Can I jumper heat and cool terminals on control board to get same fan speed for both?

Paul Formisano

I have an ICECO (Rheem) 90RJ series gas furnace and I'd like to boost the air flow on heating, which is currently on medium speed. Cooling is on high, but I don't want to just swap motor leads and decrease it to medium, so I was planning to add a jumper between heat and cool to connect both to the high speed fan terminal. Doing so will put 120VAC on the cool fan terminal when heat is called, but I don't see this as a problem because I'm assuming that the cool fan terminal is only connected to a relay contact (which would be open). The same would happen to the heat fan terminal when cooling is called for. Has anyone done this before with good results?

mattmia2

I think that is ok control wise because only one relay will be closed at a time. I would be careful that they didnt make the relay smaller for the heat stage because it didnt need to handle the full load of the motor.

HVACNUT

Why do you want to do that? Is there a problem with temperature rise?
You shouldn't need high velocity for heat. What's going on?

EdTheHeaterMan

mattmia2 said:

I think that is ok control wise because only one relay will be closed at a time. I would be careful that they didnt make the relay smaller for the heat stage because it didnt need to handle the full load of the motor.

This is unlikely. The relay contact may be in the same enclosure on the PC board or more often 2 identical relays because if you were to use this furnace further north, you may use a small AC unit with low speed fan while using high speed on the heat terminal.

EdTheHeaterMan

HVACNUT said:

Why do you want to do that? Is there a problem with temperature rise?
You shouldn't need high velocity for heat. What's going on?

I have had occasion to use the same speed for both heat and cool on a furnace in the past. Good question though. Could be high ceilings and the low speed is no throwing the warm air to the floor so a higher speed may throw the air flow a little further. But you should check the performance to see if there is an acceptable temperature rise across the heat exchanger. Blowing air too fast across the heat exchanger may not pick up as much heat as the lower speed.

Paul Formisano

HVACNUT said:

Why do you want to do that? Is there a problem with temperature rise?
You shouldn't need high velocity for heat. What's going on?

Temp rise is ok, at the furnace.  The reason I want to increase the flow is that there is a basement that is not heating as much as wanted as it is on the same thermostat as the upper floor.  The basement air is also coming from the ceiling.  The furnace is in the attic and the ductwork is flex, so it is a long run and has a significant resistance.  The basement was not used much in the past so it wasn't a problem it was 7°F cooler than the rooms above, but the plan is to renovate it and make it a more comfortable space.  Increasing the heat fan speed will assist air delivery to the basement at low cost.  Temp rise at the furnace will decrease with higher air flow, but overall energy delivery will be similar.  Air registers in the upper floor may need to be damped down to balance.  We may add a freestanding gas stove in the basement if the extra flow is not enough.

Paul Formisano

I just answered my own question by looking at the right side of the schematic. The HCR "Heat/Cool Relay" is in series with the BR "Blower Relay". The HCR is normally closed to the Heat terminal, and normally open to Cool terminal. Therefore the HCR switches to Cool terminal when using the air conditioner, simultaneously disconnecting from the Heat terminal. So, if I jumper the Heat and Cool terminals to the same blower speed wire, it is as if there is no HCR in the circuit to select different speeds and the blower is controlled by the blower relay alone.

Paul Formisano

mattmia2 said:

I think that is ok control wise because only one relay will be closed at a time. I would be careful that they didnt make the relay smaller for the heat stage because it didnt need to handle the full load of the motor.

I figured out it is actually the same relay that just moves the connection from heat to cool. Thanks.

STEVEusaPA

It's probably not going to work the way you think it will. You need to measure air flow, temp rise and TESP. You can't just increase fan speed, close some dampers and think air will be directed to where you want it. You probably need to add a supply duct, and more importantly a return to the basement.

EBEBRATT-Ed

The chances of driving air from the attic to the basement and make the basement comfortable is nill. You have different zones with different exposures and 1 thermostat.

You could add zone dampers but that will solve little and cause other issues.

I would leave it as is and add some supplimental heat to the basement.

Too much air flow on heat may cause flue gas condensation

Jamie Hall

Paul Formisano said:

mattmia2 said:

I think that is ok control wise because only one relay will be closed at a time. I would be careful that they didnt make the relay smaller for the heat stage because it didnt need to handle the full load of the motor.

