PennSilverTaco's HVAC BS MegaThread!

I've been talking with a coworker about homebuilding a lot recently; This particular coworker has a lot of experience in construction. The other day, he informed me that it might be more cost-effective to build a house from scratch than to buy an existing one. The rough estimate was that if I did a lot of the work myself and/or with the help of friends (I've got friends and relatives who know a lot about this kind of thing), I could have myself a 3-bedroom, 1,500 square foot, 1-story home with central A/C and granite kitchen countertops for as little as $215K!

Anyway, I've also been looking at smaller 2-story home plans, and after having lived in numerous 2-story homes that only have one central air-conditioner I can tell you that I'd prefer to have separate units for the first and second floor. One plan I've been looking at is a Frank Betz plan with just under 1,300 square feet; I told my coworker that if I built this specific plan, I'd go with dual 1.5-ton central A/C units as opposed to one larger unit (likely 3 or 3.5 tons). He said this would be overkill, and I'd be better off with one larger unit with the furnace/air handler in the attic that had two separate zones controlled by individual thermostats. When I asked in the title "How much A/C and heat does a house need?", I understand that a lot goes into properly sizing a residential HVAC system; HVAC has been an obsession of mine for over a decade and I know a thing or two about it.

I am asking this question because my coworker (by the way, we both work for a defense contractor that builds UAVs) and I couldn't agree on which system would be better.

Here's the floorplan in question: https://www.frankbetzhouseplans.com/plan-details/Watkins

 

not sure your exact question but, https://www.calculator.net/btu-calculator.html

 

In my area, a house of this size would require about a 3-ton A/C unit or heat pump, and a 60,000-BTU furnace if the house had gas heat. My friend @Gwyns04 lives in a 1-story home of just under 1,300 square feet that has a 2.5-ton air-conditioner; I'd add the extra 6,000 BTUs of cooling just to give the system a little extra oomph for the second floor.

Both the first and second levels of this house are under 700 square feet, which wouldn't require much; This would likely be dual 1.5-ton units, or a 2-ton for the 1st floor and a 1.5-ton for the 2nd floor...

 

Would two separate A/C units be overkill for a house this small? I don't think it would...

I already know the specific cooling requirements...

 

I called it; 3 to 3.5 tons of cooling!

 

3 ton A/C sounds huge for a 1,300sf house. 1.5-2tons max. maybe in texas you need 3 tons.

There are many ways to make a home comfortable. Most do not want to spend the money to make it comfortable.

Most comfortable is to have radiant inslab heating/cooling with a dedicated outside air unit (DOAS) for fresh air introduction/stale air removal. Good to have an A/C on this unit for dehumidification purposes. Then run a boiler for the inslab heating and for the fancoil unit heating coil. If you want to run inslab cooling, run a small chiller. If you want to go net zero energy, drill a geothermal (ground source heat pump) well and run both a water to water heatpump for radiant heat/cool and a water to air heatpump as a DOAS. Run as many temperature control zones as you can afford.

Now, the best way to do it cheap but comfortable, is to run one Furnace/AC, with multiple zones. If you want to get very comfortable, you can add a reheat coil on the zones to further temper the air to the perfect setpoint. Two furnaces is overkill, nearly double the price and double the maintenance.

If you need to control each room separately, run a VRF system with an evaporator in each room (like what you see in a mid-grade hotel). These are a bit noisier mind you.

Also, insulation and window quality make a huge difference. More insulation / triple glazed windows will cut your heating and cooling loads significantly.

 

I can understand the desire for redundancy in having two units if one fails you have the other...

these units are not prone to fail really in my opinion so, its easier to plumb in one unit nice a tidy than it is two separate units...

but shit if you lost electricity than neither unit runs..

 

Like I said, my friend and I both live in Southeast Pennsylvania, and her less-than-1,300 square foot 1-story home has a 2.5-ton unit. Also, do a lot of houses have central air in Toronto?

 

Cool - i was in West Chester back in 2007 for Tacoberfest - that was a blast.

Prettymuch every house built since the 60's has A/C up here. It can get awfully hot & humid in the summer.

If a house has minimal insulation, and single pane (or old double pane) windows, i could see a need to upsize an A/C unit.

My house is 1900sf and i have 2ton A/C unit. If you go with a larger unit, it can "short cycle". You can read about this online. Here is a simple calculator with good info https://hvacdirect.com/sizing-air-conditioner-and-heater.html
A Warning Before You Start Sizing Your Air Conditioner: Do not to make the mistake of buying an A/C that's too large! When you install an air conditioner that's too large for the space, it does what is called a 'short cycle'. A short cycle is when the compressor doesn't run for enough time to dehumidify the area you're trying to climate control. A short cycling A/C system isn't the only concern; in most cases the system will cycle on and off more often, this translates to an increase in operating costs and an overall reduction in the system's life. Proper air conditioner sizing is critical to optimize performance.

