Variable vs. Multi-Speed

broconneJune 3, 2009

I have two quotes for HVAC for new construction near Raleigh, NC and I am trying to understand the differences - primarily what the benefits are of variable speed vs. multi-speed.

All parts are Carrier

Base System:

Duel Fuel -

Furnace: 58MCB (Basement)

Furnace: 58STA (Attic)

Heatpump: 25HBS3

Quote #1:

Duel Fuel - Variable Speed 86.6% gas furnace with 14 seer heat pump with humidistat.

1st Floor:

Evap Coil: CNPVP4821ATA

Furnace: 58MVB080-f-1-20

Heat Pump: 25HBB348A003

T-stat: TP-PRH-A

Second Floor:

Evap Coil: CNPHP3617

Furnace: 58MVB060-f-1-14

Heat Pump: 25HBB336A003

T-stat: TP-PRH-A

This is a combined $4125 over base price

Quote #2:

Duel Fuel - Multi-speed 90% boost furnace with 14 seer heat pump

1st Floor:

Evap Coil: CNPVP4821

Furnace: 58MEB060-f-1-16

Heat Pump: 25HBB348A003

T-stat: TP-PRH-A

2nd Floor:

Evap Coil: CNPVP3617

Furnace: 58MEB060-f-1-12

Heat Pump: 25HBB336A003

T-stat: TP-PRH-A

This is a combined $2150 over base price

The main difference seems to be the furnaces used.

What are the benefits of variable speed over multi-speed? We primarily want a system that keeps a nice even temperature throughout the house and doesn't send out blasts of either cold air in the summer or hot air in the winter.

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You have heat pumps which can deliver a total of 7 tons of cooling (4 tons first floor, 3 tons second floor), but in Quote #2 each furnace is 60,000 BTU input, and Quote #1 first floor is 80,000 BTU, and second floor is 60,000 BTU. The quotes are not consistent, and the amount of cooling and heating also do not seem consistent for new construction.

Has the HVAC contractor done a heat and cooling calculation (Manual J) and explained it to you? I suggest you do this first otherwise you may disappointed in the performance of your HVAC systems.

    Bookmark   June 3, 2009 at 9:05AM
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Mike -
Thanks for the reply. Sorry there were two typos in my original post. In quote #2 the furnace is a 58MEB080-f-1-16. So they are both 80,000 BTUs. Not sure if hte amount of heating and cooling are consistent or not. The home is roughly 4000 square feet with about 40 window.

The second typo is that the first system is a 96.6% not 86.6%.

The home is a semi-custom, not full custom, so it is one of 8 plans (4 elevations per plan). So, I imagine (and could be wrong) the manual J calculations were done for all of the houses and not my particular build.

With those two systems in mind and the corrections. Does anyone know what the benefits of the variable speed over the multi-speed? Is that the main difference in the systems? The fact that one furnace is more efficient than the other doesn't seem too important to me because I expect the heat pump to heat the house on most days.


    Bookmark   June 3, 2009 at 9:40AM
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A friend of mine lives near Raleigh/Cary--nice area.

I think you'd be happy with the added comfort benefits of the variable speed blower in terms of humidity (if they set up the humidistat to control blower for humidity which I assume they would). It is also more efficient and can be quieter. That's one of Carrier's top furnaces but the outdoor unit is the standard grade (you may wish to ask about the 25HCB3 series units, which have better coil protection, lower noise level, and longer compressor warranty).

I certainly agree with the importance of a load calculation for sizing.

Another thing: You don't really need a 95+ AFUE furnace as backup heat, since in your climate it would not see much use. Heat pumps can be efficiently and effectively used down to below freezing outside. At that point the gas furnace would take care of the rest, but I believe your climate is fairly mild, no? So an 80% AFUE furnace, still variable speed (58CVA series) would be a wiser purchase in my opinion, and it would cut the cost a bit.

May want to ask about the Infinity controls for your new home.

    Bookmark   June 3, 2009 at 10:08AM
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Ryan -
Thanks for the reply. The furnace is 90% or higher because it is in the basement and are venting it directly outside - if they did an 80% they would be require to vent it out the attic which the floor plan doesn't support.

Thanks for the tip on the infinity controls - but I plan to replace the thermostats with HAI RC2000s as I am into home automation.

So for variable speed the benefits are:
(1) Better humidity control
(2) More efficient
(3) Quieter

Would a multi-speed, rather than variable speed system prevent large blasts of cold or hot air?
Is the multi-speed system have any benefits of a single speed system?
How many speeds are multi-speed?

    Bookmark   June 3, 2009 at 11:16AM
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I should mention that budget here is of issue. So if I can obtain 95% of the benefit of variable speed with multi-speed that may be the solution for me rather than the best solution.

    Bookmark   June 3, 2009 at 12:09PM
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The thermidistat uses the variable speed technology (by controlling the compressor and blower speed) in order to dehumidify. Without variable speed, the options for added dehumidification are limited. So, I would indeed keep the 58MVB furnaecs. That is assuming the installers go through all the setup options to enable this mode.

Multi-speed does not mean it adjusts speed like variable speed blowers. Multi-speed fanse have one speed setting for each mode/heat stage. So in low stage you will not get as much airflow, if you get a 2-stage furnace as well (58MEB).

