As much as I would like to say I am an expert, I am not when it comes to the actual performance and settings of a residential gas furnace and air conditioning unit.  So I am hoping some would be willing to share their incite or at least some resources I could go to.

So my question is about the fan speed settings for the heating and cooling system.  Two years ago I had a new Tempstar 95AFUE two stage variable speed furnace installed.  It is my understanding that even though it is a variable speed fan that you want to adjust fan speed differently for the heating and cooling system.  I ask because the service tech was at the house today on an A/C tune up & when my wife inquired about the fan speed, she was told it was on the factory settings.

So is this true, with variable speed motors you don't have to worry about the fan speed, or should there have been some kind of adjustments at time of the original install?  And for future service calls by potentially another company, what kind of testing should I ask for to be done to see if the CFM moving through the system are appropriate for the season.

Thanks in advance!


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I was hoping someone else would pop in, but as I recall some models (if not all) have different factory defaults / wires / jumpers used for when the blower / fan turns on & how fast they run for heat & cool. I know the advice on changing fan speeds for older oversized units works good in most cases, but I am not sure I would try on a newer multi speed unit. But hey hopefully an HVAC guru pops up to either confirm, deny or hem & haw


BTW, can you get access to the installers manual - that should tell you if all your jumpers are in the right place

Ha, that would mean the company doing the replacement would do a manual J.  I don't know of any local HVAC contractors that do a manual J for replacements.  However I did have one done by a friend, so I will have to pull it out and look.

Yeah that is a big issue, especially with the improvements made to not only the equipment but what people have done to their houses. Per the HAC board down here everyone has supposed to have been doing them for years (even for replacements), but most give you that dear in a headlights look

ask a test and ballance HVAC person.  look up the factory CFM, or can be called fan performance data, or called SP chart.  It comes with the new funace its part of the paper work that you use for knee pad.  I use a water collume gage or can use a blower door gage but will have to change to inch water from PA.  Most furnaces are very over sized and if 2 stage set up to run on 1st stage.   Most 2 stage funaces just cut down gas presser and then CO rates are high and total eff are just 60%, thats why you must test with a flue gas meter.   Most factory setting are under what you want.   Its all about air flow and getting the air out of funace - most 90% run high limit- most 80% do not run long so the flue does not heat up and does not get up to temp so it rains in the flue all the time.   size the furnace! to the house and make shure the duct work is large for the air flow  and keep the SP under listed on side air handler.   If the duct are to small do you size the funace to duct or size to building?

The fan speed needs to be set to provide the air flow to each room calculated by the Manual J.  It was rare to have a contractor do a manual J or adjust the fan speed until recently.  The City of Austin (Texas) required thru the energy code local amendments each and every install to measure the air flow to each room and that air flow to be within 20% of the Manual J calculation. The local amendments also set limits for the static pressure at the air handler so that the duct sizing had to calculated right and the installation didn't cripple the air flows.

Since the same variable speed air handler can be used on several different capacity systems, I wouldn't expect the factory settings to be right, most of the time.  However, they are bound to be right some times.  Maybe you were the lucky one.  But if the didn't measure and adjust the air flows, I would assume the CFM setting was just left alone and never set properly.

You are correct in stating that the blower fan is set to different speeds for heating and cooling.  For cooling the evaporator coil is designed to operate at peak efficiency with between 350 and 450 CFM per ton of cooing capacity.  Most equipment comes with the fan speed set in cooling mode at 400 CFM/ton. 

The greater the airflow over the coil, the less humidity you remove, so if you live in a dry climate, you can set the fan speed up to as much as 450/ton to get more sensible cooling and less latent.  If you live in a humid summer climate, you may benefit from slowing the fan down to as little as 350/ton to improve humidity control in your home.  Do not go much below 350/ton or you can cause the coil to ice because you aren't moving enough warm air over it to keep it from freezing!  For example, a dirty or highly restrictive air filter can reduce airflow to the point that a coil will ice up. 

On the heating side the needed airflow is also related to the BTU's being produced but this time we are looking at the firebox.  We need sufficient airflow to remove the heat being produced to keep the firebox below the safety cutoff temperature which is usually 130-135F.  If you set the fan speed too low, the firebox will get too hot because there is not enough airflow to carry the heat away and your unit will shut down on a high heat overload safety cutoff.  This is why a dirty filter can sometimes cause your furnace to short cycle and shutoff before reaching the thermostat set point.  If you set the airflow too high the air coming out of your registers will be too cool and your system efficiency and your comfort will suffer. 

We usually set the fan speed to keep the temperature rise of the air passing through the system between roughly 40F-65F depending on the age and efficiency of your system.  So, return air temperature may be 70F which means that the supply air measured in the furnace would be 110F to 135F.  The correct range for the temperature rise is usually stated on the nameplate of the furnace.  Most 90+ furnaces are rated at around 50F temperature rise for example.  By the way, the HVAC folks call this difference between input and exiting air temperatures, the Delta T of the cooling or heating mode. 

This is the formula that determines the relationship between Delta T and airflow to BTU's produced:

[(Delta T) x 1.08] x CFM = BTU output

I hope this helps.

Ideally you would set the blower up as a "2 speed" with a humidity controlled blower speed. Run 350CFM/ton during humid weather and 450CFM/ton during dry weather. Get the coil down to 35-40 degrees in humid weather for maximum humidity removal, and 45-50 degrees for maximum EER during dry weather. A simple 2 stage thermostat can be wired to do this, run low speed most of the time and boost blower speed when cooling demand is high. System must be sized correctly for this strategy to work. If the system is oversized high blower will never activate.

For heat I recommend running delta T near the bottom of the range. Keeping a high volume of air moving helps with stratification issues, especially if vents are located in ceiling. It also allows more wiggle room as the filter gets dirty over time.

I have tryed that but with the new R-410A I now get higher temps than ever.  To get very high RH humid removal I all so put smaller coils in  so the coil gets very coil, but the EER goes low at same time.    I have been adding a dehumidfier  sized to building and get lower energy use over all.  Like a Honey well or desert air then we get around 50% leaval if the building is sealed  

When used with a TXV (most 410a systems use them) you can get the coil as cold as you want simply by reducing airflow. No need to use a smaller coil

I'm always amused by HVAC contractors who blame 410a for whatever woes are at hand.

The cost delta for an ECM blower equipped air handler is now about $300, and for another $200 deploy a thermostat that senses and signals the air handler that humidity is high. Most decent AHUs now have a dehu feature that drops airflow by 15% or so in response to the dehu signal.

Don't overdo the reduced airflow thing - cabinet and nearby ductwork may sweat and ultimately evaporator will ice, wreaking all manner of mayhem.

Excessive RH is nearly always a building science issue (excess building or ductwork air leakage, poor point source ventilation) rather than an HVAC issue unless the system is oversized

Despite a swampy outdoor dewpoint of 70+, my home is at 77*F / 45% RH. That's with no central dehu.

It is my contention that separate central dehus are an expensive (both to buy and to run) band-aid masking some other screwup. I yanked two out of a home recently while in the course of right-sizing the HVAC and fixing air leakage.

Use your IR with new 16 seer coils and see hot spot. All the coil is not full.

Sounds like the unit may not be charged correctly if it's not full?


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