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# Variable Speed Blower Question

## Variable Speed Blower Question

#1
12-04-07, 06:05 PM
Member
Join Date: Dec 2007
Location: Indiana
Posts: 184
Variable Speed Blower Question

Rheem Modulating Furnace

I recently had a Rheem furnace installed which included a variable speed blower. Now, as I understand it, the blower is what moves the air through the duct work. However, the tech who installed the furnace stated on his work order that he set my blower speed A/C to 1400 CFM and Fan only to 800 CFM.

1. What does A/C mean? (My guess is heat and air conditioner air movement)
2. I’m confused with blower and fan. Are there a blower and a fan in the furnace? Or does the blower operate in different modes? If so, what are the modes?

Thanks,
Jerry

#2
12-04-07, 07:43 PM
Member
Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 16,984
Originally Posted by Jerry8A

1. What does A/C mean? (My guess is heat and air conditioner air movement)
A/C means Air Conditioner,

2. I’m confused with blower and fan. Are there a blower and a fan in the furnace? Or does the blower operate in different modes? If so, what are the modes?
You have a variable speed system, the blower is going to move different CFM of air for each mode.

-Fan = When you have the t-stat fan switch to ON, when there is no call for cool or heat, the blower is going to run at a slower speed of 800 CFM (Cubic Feet per Min.) of air.

-A/C = When system calls for cool, the blower will ramp up to 1400 CFM to move the cool air in the ductwork.

-Heat = When system calls for heat, the blower will ramp up and down along with the heat demand that t-stat calls for.

Did you get a Modulating T-stat?

#3
12-05-07, 12:14 PM
Member
Join Date: Dec 2007
Location: Indiana
Posts: 184
[QUOTE=Jay11J;1271174]

Did you get a Modulating T-stat?

Boy, you have asked a question that has bothered me. So, let me tell you what I know.

First, the installation instructions recommends a modulating thermostat, using their (Rheem’s) unit. Second, the HVAC contractor recommends Honeywell units for all their installations. I use the wireless version, T8665A, which is wired as a single stage unit. And third, this next sentence is copied from the installation instructions.
“The modulating furnace with the UT Electronic Controls IFC is also capable of operating with a single-stage or a two-stage thermostat. The furnace will operate with either single-stage or two-stage thermostats as a modulating system using an algorithm that utilizes 40%, 65% and 100% of the heating capacity. Figures 46, etc. detail how to wire the modulating furnace for operation with a modulation thermostat, a single-stage thermostat or a two-stage thermostat.”

It goes on to say:
The lack of the modulating “V” signal will automatically be sensed as a single-stage thermostat and the furnace will operate as follows:
Phase 1: 0 to 5 minutes=40% of furnace capacity.
Phase 2: 5 to 12 minutes=65% of furnace capacity
Phase 3: After 12 minutes=100% of furnace capacity

So, the statement that there is an algorithm for a single stage thermostat to operate as a modulating system and then it says there is no modulating “V” signal has me confused.

Jerry

#4
12-05-07, 03:13 PM
Member
Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 16,984
You requested a wireless t-stat, or the dealer did?

I hate to say this, you paid a dear price on this furnace and cripple this furnace with a single stage t-stat!!!!!

If you match a Mod stat with this furnace you would of gotten 13 stages, not pathic 3 stages!

With a single stage t-stat you are not getting the TRUE comfort you should be getting and paid for.

I would request to get a matching MOD stat with your equipment, unless there was some reason why they did not put it in.

#5
12-05-07, 04:01 PM
Member
Join Date: Dec 2007
Location: Indiana
Posts: 184
[QUOTE=Jay11J;1271629]You requested a wireless t-stat, or the dealer did?

The dealer recommended the wireless thermostat. When I purchased the furnace, the dealer recommended a Honeywell VisionPRO 8000 Touchscreen Programmable Thermostat, which is wired. And, I assume he recommends that unit for all of his customers. However, I do not know if it is considered a modulating type. Due to difficullty of wiring, he decided to change to wireless. However, I am sure it can be wired. I think the installer wanted an easy way out.

Now, that being said, how do you explain the algorithm that was stated?

Although I did not think it was important, I had a two zone sytem installed at the same time. Would that dictate a single stage thermostat? I faintly remember one of the installation techs saying something to the effect that he had to connect the thermostat in the manner he did do to zoning. Does that make sense?

I tried asking the modulation question to the Service Manager, and although it has three firing rates, he gave me the impression the furnace still went through the other 13 stages in getting to the firing rates. Are firing rates the same thing as the 13 stages you mentioned?

Thanks,
Jerry

#6
12-05-07, 05:32 PM
Member
Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 16,984
What make and model Zone board is used?

Honestly, you would of been better off going with a 2 stage furnace. Would of saved some money.

#7
12-05-07, 05:40 PM
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Join Date: Dec 2007
Location: Indiana
Posts: 184
[QUOTE=Jay11J;1271710]What make and model Zone board is used?