The variable speed adjusts its speed in response for dehumidification and to maintain proper airflow over the furnace/coil by adjusting RPM if necessary.

Multi-speed/single-speed motors are PSC motors (permanent split capacitor). Not as efficient, but do the job. It's the "standard blower." The variable speed motor is controlled by an ECM which ensures proper airflow should the ductwork be slightly undersized, poorly designed, etc.

    Bookmark   June 3, 2009 at 1:07PM
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Thanks Ryan -
So as I understand this. Option 2 offers really no benefits over the base model if my home is heated mostly by a heat pump?

    Bookmark   June 3, 2009 at 1:10PM
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Yes, no real benefit of the 90% 2-stage furnace. But the 2-stage variable speed has the benefit in its blower. Variable speed of course is not necessary and many new homes don't get it, but if comfort is a bigger concern, then it's what I recommend. Even the base systems, if sized AND installed properly, will heat and cool your home to a satisfactory level. With your fairly mild winters, you may want to consider replacing the base/builder model heat pump with an upgraded air conditioner (24ACB) and installing the 58MVB variable-speed 2-stage furnace, thereby forgoing the dual fuel idea. Don't the winters tend to stay around the 40s? If that's the case, that is heat pump territory and the furnaces would see very, very little use.

In the attic, can't an 80% furnace be used? I really doubt the furnace would get much use *at all* for the attic system, since heat rises, so you may not even need the furnace for back-up. And if you do, 80% is advisable.

    Bookmark   June 3, 2009 at 1:36PM
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You are under the impression a variable speed blower is more efficient. Not so. The most efficent is defined as the least cost for a given amount of cooling. In florida during the hot day you want everything running full bore, that will give you the best cost/benefit. The variable speed blower setup gives the system the opportunity to run at slow speed to extend the system run time, thereby wringing out the air (good in the early hours).....not more efficient, sacrificing max efficiency for better comfort control.

    Bookmark   June 4, 2009 at 6:36AM
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I am aware that running at a lower speed to reduce humidity is a sacrifice of efficiency for comfort, but if that means you can now set your thermostat a couple degrees higher, then what's more efficient? That's why I recommend avoiding any slow ramp-up profiles and instead using a thermostat to control humidity when needed. The DC variable speed motor, unless placed on a horrible ductwork system, should be more efficient at full CFM than a standard PSC motor. And that's reflected in the increased SEER rating.

    Bookmark   June 4, 2009 at 8:15AM
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Ryan -
Good question about the attic furnace. Does Carrier make an 80% that supports variable speed?

    Bookmark   June 4, 2009 at 9:10AM
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brcone wrote:
"Thanks for the tip on the infinity controls - but I plan to replace the thermostats with HAI RC2000s as I am into home automation."

If you do go with a Carrier Infinity furnace, then take a look at the Infinity controller before dimissing it. It has features such as reporting static pressure, motor RPMs and logging systems failures. I don't thing the RC 2000 supports this.

Also the Infinity controller communicates to the furnace and condenser via an RS-232 interface. It was not obvious if this interface can be used with the furnace and the RC 2000.

There is a version of the Infinity controller which can be controlled remotely if this is a feature you want.

    Bookmark   June 4, 2009 at 9:12AM
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"Does Carrier make an 80% that supports variable speed?"

Broconne, that would be the 58CVA I was referring to previously.

    Bookmark   June 4, 2009 at 9:45AM
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I had a conversation with the HVAC sub and I think I am going to go with the following - a lot of it driven by price:

For downstairs - The base system listed above with the minor addition that they have switched to Puron for all the heatpumps - which is a nice addition.

Upstairs, removed the furnace and gone with a variable speed heatpump/AC only.

2nd floor 14 seer variable speed r410a heat pump

Additional $960 over base.

That gets me a variable speed system upstairs for not a lot more money.

I don't think I really need a furnace upstairs - and would rather have variable speed for dehumidifying and comfort, etc.

    Bookmark   June 4, 2009 at 1:53PM
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So no variable speed for downstairs?

    Bookmark   June 4, 2009 at 4:45PM
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Based on the floorplan - basically wide open downstairs - it was the subs opinion that even without variable speed I would end up with even temperatures throughout the downstairs.

I think this option might be the best I can have without spending a lot more money. Although - I am still considering the VS speed downstairs at an additional $2k - but this is what the sub thought was the best bang for the buck considering the floorplan.

    Bookmark   June 5, 2009 at 5:50AM
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such a shame. it always troubles me to see nice new home construction qualities and then install what at best can be described as mediocre HVAC.

here is the AHRI matching number for the upstairs HP system.

3253252 Active Systems BASE 13 PURON HP CARRIER AIR CONDITIONING 25HBB336(A,W)30 FV4BN(B,F)003 34000 11.70 14.50 34400 8.10 21200

you should look at one of these condensers in person. I would at least consider an upgrade to the Comfort 15 HP condenser. see below.

3032540 Active Systems COMFORT 15 PURON HP CARRIER AIR CONDITIONING 25HCB536A30 FV4BN(B,F)005 35800 13.00 16.00 35600 8.80 21000

and I would definitely install a var speed furnace for downstairs-80% AFUE eff OK with a Performance 15 heat pump.

bang for the buck? what about comfort and mthly operating cost?



    Bookmark   June 5, 2009 at 11:25AM
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