heat pump: 2-stage w/ heat strips, or inverter-type compressor ?

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RustyShackleford

Minister of Fire
Hearth Supporter
Jan 6, 2009
1,350
NC
Feel like I'm going around in circles with this replacement HVAC decision I been agonizing over for the last 6 months or so ...

Right now, the choice is coming down to a "traditional" style heat-pump with a two-stage compressor (or is it just the valve ?), or an inverter-type compressor like minisplits have.

Two-stage would be: https://www.goodmanmfg.com/products/heat-pumps/gszc7

Inverter-style: I'm still debating between Midea knockoff (ACIQ or Blueridge) or Fuji (per another thread).

Cost is secondary, I want the most reliable (and reparable) system with efficiency a close second. I'm more interested in cooling performance, because (of course) I do most of my heating with firewood - and it seems like the main edge that inverter type has over two-stage is heating performance. And I'm still not sure the inverter-type won't need a small heat strip.

I'm still a little leery of the "new-fangledness" of the inverter type (even though of course they've been working in minisplits for years). When it comes to parts availability, for example; though that might be less of an issue with Fujitsu than with the Mideas.

It sounds like I'm trying to talk myself into the two-stage ...
 
IMHO, go with inverter. If they are a known brand they are more tolerant of bad power although they do not like line surges so make sure you have a whole house surge protector.

A two speed is going to cycle on/off and that is when most of the wear occurs in electrical and mechanical equipment. a variable speed unit will rarely start stop and run at a lower overall speed in general so they should last longer.

BTW when I used to work in an industrial facility the working life for industrial refrigeration was 10 years, the equipment could run longer but we had to pay a premium per ton installed for units over 10 years. The contractor essentially started acruing funds to start replacing compressors.

Of course, once you decide to go with a knockoff, there are going to be compromises with components to meet that price level so long term reliability is probably less.
 
I think the inverters work best as communicating units. Only install units that you can verify latent capacity at part load. Humidity control is important. Does the inverter carry blower with load/compressor speed m? Coil needs to be below indoor dewpoint, especially during shoulder season.

Don’t oversize the two stage system. Heat strips are meant to be used when temps get near design temp.

I’m very happy with my two stage system and when setup well in the tstat short cycling is not an issue. My AC never called second stage last summer.

I know I can make the two stage unit work really well with a good programmable thermostat like an ecobee. I’m not confident that I could make an inverter unit run as.

My vote is the speed system
 
Rather than start yet another thread, let me ask this question here ...

The calculators I've been using to verify system size give pretty similar results. They are:


and


For cooling, the former gives "total" load and "latent" load, while the latter gives "total" load and "sensible" load. I understand that "sensible" cooling removes the heat from the air without changing the moisture content, and "latent" cooling dehumidifies the air by causing condensation. The two calculators simply choose to display these numbers in different ways (both give a total = sensible + latent number, one breaks out latent separately and the other breaks out sensible separately).

But I'm confused about evaluating a system's specs to determine if it can satisfy the requirements specified by the calculator. I've attached a page (below) from the tech document for the Goodman 2-stage system I'm considering.

Look at the box containing the yellow-highlighted numbers. Note that for the constant indoor dry-bulb temperature of 75 degrees, as the air gets dryer (so the wet-bulb temperature drops from 71 to 59 degrees as the humidity drops from 87% to 58%), the total number (labeled "capacity") stays roughly the same, but the "S/T" numbers (sensible to total ratio) increases fromn 0.5 to 1.0.

So for the 71 wet-bulb column, the sensible number (capacity x S/T) is WAY too small (compared to the "sensible result" from the calculators). Similarly, if I look at the 59-degree wet-bulb column, the latent number (capacity x (1-S/T)) is way too small (in fact, it's zero).

Should I take all this to mean that I can use the 71-degree column for determing if the system's latent capacity is sufficient and the 59-degree column for determining if the system's sensible capacity is sufficient ? Put another way, if initially the air in the house is humid, the system dehumidifies the air, and then once the RH% has dropped into the 50s, it concentrates on actually cooling the air ?
 

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So how do the calculators report latent load? Is it figured in? I’m unsure. I think latent is a separate calc but I’m unsure.

Refresh my mind about the sq footage and year build and attic insulation
 
 
if initially the air in the house is humid, the system dehumidifies the air, and then once the RH% has dropped into the 50s, it concentrates on actually cooling the air ?

