1 HVAC unit only for entire home?

pbx2_gwJanuary 14, 2014

This is a bit after the fact - as we are in our 9th month in the newly built house - what if our multi-zoned Heat Pump had gone out or as the case this past during the cold spell we had in Central VA recently?
Would it have been better to build with more than 1 HVAC unit in a house?

When we built our house & the budget drove every decision, it was based on the premise that building a class leading tight house & using a heat pump in moderate climate would be the best bang for the buck.

& so far - the balance sheet shows that as we now spend ~70% LESS to heat & cool a 3K sqft house vs. the quarter of the size condo we were living in prior.
We are ecstatic with that as the financial modeling went better than planned.

Except this past summer when the coils went bad on the new HP & as we sweated for a week waiting for it to be repaired, wondering why we didn't get a 2nd unit to operate another zone.

So in retrospect, is there any conventional wisdom in respect to building with an individual HVAC unit that does the job excellently vs. 2 that possibly save us once in a blue moon?

The answer wasn't obvious to us & still isn't...

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cindywhitall

You could buy many portable a/c units and heaters for the add'l $ you saved by not getting 2 units. Next time it goes down just pick up one or 2 for the rooms you use most. OR, find a repair company who doesn't take a week.

    Bookmark   January 14, 2014 at 12:46PM
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mike_home

The primary reason for installing two HVAC systems is to ensure both floors of the house have even temperatures. This can also be done with zoning. The problem is most contractors don't have the expertise to install a balanced zoning system. The two system approach is easier, but there is still are still opportunities for a bad installation.

My vote would have been to install two systems for a 3000 sq. foot house. In addition to having a built in back system, you would have two smaller units which may run more efficiently and less often. The less run time translates to a longer life.

My old AC failed twice since I have owned my house. It was nice having the second AC keep the house comfortable while I waited for the service tech.

    Bookmark   January 14, 2014 at 1:39PM
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sss120396

Deleted

This post was edited by sss120396 on Tue, Jan 14, 14 at 17:40

    Bookmark   January 14, 2014 at 3:15PM
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energy_rater_la

I'm a fan of zoning..that is zoning with one system
and multiple zones & tstats.
number one is a contractor who understands zoning.

systems break down, that is just life.
but if you have spent a couple of thousand
more for zoning, & high effieicney equipment
then you have a relationship with the hvac co.
they are going to get out there & get it fixed.

the companies I know that do zoned systems
laugh off the 'cool place to go' pov ...as do I,
as hvac speak for not understanding how to
do zoning..or just greed of selling 2x the equipment
& service contracts.

to me the cost of a second system, second duct
system, twice as many supply /return plenums.
twice the electrical or gas setup costs..
and then maintanence & operating cost of
two systems...why?

for that week that the system might break down
one year? that is like sizing for the 5 days a year
of extreme temps...and paying for the cost to
operate short cycling for the other 360 days.

I'm biased against inefficiency, and to me
two systems when one would do...is
the epitome of wasteful.

but then lots of co's here put two 3 tons on
2500 sq ft. same as cool space to go folks...
selling more tons than needed...because they
can.

oversizing hvac systems is something that is
very problematic. but as long as people/hvac
companies believe that bigger is better...
it is what it is. wasteful.

just my personal opinion.

best of luck.

    Bookmark   January 14, 2014 at 5:14PM
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pbx2_gw

Thanks for all the feedbacks. It's very good food for thought!

When we were starting out the project in 2012, I visited this board a lot to get the sense of what was needed to make the proper decision.

The resonating message was to make the house tight, air leak proof & HVAC equipment sized properly to maximize the efficiency & savings. Get duct testing & zoned properly.
Of course energy_rater_la was one of the voice I listened to.

Only recently have I thought having a backup system would be nice...but @ what costs? Just to have a suspender for the belt?

I forgot to mention that we also have a 36K BTU double sided fireplace that we have learn emits very nice heat to even the upper floors.

So as energy_rater_la said - things break & mine broke during it's first few months in operation. Which by all accounts - was good that it broke early & extremely rare that a Trane coil on a XR16/17 broke at all.

So I think my 3 tons - 3k sqft home asized HVAC system has provided me great financial savings & except for that 1 week where the coils were being shipped this past summer, great comfort.

I think that is all I can ask for. Everything else is just pure dumb luck.

    Bookmark   January 14, 2014 at 6:01PM
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tigerdunes

There is nothing wrong with zoning. The key is finding an experienced HVAC dealer both in equipment selection and ductwork design/sizing.

If at the very best install, you do lose some small measure of control and comfort plus the backup of at least the second system for the other zone.

With a zoned system, there is the issue of more wear and tear with possibly a shorter operating life. Small issue but still there.

IMO

    Bookmark   January 15, 2014 at 8:08AM
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pbx2_gw

@tigerdunes - another poster I listen to LOL!

Can you clarify what you mean by:
If at the very best install, you do lose some small measure of control and comfort plus the backup of at least the second system for the other zone.

Are you saying that even with the best zoned install & equipment - I would still lose out on some control, backup & longevity?

    Bookmark   January 15, 2014 at 11:45AM
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tigerdunes

The statement speaks for itself. You obviously have no backup/ redundancy.

I am not sure where this thread is going unless you are having some buyer s remorse.

You do have emergency strip heat for a condenser malfunction. That would help you on the heating side. Surely this would have been enough in this past week's extreme cold to keep you from freezing. But you will be out of luck for cooling during a condenser breakdown. As I recall, it can get mighty hot and humid in the Old Dominion.

IMO

    Bookmark   January 15, 2014 at 11:58AM
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pbx2_gw

Ok - appreciate the clarification.
Heating redundancy wasn't too much of a problem with the Aux heat strips but as I mentioned originally, the coils did go bad in August - which had us down for 1 week while it was being shipped & installed.

