/blog/50152/If-You-Think-Thermostat-Setbacks-Don-t-Save-Energy-You-re-Wrong.  While he does not cover the life expectancy of the HVAC unit, I know on an August day my southern home runs over 80% of the time during the daylight hours.  I imagine setting back the temp helps the unit by allowing it some down time, but I am open to a reason why that is not the case.

Depending upon the temps that we are talking and the part of the world that you are in each additional degree costs you 8-10% more in HVAC  costs.  Your HVAC costs could be 40% of your electric bill, or in my case it is 64.3% of my total bill this month (so far).

The energy monitor has shown me that much of the commonly repeated bits of advice found on the internet is simply false.  It is either dated or based upon some different climate with a different set of assumptions.  The advice from local experts has been the worst, at least for me.

A monitor runs 300.00 and if you use the data you can easily save that much through cost avoidance or conservation.  Here is the one I use, there are other brands available as well: https://www.amazon.com/sense-Energy-Monitor-Electricity-Usage/dp/B075K6PHJ9

« Last Edit: June 25, 2018, 09:56:54 AM by MMM98 »

Jon Bon

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Re: Efficient Use of a Modern HVAC
« Reply #3 on: June 25, 2018, 12:07:46 PM »
Ok I have heard of only one specific example of this being true. * My guess is this kind and honest HVAC person heard this and misunderstood what was happening.

The only way this is true is with heat pumps.

So a heat pump uses magic, and pull heat out of the cold air. However at some point if it gets cold enough the heat pump goes into emergency mode and kicks on basically a giant ass space heater and is expensive as hell to run. So when the heat pump can no longer heat up the air enough the emergency space heater will kick on. Frozen pipes are bad and all, so it makes sense they are designed this way.

I have heard about a thermostat being stupid. In that you set the temp for 62 degrees and night and 75 degrees during the day. So when that change happens the thermostat notices an alarmingly large 13 degree gap between the house and the set temp. So it kicks on the emergency heat to make up the difference in a hurry.

But yeah, all modern appliances are designed with their human users in mind. I am not sure what he is talking about!




*Even this might be outdated info


MMM98

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Re: Efficient Use of a Modern HVAC
« Reply #4 on: June 25, 2018, 12:16:07 PM »
Ok I have heard of only one specific example of this being true. * My guess is this kind and honest HVAC person heard this and misunderstood what was happening.

The only way this is true is with heat pumps.

So a heat pump uses magic, and pull heat out of the cold air. However at some point if it gets cold enough the heat pump goes into emergency mode and kicks on basically a giant ass space heater and is expensive as hell to run. So when the heat pump can no longer heat up the air enough the emergency space heater will kick on. Frozen pipes are bad and all, so it makes sense they are designed this way.

I have heard about a thermostat being stupid. In that you set the temp for 62 degrees and night and 75 degrees during the day. So when that change happens the thermostat notices an alarmingly large 13 degree gap between the house and the set temp. So it kicks on the emergency heat to make up the difference in a hurry.

But yeah, all modern appliances are designed with their human users in mind. I am not sure what he is talking about!




*Even this might be outdated info



It's true that most thermostats have a default setting for large changes in the heat mode: if the temp is raised more than three degrees the emergency heat (heat strips, resistance heating) kicks in.  Many thermostats also allow you to shut this feature off or set a different point at which the resistance heating kicks in.   Even if your thermostat does not have that feature, set the temp up in two degree increments, twenty or thirty minutes apart.   In those cases the heat pump/ efficiency argument vanishes.
« Last Edit: June 25, 2018, 12:20:51 PM by MMM98 »

Systems101

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Re: Efficient Use of a Modern HVAC
« Reply #5 on: June 25, 2018, 03:13:32 PM »
Let's start with: bacchi is right on the thermodynamics and MMM98 has the data.

But let's get into a bit more of the electronics and physics.

