Is my HVAC Guy correct? 2 systems instead of 1 system with Dampers

I'll soon be working on converting a second floor of a residential building that was never finished and is being used as an attic for the last 80+ years. Building Size 36'x36', 9 1/2' 1st floor ceiling height, 8' planned ceiling height in 2nd floor with plenty of insulation and air barrier planned.  This is also a DayCare Facility.

I will be adding an open stairway to the floor plan to provide access to the 2nd floor. (Clients request for capitalizing on the natural daylighting.)

The HVAC guy informed me that: having 2 separate HVAC systems is the route he would use because-it was how he learned and is the norm in St Louis:

  • Existing HVAC system for  1st floor is undersized 1 1/2 Ton compressor should be 3 Ton unit. (located in the Basement)
  • 1 system with dampers for climate control could  lead to future mechanical problems if one big system was used.  (I don't believe he had faith in a damper system to work over long periods of time.)
  • Controlling Costs and Not disrupting existing downstairs.  We would have to build a chase for the hvac ducts from the basement to the 2nd floor. (closet space is available to do this)

The RED warning light that is going off in my head and is contrary to all I've learned is that: "What will keep one system from robbing the climatized air from the other system?"  The open stairway will allow air movement up and down.

I agree with him partially on 2 systems if there was not an open stairway planned.

Since this is in design stages to capture the natural lighting I've mentioned a French door with a wall would allow lighting and separate the floors.  Which makes a dual system more appealing to me.

Thanks in advance for any advice.

Build Green,


The following CAD drawings were made using Sketchup and what I've found is an easy way to convert plans into picture files for showing my clients what the finished product will look like.  Yes there will be rails on the stairs, omitted so that it doesn't clutter up the drawing.

Instead of this railing set up a wall with a French door would allow the natural light into the room.  And also alleviate my fears of a child climbing the rails and falling to the 1st floor.  If this design is chosen I plan to build the railing 48" tall.

Tags: Air, Energy, HVAC, Movement, Split, Systems

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Total consumption.  EE doesn't change the meter fee.

Yeah, 25% total, 1/3 heating.  LOT will be due to sizing.  Be fun to see, won't it?

You also lose heat from ducts.  Cooler ducts lose less heat.  

Longer cycles mean even heat to long runs.  Even temps mean less stupid pet tricks chasing comfort. 

Lower output means lower static.  Lower static dramatically decreases delivery cost (fans laws or some such), and it seems delivery efficiency too (better transfer x heat exchanger maybe?)

Too bad you didn't go hybrid.  24,000 btu would have made a nice low stage.  

Get tired of hypothesizing about the why.  So long as savings and comfort show up, and I can predict with decent and improving accuracy, that's what matters.  

People want what they're promised for the price promised.  Savings that doesn't show up means you charged more than promised.  A little variance no reasonable human will hold against you.  A lot of variance, particularly if it's all weighted on the "consumer loses" side, is theft.  

Hybrid doesn't make sense for our area. Electricity is 10 cents per KWH, gas is 47 cents per Dth. $2.94 for 100K BTU@ a COP of 1. With a COP of 3 a heat pump costs about $1 for 100K BTU.

... and certainly those prices will NEVER change.  

Do you think if you lock the pump out at 45f your COP will only be 3?  

When NG ports come on-line and gas price reflects world supply and demand instead of continental, what will the cost per btu be to you then?  

And finally, if jumping from 80 k to 40 k added comfort, what comfort are you missing not having 40k and 24 k.  

Sure, limit your options.  Don't consider things might change.  Make 20 year decisions based upon current conditions assuming gas won't get expensive, and electricity won't get cheaper even though there is fair evidence both will occur.  Don't imagine you might want to some day produce all your own energy.  

Lock yourself into FF slavery.  Gas companies LOVE thinking like that.  

I never recommend, and to date have never sold ac.  Only heat pumps.  I recommend everybody only recommend them.  

That doesn't mean you don't SELL ac.  If someone doesn't want to follow that advice and regrets it later, you are blameless.  They harmed themselves.  If you don't recommend and knew better, they can blame you.  Why carry that weight?  

The dual fuel setups we have in our area are generally not used in HP mode. Most simply use the gas because it's cheaper to run. I considered spending the extra $250 to get the HP instead of the AC condenser be cause of the possibility of future rate changes. We pay cost for fuel and it does vary. It's never been over $10 per Dth which would make it equal to a COP of 3 on a HP. I looked at a 45f lockout for the HP, but the heat runs so little above 45f I couldn't see the value of it. We have to get down into the 30's before the heat runs much.

If I was going to go 2 stage for comfort I would have bought the 2 stage furnace or used a hot water coil setup. I bought an "old stock" (2004, brand new in the box) furnace for $250. At that price I couldn't pass it up.