I am not an HVAC pro even though I used to mess with big chillers but your restatement is correct. One way to think of it is the air in the room is a mix of two distinct parts. Dry air and water vapor. The inside coil in the AC unit will be colder than the room temp , water vapor in the air will condense on the coil and down into the drip pan and hopefully outdoors as long as the vapor in the air is above its dewpoint. Once the air is below dewpoint then the amount of energy used is far less as the latent (hidden) energy being used to condense the water is far less. This latent load sucks up a lot of energy 970 btus/lb of water in the air. In order to know how much water in the air you need some basic room measurement tools and psychrometric table, there are lots of references on the internet but here is one https://extension.psu.edu/psychrometric-chart-use. It looks intimidating but it tells you a lot, once you figure it out a you can establish the exact worse case room air condition and also the desired room condition and then calculate directly the cooling and dehimidfication load. Dry air is very light so it takes far less energy to cool the air but its enthalpy changes with temperature. Manual J estimates the sensible heat load coming into or leaving the room but does not deal with infiltration which can vary quite a bit depending on construction. A blower door test can measure the infiltration very accurately. Most people guess and usually guess wrong by guessing too low.

When you first turn on the unit, its going to be running full bore to drop down the humidity in the space and then it will start cooling the air. Ideally the unit is run early to knock down the humidity and then run on low to keep the space cool and deal with any humidity that leaks in or is introduced into the space. Folk like my deceased parents only had AC late in life up north and they always waited until the conditions in the house were near intolerable before turning the AC on and then my mom would complain that she didnt like it as it was "too noisy" It was noisy as it was struggling to deal with all the moisture. If I convinced them to run it earlier in the day, it would make less noise and the room never got uncomfortable. You are not just dealing with the humidity in the air, all the "stuff" in room that is permeable will have absorbed moisture and it needs to dry out so as the room humidity drops, this extra moisture enters the room and needs to be dealt with. Dealing with a bathroom with tub and showers can be a real PITA, in many cases its better to exhaust the damp air outdoors and let infiltration bring in outdoor air to replace it.

Note that if the space is too "tight" the air can get stale inside the room and some folks will run with a window open. Generally, if its a designed system there will fresh air inlet built into the system but usually a window shaker type unit does not and depends on air leaks in the room.

You will see warnings on occasion not to oversize an AC unit. It is somewhat rare for homes to be an issue but in commercial and industrial it can be an issue if there is lot of humidity in the room. The temperature of the indoor coil can drop below freezing and any water condensing on it can freeze and plug air flow through the coil rather than flowing to the drip pan. It is called "freezing" the coil. If a house is freezing a coil, its usually a sign that it is a very "leaky" home in a very humid area or something inside of conditioned space like cooking is adding lot or moisture. Single speed units tend to freeze up the most, two speed units are better and inverter units probably the least. There can be internal controls to prevent frozen coils but that usually means slower moisture removal. An inverter style unit has a wide range of cooling while a two speed unit only has two.

There also is the "dirty secret" of cooling in the past and that is reheat. Hospitals and public buildings want a lot of fresh outdoor to be circulated through the building to reduce odors and to keep nasties from recirculating the building. That fresh air is loaded with humidity that needs to be removed as they also want dry air as nasties in the air and on surfaces don't like dry air and they don't want any spots where mold might grow. That means there is a lot of latent cooling load dealing with all the humidity coming in. The approach used to deal with this was to overcool the air coming in and then reheating it with steam and electric coils to bring the air back up to comfortable. It works great but not very energy efficient. This was standard in most big buildings until energy efficiency came in and then buildings got tightened up and air was recirculated. This on occasion lead to "sick building syndrome". It is an issue with new super energy efficient homes like Passive Homes and the solution is a separate fresh air make up system that pulls air out of the building and runs it through a heat exchanger to pump back into the building. The heat exchangers used have two types, for a cold climate they use a latent and sensible exchanger to grab any moisture that may be in the indoor air being exhausted and put it back in the house while in cooling dominant climate, they use a sensible exchanger that allows the moisture to go outdoors while the sensible cooling is used to precool the incoming air.

In some cases, older residential system had electric reheat coils. Note this is not the same as strip heat which usually kicks in when outdoor temps are low and a single speed refrigerant system cant deal with the low refrigerant temps. Most folks do not know the difference and most residential techs are either mechanics who keep the refrigerant system working outside or they are duct guys. Few are really good with both skills and many systems are built to "rule of thumb". To actually design a system from scratch takes some technical background that most techs and small contractors skip. Few residential systems can afford to be designed by an engineer so in many cases the systems installed are just a guess based on similar systems.
 
You will see warnings on occasion not to oversize an AC unit. It is somewhat rare for homes to be an issue but in commercial and industrial it can be an issue if there is lot of humidity in the room. The temperature of the indoor coil can drop below freezing and any water condensing on it can freeze and plug air flow through the coil rather than flowing to the drip pan.
It’s way more common down here. Tech can up sell the owner a more expensive unit that offers more cooling….