I figured out it is actually the same relay that just moves the connection from heat to cool. Thanks.

Well... I'd be very curious as to what you were shorting out in the IBM doing that. It would probably be OK if you disconnected the heat connection entirely, but not if you short them.

But -- as has been said, it's not going to solve your problem. Even if the additional flow and decreased temperature drop across the coil doesn't cause an exhaust gas problem -- which it may -- unless you severely restrict the flow to upstairs you'll not get rid of the temperature differential, although it may not get much worse.

Paul Formisano

STEVEusaPA said:

It's probably not going to work the way you think it will. You need to measure air flow, temp rise and TESP. You can't just increase fan speed, close some dampers and think air will be directed to where you want it. You probably need to add a supply duct, and more importantly a return to the basement.

There are three supply ducts and one return duct to the basement.
This modification is almost no cost to try it out, which is why I am doing it. The temp rise will go down but the total energy extracted from the flue gases will not decrease, in fact it should increase slightly with the increased convection coefficient.
The high blower speed is a 20% increase from medium, per the spec, with an expected corresponding 20% total air flow increase, which will increase the available air flow everywhere. To get more air in the the basement we will adjust the upper floor air registers to balance the system after the change.
This is a condensing gas furnace.

pecmsg

Basements have a completely different heat load / loss then the rest of the house because of being surrounded by earth at 55*F. 

In reality you’ll run a heat pump with a dehumidifier while running the A/C upstairs!

Paul Formisano

Jamie Hall said:

Paul Formisano said:

mattmia2 said:

I think that is ok control wise because only one relay will be closed at a time. I would be careful that they didnt make the relay smaller for the heat stage because it didnt need to handle the full load of the motor.

I figured out it is actually the same relay that just moves the connection from heat to cool. Thanks.

Well... I'd be very curious as to what you were shorting out in the IBM doing that. It would probably be OK if you disconnected the heat connection entirely, but not if you short them.

But -- as has been said, it's not going to solve your problem. Even if the additional flow and decreased temperature drop across the coil doesn't cause an exhaust gas problem -- which it may -- unless you severely restrict the flow to upstairs you'll not get rid of the temperature differential, although it may not get much worse.

I think I wasn't clear. I will connect the Heat and Cool terminals of the control board to the high speed blower motor wire. All other blower speed wires will be disconnected. Therefore only high speed will be activated regardless of heat or cool.

The idea is to move more thermal energy into the basement with the existing system and see what happens before investing in another heat source, as this is a no cost test. The upper air registers need to be restricted 20% to keep them the way they were, the balance of the air will be directed to the basement.

This is a condensing gas furnace. More air flow will increase condensation in the secondary heat exchanger and improve overall heat transfer.

STEVEusaPA

Paul Formisano said:

...There are three supply ducts and one return duct to the basement.
This modification is almost no cost to try it out, which is why I am doing it. The temp rise will go down but the total energy extracted from the flue gases will not decrease, in fact it should increase slightly with the increased convection coefficient.
The high blower speed is a 20% increase from medium, per the spec, with an expected corresponding 20% total air flow increase, which will increase the available air flow everywhere. To get more air in the the basement we will adjust the upper floor air registers to balance the system after the change.
This is a condensing gas furnace.

This isn't how air flow or balancing works. That 20% increase is at the same static pressure. As your static pressure goes up from increasing the blower speed, your CFM's go down, then you close off some registers, and your static pressure goes up higher, and your delivered CFM's get even lower.
As an exaggerated example, lets say your system is properly designed to deliver 1400 CFM's @ .50 TESP, 60° temperature rise. Main truck goes in 2 directions, 8 branches, 16 registers. If you close off all the registers except 1, do you think you're getting all 1400 CFM's out of that 1 open register?

But, it seems like you're going to try. Good luck to you.

Paul Formisano

STEVEusaPA said:

Paul Formisano said:

...There are three supply ducts and one return duct to the basement.
This modification is almost no cost to try it out, which is why I am doing it. The temp rise will go down but the total energy extracted from the flue gases will not decrease, in fact it should increase slightly with the increased convection coefficient.
The high blower speed is a 20% increase from medium, per the spec, with an expected corresponding 20% total air flow increase, which will increase the available air flow everywhere. To get more air in the the basement we will adjust the upper floor air registers to balance the system after the change.
This is a condensing gas furnace.