What boggles my mind is that 75% of all electric homes in our area use STRIP HEAT as the primary heat source. Yes, 20KW air handlers as the PRIMARY heat source. Even at retail a HP is only $500-800 more than a standard AC with electric heat. Customer would save that in electricity costs in 2-3 winters. Selling a HP SYSTEM means selling a matching air handler and condenser, many customers just want to do condenser swaps.

While we sell nothing but heat pumps in North Florida, South Florida is full of straight cools and strips. Down there heat is needed just a few days per year, though those can be VERY hard days for the utility!

Aside from the $500-800 retail savings for a straight cool vs HP, there is another issue in play. Losing the reversing valve required for HP adds about a full SEER point in cooling mode. In a climate with 10+ months of cooling with 3000+ equivalent full load hours, that 1 SEER point starts to add up. The same reasoning could apply to hybrid systems. It may be hard to justify the SEER loss, and additional expense and complexity when NG is available.

I never have bought into the whole SEER thing, EER is what I'm concerned about. Most people keep their AC @ 75, not 80. Adding 15 degrees puts you at a 90 degree outdoor temperature which is very realistic. The 2 degree difference that SEER uses only puts you at 77 outdoors. AC's spend a LOT more time running @ 90F+ outdoors than they do at 77F or less. How much of a hit does a HP take vs. straight cool when it comes to EER?

As far as utility strain It's 2-7pm weekdays during the summer that our grid works the hardest. The winter load doesn't even show up on the radar even with all the 20KW air handlers running. Too many homes have gas heat.

While I agree that EER is useful, SEER is also useful for comparison. The 80 degree EAT is not intended to reflect thermostat setpoint but instead to compensate for leakage into return air ductwork, AHU's outside conditioned envelope.

Everybody THINKS about AC whenever it is 90+ outside, but bin data shows that systems in the eastern half of the country spend many more hours dealing with humidity and 75-85 degree outdoor temps.

A brief look at my Carrier xls suggests that HPs lose 1/2 to 3/4 EER point over comparable ACs.

Accumulator would have a small impact, and I'm not sure crankcase heaters figure into SEER or HSPF calculations.

So my advice to clients with NG for heat would likely run to straight cool - lower installed cost, higher operational efficiency, less complexity.

I have a hard time believing a 5 degree increase on return air temp due to ductwork when it's 82 outdoors. You would be looking at a 10F+ increase on 95F days if the ductwork was that leaky.

Getting high SEER ratings without increasing EER is done mostly by reduction in cycling losses. Unfortunately the high SEER systems tend to be the ones most oversized which negates much of the real world savings...

A 75 degree room may have 77-78 degree air into ceiling returns. Add a bit of duct leakage from a warm attic or garage and 80 is not unreasonable.

I will not further quibble with you over SEER parameters - my overall stance is that it is useful for comparison, just like MPG ratings. If my AO was in the desert southwest, I might pay more attention to EER. Besides, EER and SEER tend to track each other more or less, with EER running 75-80% of SEER.

Where high SEER systems really shine is during part load, low stage operation. We try to design zoned systems to keep a system in low gear more often by prioritizing different parts of the home during different times of day.

I'd be curious to review data supporting the premise that high SEER systems are more often oversized - do you have any?

EER and SEER do track each other until EER hits about 12-13. EER then stops about 13 while SEER continues to climb. Multistage certainly makes sense for zoned operation. Forcing full output into 1/2 the ductwork rarely will perform well.

"Bigger is better" is a hard mentality to break and the equipment is sized based on low stage with stage 2 as "reserve capacity". Rarely will you find 14+ SEER systems installed that are less than 3 tons capacity, regardless of what the load is. The 2ton 14.5SEER I installed summer of 2012 had a mfg date of 2009 on it. The supply house said they don't sell many high SEER low tonnage units.


If anyone has measured or can calculate the value of 1 seer, it would be Curt Kinder.

Don't make me beg, tell. How do the numbers look. If you don't know for sure, I'd be comfortable with a guess from you. Also, can you explain why the eff loss?

I know there's endless conflict about SEER vs real world efficiency, but SEER provides a useful comparison between alternative systems. If SEER goes up by 1.0, then I'm confident we can expect an efficiency improvement in keeping with % delta SEER as long as everything else is held equal.

 The increase in SEER linked to AC vs HP arises from deleting the reversing valve. The RV and sundry copper plumbing bits both restrict refrigerant movement and transfer heat to / from outdoor conditions near the plumbing. Keep in mind HP or AC refrigerant plumbing is bare copper tubing installed in a very windy environment - the interior of an outdoor unit with 2000-4000 CFM air being sucked throught it by the condenser fan.

The RV itself consists of a horizontal copper cylinder with 4 ports very close to each other. Refrigerant temps in adjacent, metallically connected ports maybe 100*F apart, greatly increasing unwanted heat transfer between two refrigerant streams.

RVs are ingenious devices, but have their shortcomings.


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