During part load conditions temp is satisfied before the humidity drops. Friends got a new system then a few weeks later left for 3 week vacation. Temp set at 80 or 82. I forget. Came home to mold everywhere.

Two stage systems only come in single ton capacities. For us down here it can work better as stage one is close to cooling and stage two is closer to heating load.
 
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It’s way more common down here. Tech can up sell the owner a more expensive unit that offers more cooling….

During part load conditions temp is satisfied before the humidity drops. Friends got a new system then a few weeks later left for 3 week vacation. Temp set at 80 or 82. I forget. Came home to mold everywhere.

Two stage systems only come in single ton capacities. For us down here it can work better as stage one is close to cooling and stage two is closer to heating load.
You are describing what I am fighting through right now. I have a 3 ton, 2 stage HP system in a 1800sq ft build. The wildcard in the build is I have 3 inches of closed cell foam pretty much shot all the way through, sealing the place to the slab. Add a silver steel roof and on a 95deg day the ac won't come up till mid- late afternoon. 95%-98% humidity is normal up here and the ac struggles to pull down the indoor humidity since it runs so little. The system is set up with a hard bias to the first stage and a 2 degree overshoot in dehumidifying mode. The ERV unit turned off so it doesn't drag in more humidity the place hovers around 70RH.

Last summer I ran a dehumidifier to keep the RH down in the 50s. On your friends situation we left last summer with the place at 50%RH and the stat set to 80 and the RH didn't spike and stayed stable. As soon as we got home just living, walking in and out caused the RH to jump back to 70 and i had to go back to the dehumidifier. Mold is a concern for me.

Rusty, after all this be looking hard at the ability for humidity control. I seem to be on the line where 3 Ton is just a touch big but a 2 Ton is a touch small. Ultimately I will probably have to go to a whole house dehumidifier system with ventilation. The ERV system with its passive dehumidifying system is not useful down here during the summer.
 
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You are describing what I am fighting through right now. I have a 3 ton, 2 stage HP system in a 1800sq ft build. The wildcard in the build is I have 3 inches of closed cell foam pretty much shot all the way through, sealing the place to the slab. Add a silver steel roof and on a 95deg day the ac won't come up till mid- late afternoon. 95%-98% humidity is normal up here and the ac struggles to pull down the indoor humidity since it runs so little. The system is set up with a hard bias to the first stage and a 2 degree overshoot in dehumidifying mode. The ERV unit turned off so it doesn't drag in more humidity the place hovers around 70RH.

Last summer I ran a dehumidifier to keep the RH down in the 50s. On your friends situation we left last summer with the place at 50%RH and the stat set to 80 and the RH didn't spike and stayed stable. As soon as we got home just living, walking in and out caused the RH to jump back to 70 and i had to go back to the dehumidifier. Mold is a concern for me.

Rusty, after all this be looking hard at the ability for humidity control. I seem to be on the line where 3 Ton is just a touch big but a 2 Ton is a touch small. Ultimately I will probably have to go to a whole house dehumidifier system with ventilation. The ERV system with its passive dehumidifying system is not useful down here during the summer.
One year ago I installed a whole house dehumidifier pulls from a return dumps into a supply after a back draft damper. Was it worth the extra 5k??? Maybe. I’m super glad we have it. In your case is should be set up as a ventilating dehumidifier bringing 100-150cfm of outside air in. (Mine needs this but I didn’t ask for it at time of installation). It’s really nice to have the house at 58%RH no matter the season. (It dropped to the 40%s during the cold snap this year) Mine runs every month of the year. You get some reheat in the summer and free heat in the shoulder seasons.
 
One year ago I installed a whole house dehumidifier pulls from a return dumps into a supply after a back draft damper. Was it worth the extra 5k??? Maybe. I’m super glad we have it. In your case is should be set up as a ventilating dehumidifier bringing 100-150cfm of outside air in. (Mine needs this but I didn’t ask for it at time of installation). It’s really nice to have the house at 58%RH no matter the season. (It dropped to the 40%s during the cold snap this year) Mine runs every month of the year. You get some reheat in the summer and free heat in the shoulder seasons.
I am going to move that direction soon. I the big picture I need to get the humidity under control, I don't want to saturate the structure of the house with moisture due to mold issues as time goes on.

We JUST got a lemon HP issue resolved 2 weeks ago so I didn't want to add confusion with the HVAC company. My HP has never produced heat and Trane Rep didn't want to ding his reputation with replacing it. The HVAC company took care of the replacement so now we are talking the WH dehumidifier. The owner feels bad about the system not dehumidifying well so I got a cost plus labor quote for $3800 for a 120pt system with ventilation. The outside air intake are already in place. I just need to pull the trigger on it.

We moved from a high humidity area but MAN summers are a different world all together up here.
 