It's a Jackson System, Z-300-HC-MS

Since you asked the question, I looked through the information I had.

First, the information on Jackson’s Web site states, “Works with any thermostat.”

But, the sheet that came with the zone equipment states, “The zone control system works with standard, single-stage or multistage heat/cool thermostats. The thermostats can be either manual or auto changeover.”

So, maybe that is my answer. Would you have any idea why a modulating thermostat would not have been mentioned on their sheet or why it apparently will not work with their zoning system?

Jerry

Last edited by Jerry8A; 12-05-07 at 06:22 PM. Reason: New Information
#8
12-05-07, 07:58 PM
Member
Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 16,984
Hope they put a high limit sensor on this board???

This zone control is an OK system.. It does not have a set up say like Honeywell Zone system would do..

Honeywell will only allow one stage to come on when only one zone is called for. 2nd stage as needed when 2 zone are open, and 3rd if needed on both zone open.

Your zone board won't do that.. It's only set up to run single stage, and uses the furnace's timer to run stages..

So if one of your zone needs heat, and 2nd zone never calls for heat, in about 12 min, the furnace fires off in high heat (3rd stage) you are going to over heat the heat exchanger, and may shorten the life of the equipment.

Do you request the Mod??

#9
12-06-07, 06:39 AM
Member
Join Date: Dec 2007
Location: Indiana
Posts: 184
Originally Posted by Jay11J
Hope they put a high limit sensor on this board???

Do you request the Mod??

Here is what I can tell you.

I found out that limit switches were not put in with the zone system, so I requested that from the HVAC contractor.

His work order says: “Installed high limit control, freeze stat for zone panel.” So, I think that may answer your question.

I tried to follow your explanation of Honeywell’s system. For the Jackson system, as I understand it, only the zones that call for heat will be open. I have a two zone system, so that means Zones 1 and 2 are either on independently or are both on, depending what thermostat(s) is calling for heat.

I did not request a modulating thermostat, since I did not know I had to. I just assumed the contractor would install the correct thermostat to work with the Rheem furnace. However, I was never told that a modulating thermostat would not work with a Rheem furnace that had zoning until after it was installed.

Now, your point about one zone calling for heat and having the furnaces’ third stage come on in high heat is interesting. Based on my earlier post where I stated the installation manual stated Phase 1 as being 40% of furnace capacity for the first 0 to 5 minutes seems to tell me I should be okay. What do you think?

Jerry

#10
12-06-07, 07:09 AM
Member
Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 16,984
If only one zone is called for heat, and been calling for more than 12 min, the furnace is going to go 100%

I don't know how big your ductwork is for zone. but mos zone I've seen is not big enough, so when it times out to 100% you got a lot of heat and air pushing in one zone. If both zone was calling, then you'd be OK.

I ment, did you request for the Mod furnace? Was the house already zoned before with the old furnace?

Mod t-stat will not work with zone system.

#11
12-06-07, 07:52 AM
Member
Join Date: Dec 2007
Location: Indiana
Posts: 184
[QUOTE=Jay11J;1271951]If only one zone is called for heat, and been calling for more than 12 min, the furnace is going to go 100%

I don't know how big your ductwork is for zone. but mos zone I've seen is not big enough.

I ment, did you request for the Mod furnace? Was the house already zoned before with the old furnace?

-----------------------------------------------

Sorry, I overlooked your earlier post about going to high heat after 12 minutes.

No, the house was not zoned before the new furnace was installed. However, I was told I have large duct work. The damper sizes are 24 X 8 and 36 X 8. What do you think?

Jerry

#12
04-11-10, 10:43 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 4
Zoning a Rheem/Ruud Modulating furnace

Originally Posted by Jay11J
If only one zone is called for heat, and been calling for more than 12 min, the furnace is going to go 100%

I don't know how big your ductwork is for zone. but mos zone I've seen is not big enough, so when it times out to 100% you got a lot of heat and air pushing in one zone. If both zone was calling, then you'd be OK.

I ment, did you request for the Mod furnace? Was the house already zoned before with the old furnace?

Mod t-stat will not work with zone system.
A Mod Stat WILL work in a Zoned application with my new interface board.

Allow me to explain myself with an except from the installation instructions:

"There is a PWM (Pulse Width Modulated) signal output from certain thermostats that control the firing rate of
the Rheem/Ruud Modulating Furnace.
The "PWM" signal can vary depending the temperature difference from the set point of the thermostat to
actual room tempersture. Typically it varies from 350 to 950 million pulses per second which determines the firing
rate of the furnace
This signal, presently, can not be controlled by any device on the market except for the "Four Zone
Modulating Furnace Interface Board". Without this board on a Rheem/Ruud Modulating Furnace Zoned system, or
any furnace that utilizes a "PWM" signal, the system is incapable to attaining Full Modulating capability due to the
"V" signal from the thermostats cannot be combined together to one input to the furnace because each thermostat,
typically, outputs a different "PWM" signal at different times and will cause the furnace to error out or not function at
all in Modulation mode, or may not function at all."

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