First, there are 2 sets of electronics here: The thermostat and what I'll call the A/C controller.  You interact with the thermostat and it sends 12V signals to the controller to let the A/C know what it needs to do.  Turning the thermostat off (which is changing a control signal) is very different from turning the A/C off.  The latter can be bad for the A/C unit, since it wants to have a less stressful ramp downward after it's running.  It's kind of like an automobile fan that keeps running after you turn off the car (under really hot conditions).  The AC wants a similar thing and has a very delicate balance for how it starts up and shuts down the subsystems to avoid damage - so sending signals via the thermostat is critical.  Literally turning off the A/C would be like taking a hot engine and not only pulling the key but also pulling the battery... so the fan couldn't run.  Over time, that won't end well.

That's significant, because many years ago, there was a window of time/brands where the thermostat was smart but the AC controller was dumb so the mad rush to reduce the home 10 degrees or whatever was very stressful on an A/C unit (like trying to sprint a marathon). This could cause various problems, including having an A/C unit completely freeze (as in ice) up.  The controllers are now smart enough to run for some time and then cycle down and back on, because engineers learn things when products fail and they design them not to fail the same way next time. :)   It's likely that bad experiences from this time period are now burned in as urban legends.

There is also a difference between turning it up and turning it off.  The thermodynamics states "off" is more efficient, but remember an A/C is not only cooling - it is also dehumidifying.  So going up 5 degrees (or 10) may allow the unit to cycle 1-2 times while you are gone, helping to control humidity.  This is especially important in certain climates.  Under the right (arguably wrong) conditions, having a very humid house can encourage mold growth (70+% humidity, better if still air, warm and dark - like a house where you closed the doors and pulled the shades).  Having cold air blowing into a warm and humid room can cause condensation in places where the A/C unit is not designed to dry out/drain the moisture.  If it runs consistently, it will evaporate.  But if it's a more rapid cycle, i.e. out to work, home for a bit, back out to eat/shop, this can encourage mold growth and provide a spore factory.  Then leaving a sealed house as above and it going up to ambient temperature/humidity in an area that is regularly 70+% humidity can result in issues across the entire house.  So the other point here is to always run through a condensation/evaporation cycle (For example, running a car for only 3 minutes in the winter is asking for rust in your exhaust system because the system never gets to evaporate the moisture.... in this case, if the A/C takes an hour at night to bring the house back to temperature, having it on for 30 minutes and then going back out and turning it off is a recipe for moisture in the system.  Once you turn it on, leave it on long enough to bring the house to temperature)

So in summary:
- Yes, smart thermostats are great, especially with newer HVAC units - let the temp go up during the day and save some money
- Never circuit breaker off the A/C unit unless you first signaled via the thermostat that was your intent (and preferably did so 15+ minutes before you throw the breaker)
- Consider carefully whether you go up in temp or completely off (balance here will depend on your local climate, but in higher humidity climates, I'd strongly advise up not off)
- Be careful about rapid cycling, especially if you are in for a short period and then back out. 

Nicholas Carter

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Re: Efficient Use of a Modern HVAC
« Reply #6 on: June 26, 2018, 08:33:07 AM »
That's significant, because many years ago, there was a window of time/brands where the thermostat was smart but the AC controller was dumb so the mad rush to reduce the home 10 degrees or whatever was very stressful on an A/C unit (like trying to sprint a marathon). This could cause various problems, including having an A/C unit completely freeze (as in ice) up.  The controllers are now smart enough to run for some time and then cycle down and back on, because engineers learn things when products fail and they design them not to fail the same way next time. :)   It's likely that bad experiences from this time period are now burned in as urban legends.
Thanks everyone. It sounds like this is probably what happened. I live in a LCOL area, so more people are paying to have outdated machines repaired than replaced. My friend's experience is probably with baby-sitting these dysfunctional models.

Fishindude

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Re: Efficient Use of a Modern HVAC
« Reply #7 on: June 26, 2018, 08:55:56 AM »
Set it at a temperature range that keeps you comfortable.   Modern HVAC systems are one of life's simple pleasures.
If you are unhappy with the utility bills related to it, your efforts will go a whole lot further working on things like adding some insulation, better weather stripping, etc.