Any HVAC experts?
 

So I have a question about my air exchanger. When we initially built the house, they told us they installed an air exchanger as these new houses are somewhat air tight. They said to be careful how much I use it as it could increase my electric bill if I keep blowing all my hot air outside and my furnace has to run more often to keep the house at a certain temperature. Anyways, years later we had a A/C unit installed and connected to the furnace. We still hadn't really used the air exchanger up to this point. I recently turned on the air exchanger while my furnace thermostat was set to off just to circulate the air in my house. I noticed that the fan on my A/C unit would run when the air exchanger was on. It was not cooling the air but it was running continuously while the air exchanger was on. Is this normal? I don't know much about these things.

Depends on a few things. First, if it is JUST an air exchanger then yes, you have to reheat the incoming air (or cool it). If it is an HRV then those units recover much of the heat (cold) so that only a portion has to be reheated or cooled.
Most exchangers now are plumbed to the furnace to give distribution throughout the house so the fan will come on for that distribution.
My HRV is installed so that it can go throughout the house or only in the basement where all my tanks and cultures are, but I have to physically change the HRV output to just the basement or to connect it to the cold air return of the furnace.

Yes if you are running the air exchanger the furnace fan will come to replace the air in the house that the exchanger is removing from the house. Air going out has to be replaced with air from your furnace it will have a fresh air outside connection that pulls in the fresh air to accomplish this. This is a typical hook up in the newer homes. :biggrin:

Depends on how it's setup, quite often if you have forced air heating the air exchanger ties into the cold air return and heating ducts of the furnace. If this is the case the furnace fan needs needs to run with the exchanger fan. I'm not sure about the AC fan, central air units have a fan on the unit placed outside and the coil is just placed in the furnace and uses the furnace fan. So which fan is on exactly?

But should my air conitioning unit fan come on as well. I am okay with the furnace fan being on. Would that mean I cannot run my air exchanger in winter as my air conditioning unit would be covered and I am assuming you shouldn't have the fan running under the cover.

I don't think the fan on the outdoor A/C condensing unit should be running...

If it's running, it probably means the compressor is running as well, which means your A/C is effectively ON.

That is what I thought, but when I went to the registers it was blowing warm air not cold air and the thermostat was set to off. As soon as I turned off the air exchanger the fan stopped turning on the a/c unit. Since then I have had the a/c on and it blows cold air just fine. I am at a loss. Maybe I will go turn the thermostat to heat and see if the air exchanger still turns on the a/c unit.

That seems really weird to me - can you call the place that did the installation?

Unfortunately, they were not that great to deal with in the first place and it was so many years ago they wouldn't do it for free now. I just talked to an electrician friend and he suggested that if everything else works fine, he would just turn off the breaker for the air conditioner in the winter and not worry about it although it did sound to him as if they wired something wrong.

I had something like this in my last house. When we turned on the circulating fan the AC compressor would start. I was told it had to do with the relays for the fans and they would need to be changed. I never did, I just tuned the breaker of for the AC off.

The fan on the ac unit is turning. But is it turning because the motor is on, or is the fan turning because the air is being pulled through the duct to replace the air that the air exchanger is pushing to outside?

I would say most definitely the motor is on as you can hear it going from quite a distance. I will test that theory when I turn off the breaker tonight.

I'm not real clear on this yet. Is the AC fan turning the one in the outdoor unit?
I've not seen an AC motor inside.
If it is the outdoor unit, is it just the fan coming on or is it the fan and the compressor?

The outdoor a/c unit's fan is turning on when I turn on my air exchanger (With the thermostat set to off) but the compressor is not turning on so the air circulating in my house is room temerature.

If you ac is covered in the winter you should be able to just flip The breaker to the ac (should be independant) and the fan will not run. I know this is kind of putting a bandaid on a bullet wound but it should work as a temporary fix if all else fails.
But believe me when I say I am no expert lol

there is no conection between the outside AC fan motor and an air duct, the outside AC fan is just to cool the condensing unit.

I would get a real HRV guy and bite the bullet for a few bucks and get it fixed properly. this should not be comming on and a AC unit fan is not a cheep one to run. Also you are using the aire exchange in the summer that fan will be running twice as much and I imagin replacing that fan will be a lot more expensive than getting the aire exchange hooked up properly.

Steve

I didn't know we were talking about the outside fan motor. I thought the fan was on the inside of the house, blowing through the evaproator

Well no luck. No matter if the thermostat is set to off, hot or cold as soon as I turn on the air exchanger, the outside a/c fan comes on. Now to decide whether I bite the bullet and pay an arm and a leg to fix it now (As you know they will overcharge me somehow) or just turn off the A/C in winter.

You should be able to trace the wiring from the AC and from the air exchanger to where they interconnect and fix the problem there.

call a reputable company then because not all heating and AC companies are crooks

my companies AC service call is $135 and $95 an hour after

if you dont wanna pay a company your going to have to learn to do it yourself

You won't fix it yourself unless you can understand what everything does. Before you had the AC installed the HRV was interconnected to your furnace/airhandler fan. The a/c guys tied into this same system. When your tstat calls for a/c, an inter connection in the tstat brings the air handler fan on also. What is happening to you is it is back feeding so when you turn on the HRV, the a/c turns on as well. This is very bad. To verify if i'm right, turn on your hrv and if the a/c is on, flip the fan switch on the tstat to "on". The a/c should turn off. At best it will cost a boat load of power. At worst it will ruin the compressor. Let the a/c guys know and they will likely fix it up for you.