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Interesting discussion, and seeing the woes some of youv'e had with humidity. I don't feel like this new system will be oversized at 3-tons, since that's my existing system and I haven't had a problem with humidity. And the new system will be two-stage, so capable of running at 70%.

My house is 1400 sq-ft, not counting a 300 sq-ft addition with its own minisplit. Built in 1988. Pretty well insulated, 9+" attic insulation. But, it's also meant to be somewhat passive solar, so there's a LOT (like 192 sq-ft) of glass on the south side of the big room, with 16ft vaulted ceiling.
 
Thanks for the thoughtful reply, @peakbagger . Sounds like you're agreeing, that if the latent capacity shown in the 71-degree IWB column is sufficient, and the sensible capacity shown in the 59-degree IWB column is sufficient, then I'm good to go.
 
Interesting discussion, and seeing the woes some of youv'e had with humidity. I don't feel like this new system will be oversized at 3-tons, since that's my existing system and I haven't had a problem with humidity. And the new system will be two-stage, so capable of running at 70%.

My house is 1400 sq-ft, not counting a 300 sq-ft addition with its own minisplit. Built in 1988. Pretty well insulated, 9+" attic insulation. But, it's also meant to be somewhat passive solar, so there's a LOT (like 192 sq-ft) of glass on the south side of the big room, with 16ft vaulted ceiling.
Rusty, sounds like you have the solution. Replacing an existing 3ton is the real kicker in the equation. The old one works, the new one will be better.
 
Given I am in heating dominant climate, I think some other "expert" should weigh in but its sounds right to me.

Lot to be said for adding exterior shading. The mylar based foil may not look great but it sure does a nice job on exterior windows.
 
Interesting discussion, and seeing the woes some of youv'e had with humidity. I don't feel like this new system will be oversized at 3-tons, since that's my existing system and I haven't had a problem with humidity. And the new system will be two-stage, so capable of running at 70%.

My house is 1400 sq-ft, not counting a 300 sq-ft addition with its own minisplit. Built in 1988. Pretty well insulated, 9+" attic insulation. But, it's also meant to be somewhat passive solar, so there's a LOT (like 192 sq-ft) of glass on the south side of the big room, with 16ft vaulted ceiling.
I wouldn’t recommend a 2 ton (unless you do a mini split in the big room). (3 ton single stage would be too big for cooling). It’s kinda a no brain for sizing. Can’t do 2 and 4 is way too big. Get a decent thermostat that sets second stage to come on when house is 2 degrees or more from set point. Address any humidity issues with a portable dehumidifier until you can’t take that anymore.

What is the air flow in stage 1?

Point of reference. My 2000 sq ft with 1100 sq ft basement in Wilmington built 1968 new windows original insulation, was cooled all last summer on stage 1 of a 3 ton unit.
 
Deciced to go with the two-stage Goodman GSZC7. Hvacdirect, seem like good folks, price-matched another site and then some.
 
Deciced to go with the two-stage Goodman GSZC7. Hvacdirect, seem like good folks, price-matched another site and then some.
Hvac direct is right down the road from where I lived. I used them alot and yes, they are good people to deal with.
 
Deciced to go with the two-stage Goodman GSZC7. Hvacdirect, seem like good folks, price-matched another site and then some.
I'm not sure any more what model Goodman I had in TN, but it looks a lot like mine.
I was told this was a "cheapo brand" (I guess they were trying to sell me a more expensive brand b/c the same margin would pay the installer more profit) - but it did very well for us for the 8 years after I owned it.

I had natural gas for the emergency heat (below 40 F...) instead of the resistive coils as we had a gas line in the road. That was a fantastic combo.
 
Dry air is very light so it takes far less energy to cool the air
Great explanation @peakbagger thanks, but I must take issue with the first part of the quoted sentence. The density of a gas is proportional to its molecular weight, and the molecular weight of water is 18. The molecular weight of dry air is roughly 29 (4 parts nitrogen at 28 and 1 part oxygen at 32). Nonetheless, of course the second part is true due to the heat released by condensation, as you describe).

I was amzed by the figure of 900 btu per pound of water condensed, That is, of course, the energy needed to raise the temperature of that same water by 900 degrees (though of course it would boil first)
 
I'm going to need to add an electrical circuit since I decided not to go with the inverter-type compressor. I already have 6awg running to a little outside subpanel, currently running the compressor on my existing dual-fuel system and a minisplit in an addition. I am sorely tempted to re-purpose that 6/2 cable for the air handler (including heat strips), and to run new 8/2 to the outside subpanel.

Here's the rub though. The MCA of the minisplit is 16 and of the new Goodman compressor it is 24.1; that sums up to 40.1 amps. Would I be crazy to feed that with 8/2, protected by a 40-amp OCPD of course ?