HVAC question

[LDB415]

We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?

[K.Mooneyham]

Maybe it's an incorrect time delay relay installed?

[03Lightningrocks]

We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?

All these new units run off a circuit board. Call the installing company. It is likely a simple pin change at the circuit board. Maybe micro swithes, depending on brand, but it is adjustable at the air handler.

[bbhack]

That air is not coming from the attic but from the return. The return air should be 70F or whatever the air temperature was when the thermostat said stop. If it's getting that cold between the return intake and the supply registers, it sounds like you really need the ducts to get reinsulated. I guess it's possible that they were not insulated initially, which would make your bills crazy.

[RPBrown]

We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?

All these new units run off a circuit board. Call the installing company. It is likely a simple pin change at the circuit board. Maybe micro swithes, depending on brand, but it is adjustable at the air handler.

You do want some time delay there but it is usually adjustable at the circuit board.

[Tex1961]

Even though the compressor has stopped, the coils in the attic are still cold, so the system will run the fan for an extra few minutes to take advantage of the free cooling. This is part of the newer energy saving systems. One of the cheapest upgrades to an old air conditioning system is to install a smart thermostat. The new thermostats take advantage of that very type of set up. It will continue running your fan, even though the compressor has stopped to continuously give you cold air for another 5 minutes or so.

[troglodyte]

That air is not coming from the attic but from the return. The return air should be 70F or whatever the air temperature was when the thermostat said stop. If it's getting that cold between the return intake and the supply registers, it sounds like you really need the ducts to get reinsulated. I guess it's possible that they were not insulated initially, which would make your bills crazy.

or, if the return duct or unit is in the attic, the return duct work is breached in the attic.

We had something similar happen at work with an attic unit. The AC would run normally in the cool of the morning then by the afternoon wouldn't cool enough to turn off. After crawling up in the attic I found panel had fallen off. Reinstalled and all was good.

[03Lightningrocks]

That air is not coming from the attic but from the return. The return air should be 70F or whatever the air temperature was when the thermostat said stop. If it's getting that cold between the return intake and the supply registers, it sounds like you really need the ducts to get reinsulated. I guess it's possible that they were not insulated initially, which would make your bills crazy.

or, if the return duct or unit is in the attic, the return duct work is breached in the attic.

We had something similar happen at work with an attic unit. The AC would run normally in the cool of the morning then by the afternoon wouldn't cool enough to turn off. After crawling up in the attic I found panel had fallen off. Reinstalled and all was good.

Nope. He simply needs the board adjusted on the new unit he just put in. When room temp air is being sucked in and blown out ducts, the body senses it as cool air. The fan is simply running for too long after the strips and/or heat pump outside shuts off.

[LDB415]

It's cooled enough outside it's running again. I timed it and from the thermostat click to signal end the cycle until the fan stops is 2:30. Seems like a very long time. I'll have to ask the a/c people that did the job.

[03Lightningrocks]

It's cooled enough outside it's running again. I timed it and from the thermostat click to signal end the cycle until the fan stops is 2:30. Seems like a very long time. I'll have to ask the a/c people that did the job.

In the heating cycle, you don't want the fan running much at all after strips/heat pump shuts off. No more than 10-15 seconds. I have also had our installers not connect the high voltage properly when changing an electric heat air handler. Depending on the heat kit they put in the air handler, it may run off one 220V circuit or it may require two 220V circuits. It is a common screw up on change outs with air handlers. So the installers, in all their infinite wisdom, will simply hook up one set of strips and not hook up the others. This can leave the fan thinking the unconnected set of strips is still on. Thus, the reason the fan is still running so long. My bet is that a set of strips are not connected or they put a smaller heat kit in than what you had before. I hope they are honest with you if this happened because you are at their mercy unless you get some other company to come look at the install.

I have no idea what your set up is but to further clarify. All air handlers are supplied with no heat strips. We order the "heat strip kit" separately from the air handler, depending on what the high voltage electrical to your air handler requires. But just take a 4 ton air handler for example. Let's say your air handler has a single 100 amp 220V circuit feeding it. Typically, it would use a 20 KW kit. 4 heat strips. Each strip pulls 20 amps. I can order that kit as a single circuit kit for your 80 amps worth of heat strips. I can also get the same 4 strips in a kit with two separate circuits, 40 each circuit or 60 on one circuit and 20 on the other circuit or even 2 strips at 40 amps each. The reason all those add up to 80 amps is the fan motor will come on with one of those circuits and accounts for the missing 20 amps in my examples. The motor doesn't pull 20 amps but you need the extra 20 amps as wiggle room in total amps pulled. The kit I order depends on whether you have a single circuit of 100 amps or two circuits with that 100 amps divided between the two. This same situation applies to all air handler sizes. Sometimes a company will replace your old 4 strip unit with a 3 strip kit for your new air handler. It can be done a myriad of ways depending on the way they decided to set it up. But the single restricting factor is the high voltage circuit supplied to your existing air handler at the time. Is it one large circuit or two smaller circuits? Hope I did not confuse the issue. I was just trying to explain how the kit could be different than what you had and this will affect the units operation. The blower fan is either on the one single circuit kit or if it is a two circuit kit it is on one of the two. In any case, the fan won't shut off until it knows the last strip is off. It sounds like your fan thinks one or more strips are still on for that 2.5 minutes.

This is not typically something we discuss with the home owner because it is more of an application issue than an optional issue. It is our job to match equipment to electrical supplied. If we get it wrong, we get situations like yours and have to replace the kit with one that matches your existing wiring without overloading the wire\breaker size.

If you can tell me what size unit you have and what size breaker in your panel is feeding it, one breaker or two separate breakers, I can tell you specifically what has happened that is creating the issue. I have to go to work at 10:30 this morning and won't be back until around midnight. So I will reply later if you need more clarification.