This isn't how air flow or balancing works. That 20% increase is at the same static pressure. As your static pressure goes up from increasing the blower speed, your CFM's go down, then you close off some registers, and your static pressure goes up higher, and your delivered CFM's get even lower.
As an exaggerated example, lets say your system is properly designed to deliver 1400 CFM's @ .50 TESP, 60° temperature rise. Main truck goes in 2 directions, 8 branches, 16 registers. If you close off all the registers except 1, do you think you're getting all 1400 CFM's out of that 1 open register?

But, it seems like you're going to try. Good luck to you.

Increasing the blower speed will shift its performance curve so that it produces more flow at the same static pressure, or more pressure at the same flow, the net PV work having increased. This results in an increase in static pressure across the blower as well as an increase in CFM per the system load, they are not independent variables, each will rise a little and land at a new operating point at the intersection of the blower curve and overall system curve, the latter which has a positive slope for ductwork.

Now, assuming the fan is now running on high, if I start to restrict the three 2nd floor registers by 20% the overall system curve shifts and now the static pressure has increased per unit of flow, but now that I have more CFM this reduction in CFM is not as much as the overall gain in flow that came from the original boost in static pressure. The bottom line is that any increase in blower speed results in more pressure and more flow for a given system.

The three basement registers will not increase by exactly the 20% that the first floor dampers were reduced, but they will indeed be flowing more than they were prior to the blower speed increase.

STEVEusaPA

Well you know more than me. Surprised you came here at all...
Here's a pic of a blower chart. Med-High @ .25" wc is 1030 cfm. But if you move it to High and your static pressure goes up to .50 wc, your cfm is 1050, not 1170 cfm. So your cfm didn't increase 12% because your static when up. It only increased 1%.


Of course you could just do what you want, then check all the ductwork with a flow hood to see what you're actually delivering.

Paul Formisano

I just ran some numbers and using my blower data of 0.4”WC at 1005 CFM on medium I would have a system constant of 3.96x10^-7 “WC/CFM^2.  If I increase from medium to high my system should see a new pressure of about 0.53” WC at 1166 CFM, a 16% increase in flow.

Think about the existence of three or more blower speeds.  If changing blower speeds resulted in only 1% difference in flow, ask yourself why manufacturers would go through the trouble of making different speeds available when they could save money by making one speed.  

The key thing to understand is that static pressure will go up with CFM, but not at the same rate.

Larry Weingarten

Hi, I'm probably stepping in over my head. So, you said it's flex duct that's creating resistance. I
understand that it can create a LOT of resistance unless it's pulled tight, not strapped too hard and not bent too tight. Are there any modifications you can make to the flex duct to improve airflow? Or, should those changes come later?

Yours, Larry

Paul Formisano

I’m doing this job as a favor for a friend.  So I’m trying to keep it simple.  Half the flex duct is behind walls.  If this mod doesn’t work I’m suggesting a direct vent gas stove.

Paul Formisano

Here is my easy solution to make both heat and cool run on high speed:
1. Connect high speed blower motor wire to EAC terminal.  

2. Remove medium speed blower motor wire from heat terminal and insulate and secure it.

No jumper required!

ratio

Just use a jumper! I've seen this very thing in more than one manual when high fan speed is needed for both heat & cool. The schematic indicates that there's no way for power to back-feed through the controller, so no harm to jump them together.

While it's unlikely to completely fix the problem, I can't see it hurting anything (assuming the ΔT stays within range) and it may make it good enough, so take the 2 minutes & try it out.

wmgeorge

I just put my Trane furnace ECM motor on constant fan. My system is balanced with balance dampers that you get when you install proper hard pipe instead of flex. Basement is just within a degree or two of the upstairs. Same either on heating or AC.

PRR

FWIW: I have both heat and cool jumped to LOW. I have a very small furnace, which seems to have the same blower as the next size up. Even on low I only have 105 degrees (35 deg rise). More blow would be cool and windy. The blower is sized for 3.5 tons of cooling but in Maine a 1.5t cooler makes much more sense (run it long and easy to work the humidity down).

realliveplumber

Can I jumper heat and cool terminals on control board to get same fan speed for both?

Yes.