Not as much buyer's remorse as actually doing a review of all systems setup & where there are holes & at the very least - manage expectations & prepared contingencies.

    Bookmark   January 15, 2014 at 12:08PM
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tigerdunes

With a zoned system like yours, it's important to you and members of your household to have it running efficiency. I would invest in a yearly PM, not as important with a home having multiple systems.

IMO

    Bookmark   January 15, 2014 at 12:20PM
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pbx2_gw

Yes - words of wisdom there tiger. Pound wise is better than penny save. Thanks!

    Bookmark   January 15, 2014 at 1:05PM
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mnk716

I have a one zone 3ton unit AC system for a 2400sqft house built in 2006. House is very tight. I was very concerned when it was proposed and the HVAC contractors said it would not be a problem to maintain a temperature i am comfortable with. i like it at 70-71 degrees during the summer and it has always maintained that temperature.

the thermostat is one 2nd floor in the hallway by the stairway. no complaints and the 1st floor gets about 1-2 degrees cooler than upstairs.

i am convinced that one unit can maintain the temperature if it is installed correctly. I live in CT and spend about $160-180 @ month during the summer with AC.

    Bookmark   January 15, 2014 at 4:56PM
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countryboymo

Miy 1850sqft used to be 3.5 tons and 20kw of unstaged strips with a heat pump.

I topped 6" of cellulose with one layer of r19 insulation and someday will go with 2 after I complete more attic sealing. I sealed and reconfigured my duct system in the basement and insulated my rim joist and sealed almost all basement gaps and holes around pipes.

I upgraded my system and downgraded the size to 3tons and my strips to 10kw. At zero the heat pump might be off but it still cycles on the strips. At 10 below the strips run constantly but maintain temp. I still have a considerable amount of sealing to do in the attic and more insulating.

What is upsetting to me is the materials and labor to do what I have done and plan to do would be under 500 bucks at the time of building in 06 AND they could have saved a good portion of it on the smaller system.

A quality envelope and decent system that is zoned will keep money in your pockets. Being 'green' the way the government is going about making it happen is going to cost a bunch of it in the long term.

    Bookmark   January 15, 2014 at 11:32PM
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pbx2_gw

mnk716 (My Page) on Wed, Jan 15, 14 at 16:56
I have a one zone 3ton unit AC system for a 2400sqft house built in 2006. House is very tight. I was very concerned when it was proposed and the HVAC contractors said it would not be a problem to maintain a temperature i am comfortable with. i like it at 70-71 degrees during the summer and it has always maintained that temperature.
the thermostat is one 2nd floor in the hallway by the stairway. no complaints and the 1st floor gets about 1-2 degrees cooler than upstairs.

i am convinced that one unit can maintain the temperature if it is installed correctly. I live in CT and spend about $160-180 @ month during the summer with AC.

Wow - that's what I am talking about! Has it ever broken down? Or if it did what is your philosophy?

Posted by countryboymo: I upgraded my system and downgraded the size to 3tons and my strips to 10kw. At zero the heat pump might be off but it still cycles on the strips. At 10 below the strips run constantly but maintain temp. I still have a considerable amount of sealing to do in the attic and more insulating....A quality envelope and decent system that is zoned will keep money in your pockets.
Again - inspirational!

    Bookmark   January 16, 2014 at 12:51AM
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mnk716

@pbx2 has never broken down, i did have a power surge during Storm Sandy that blew out the relay in the blower but it was repaired easily. I have a HEIL 13SEER system. I leave at 76 during work days and 70-71 in the evening and overnight. I do leave the fan running all night for circulation.

    Bookmark   January 17, 2014 at 4:08PM
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pbx2_gw

@ mnk716 - thanks for the feedback. I am more than convinced now that I did it the right way with the install, contractor, & planning. My worries are from not knowing all the context of having a 1 unit zoned vs. 2 unit HVAC.

    Bookmark   January 17, 2014 at 5:06PM
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countryboymo

The majority run the calculations of replacing a system vs more sealing and insulation without looking deep enough into the picture.
You can save X in efficiency with more insulation/sealing.
You can save Y amount with upgrading the system.
These are both valid calculations but if you look deeper.

You can save X amount insulating and sealing and
You can save Y amount upgrading the system.....
The biggest savings is calculating the two together and size the system accordingly.

That unit will draw less current and on average less run time.

    Bookmark   January 17, 2014 at 11:32PM
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pbx2_gw

Posted by countryboymo
... but if you look deeper.
You can save X amount insulating and sealing and
You can save Y amount upgrading the system.....
The biggest savings is calculating the two together and size the system accordingly.

That unit will draw less current and on average less run time.

Exactly the point I was looking for.
In building science, if one can get the right calc & triangulation of the components involved, the payoff in savings could be huge.

In my case, I was worried that I would be far too dependent on just 1 unit & would be subjected to pain if it should go down.

What I failed to realize earlier was that if I am insulated right & my ducting & equipment are correctly sized, then my home requires less run-time than a setup with 2 units & poorly sized.
With properly sized equipment & a tight house, I am able to retain my conditioned air longer while keeping out more of the bad air = less 'hard' run time on my 2 stage HVAC. Ergo, less wear & tear & more comfort.

Sure things break...but as a wise man said recently, 2 units setup can break too.

    Bookmark   January 18, 2014 at 1:12AM
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countryboymo

I think you have it PBX. As time goes on and brushless motor technology evolves and also becomes cheaper the higher end equipment will get even better and also less expensive.

    Bookmark   January 19, 2014 at 4:53PM
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