It is impossible for the condencer fan motor to run without the compressor. It runs on the same contactor. Though there is an internal overload that can shut the compressor down for heat/over amp issues.

Well you must be a HVAC installer as you were right. If the fan is going on the a/c unit after I turn on the Air exchanger and I turn the fan from auto to on, the fan does shut off. I am assuming this is a simple fix and it shouldn't cost me very much?

I wouldnt be quick to call it simple and cheap is a relative term. It would take me a relay and service call, shy of 200 would be a guess. A reputable company in Calgary would likely be just over 200. A guy that scratches his head lots and lets the smoke out of electronic boards could cost a thousand.

Do you have any electrician friends? Any experienced electrician should be able to help you out.
However, you now know the fan doesn't go on with the exchanger when the selection is made to "on" instead of "auto" so as long as everything still works correctly when the furnace or AC is working along with the exchanger, just leave the selection where it is.

A Psc fan running steady will eat $200 in 3 months with today's power costs.
If you have an ecm motor ( commonly called "DC" or "variable speed" it might take 6 months. Fixing it will be far cheaper.

This is an Hvac issue and we deal with it all the time. The a/c guys should have seen wires and fixed it when they installed it.

I don't use the air exchanger very often which is why I didnt notice this happening until years after the ac was installed. I just thought it might help my heat differential from top floor to bottom floor in the summer hence how I found out it was doing it.

I may have a look myself and see what is up in there.

Yep, this is totally accurate.

It sounds like the backfeeding thing as mentioned above. The original intent was to turn the exchanged on automatically when the A/C was run. But the opposite is also true.

You could have already done some serious damage to your A/C condensing unit if you've had the cover on. And besides, it's costing the same amount of money to run it right now as if it was the middle of summer, since the A/C and exchanger are both running at the same time.

I would call in a qualified HVAC person ASAP before any further damage is done.

EDIT: You may need to/want to invest in a good thermostat (I.E. Honeywell Vision Pro) that has a separate programmable output for the exchanger.

I don't think any damage is done as I have never really used the exchanger. It's on a manual switch which is almost always left off. We actually turned it off right by the furnace for most of it as we had little fingers going around turning on random switches in the house and we didn't want them to turn on the air exchanger. I am just finally realizing that maybe I should use it more and have come accross this problem.

Sounds like a wiring issue. Look inside the furnace, there should be 2 wires from the hrv to furnace, 2 from a/c to furnace and 3,4or5 from T stat to furnace. From the hrv to furnace one wire should go from the r terminal and one wire should go to the g terminal. If this wire went to the y terminal by accidient, this is the problem you would have. The wire to the Ac should go from the c terminal and one from the y terminal. If this wire was accidiently ran to the g terminal, same problem would happen. As well if this was the case, Ac would run if you switched the furnace to fan on position.

It is as you described

I heard it also may be related to an old tstat. However I have no idea how to tell how old is old.

So hrv is going to r and g
Ac is going to y and c
Make sure there is not a jumper wire between y and g
If that checks out look at the back of the t stat make sure there is no jumper on the t stat from y and g. Check the wiring on the back of the t stat. Note the colors going down to furnace and make sure they match up. Ie w-w
R-r and rc if ther is one y-y and g-g

Make sure there is no jumper from the hrv to the furnace. There shouldn't be but some people's children are idiots

Hrv is going to r and g
AC is going to c and y
Tstat is going to w r y and g and they match the same color as at the tstat
I don't see any jumpers at the furnace or tstat except the r on the tstat to an empty r

That is really odd. Everything sounds correct. Somehow the circuit is connected from the r to y via your hrv. Try disconnecting the t stat wires and swing if the problem still occurs. If so it is in the furnace wiring if not the problem is in the t stat wiring or the t stat itself. The wire from the hrv to the furnace is to make a contact from r to g to force the furnace fan to run when the hrv is on. Write down the wire colors to terminals so you sent chasing your tail when you re connect everything. So you know what your dealing with, the r is the positive feed off the furnace, when it is connected( via tstat) to w furnace calls for heat, g furnace calls for fan y furnace calls for fan and Ac contractor is energized

I have read a lot about an air exchanger back feeding (incl those who help me on this board). It looks like I just need to get an isolation relay to stop the back feed. Unless you think that is incorrect?

Most heat/cool tstat have a build in jumper. so that when the stat is turning on the ac, the furnace fan turn on.

On a heat/cool stat.
RH is for heating, the high limit control the fan in the furnace.
RC is for cooling, the stat turn on the furnace fan and the ac.

You need to add a relay to prevent the ac turning on when you run the HRV.

If your HRV ducts are connected to return air of the furnace then the furnace fan should run when the HRV is on.

exactly, when fan switch is in auto, y and g are jumped, this back feeds to turn on the AC. Common problem all Hvac guys run into.

Why don't you ask your electrican friend to install a relay for you?

I alway turn off the breaker to the ac in the winter.

Yes this would make sence except he said this still happens when the t stat is switched off. Also I don't think most t stats jumper the y and g when in auto. I'm pretty sure the furnace board takes care of this. Or else when the fan switch was in off, and the t stat was in cool, the fan would not run. Sometimes on older furnaces without a y point people will jumper these points but op said thier was no jumper. The r and the rh need to be jumpered out unless you have a system with separate heating and cooling transformers. Which you don't. If the unit does not do this with the t stat wire disconnected at the bored then double check the t stat wiring and replace the t stat. I do not believe a relay is the way to go. I Hooked 2 of these up last week alone. The only way you should need a relay is if it is an older furnace with no y point.