 

My parents' old house was a 2-story, 3-bedroom end unit townhouse; It was about 2,500 square feet total, maybe a tiny but more. One 3.5-ton central A/C was standard, and my parents originally did not want to spend the extra $6,000 for a second system (the house was built in 2007, so it is likely more expensive), but after being inside the model on a hot day they quickly changed their mind; They spent the six grand and it go us dual 2-ton A/C units matched with dual 40,000 BTU gas furnaces. My bedroom was located directly above the garage, and it was FREEZING even in the middle of a heatwave; I LOVED IT!

Funny you mention the "if one unit fails" scenario; The run capacitor on the upstairs unit went in the summer of 2014, and we had to wait a night for the part to come in. I experimented once and found that the upstairs unit could keep the whole house tolerable, but since cold air falls, the downstairs unit was utterly useless for keeping the whole house comfortable. I sleep great when it's cold, but not when it's warm...

 

My aunt works for the CDC and lived in Kenya from 2016 until last year; Before she moved to Kenya, she lived in the Atlanta suburbs. Her house was a 2-bedroom brick rancher built in the 1950s. It had a straight-cool central air-conditioner and a gas furnace. I have no idea if the house originally had central air, but considering it was in Georgia I would not be surprised; My aunt bought the house in 2006, and the A/C unit that came with it was from 1996 according to the serial number. The previous owner's dog had lifted its leg on the poor thing, and the urine had eaten away the coil. The unit was a Goodman/Janitrol builder-grade unit, about 10 SEER, and I'm amazed it ran as good as it did because these things are junk in my opinion! So, the unit was a 4-ton, and even in Georgia it was oversized for the house! My aunt stated that house felt "cold but clammy", which is an obvious indication of a too-large air-conditioner; Basically, the unit had no problem getting the house to the desired temperature, but it reached that desired temperature so quickly that it didn't have time to properly dehumidify the house. The run capacitor went in about 2014, and one contractor was ready to sell my aunt a new Trane system; Trane is great, but not cheap...

I told my aunt that based on the symptoms she was describing, it was likely the run capacitor and that she should get a second opinion. She did exactly that, and go figure, it was the damn run cap. The system was ten years old when my aunt bought the house in 2006, fifteen years old when I saw it in person for the first time, and twenty years old when my aunt sold the house in 2016. The house passed the pre-purchase inspection and as far as I know was still going strong when the new owner took delivery; I would be surprised if that thing is still there now though!

 

It might be better to focus on the efficiency of the house first. Max out your R values first then size the unit.

 

For the record, my parents' current house is a roughly 2,900 square foot colonial, 2 stories with 4 bedrooms, 2.5 bathrooms, and an unfinished basement. It has a 2.5-ton Goodman A/C unit for the first floor, and a 2-ton unit for the upstairs. Each unit is matched with a 92% efficiency gas furnace, and I don't know what size the furnaces are. What I can tell you is that due to an issue with the flex duct in the attic, my room can get incredibly warm during the summer unless the thermostat is set to 66-67 degrees.

 

I wouldn't put much faith in that calculator. It has very generic inputs and doesn't even consider single story vs multi-story dwellings.

 

The first calculator has room-by-room, first floor, second floor, and even whole-house calculations...

 

If you are looking at building, take a hard look at manufactured housing. The quality is probably better than houses built on site, since they generally have pretty decent quality control. General contractors are only as good as the subcontractors they use, and that can be a crapshoot. The reason I mention that is they are manufactured with SIP panels, which are generally better insulated. That reduces your heating and AC requirements.

If your state is fully embracing "green energy," probably smart to add a wood stove. Just look at Texas for a perfect example at what happens when you place too much emphasis on wind generators and solar panels.

 

YouTube is blocked here at work, but google: "Matt Risinger builder's own insanely efficient house!" and watch that video. This builder heats and cools his entire house (I think its over 1800 sq ft, cant remember exactly) on one of those Mitsubishi wall mounted AC's in the kitchen. So it depends on how efficient you build your house but remember that this level of efficiency will come with a price. Plan on spending more up front for an efficient house to spend less over the course of owning the house. Bear in mind that the average time a home owner lives in their home is ballpark of 5-7 years. Unless you plan on hunkering down for 25+ years, you may not see the return on investment. Just something to keep in mind. Matt Risinger is one of those building science guys that is always looking at different home building methods and technologies so its a good place to get some ideas, Lord knows that I've already jotted down a bunch.

-J

 

My house is roughly that same size and just had the new AC and Furnace installed last year and I already forgot the size, think it was either 2 or 2.5 Ton AC.

 

Just something to think about when it comes to one versus two units. Redundancy.

Lots of reasons, pro and con, but when something breaks, summer or winter, it's nice to have a second unit still working. No, it probably won't handle the whole house, but half a cool/warm house is better than nothing. For heat, I want two sources of heat as well. Preferably one that does not rely on electricity.

Our current house has over 4,000sf and two floors. The one unit we have is more than big enough and works well, when it works. It has broken in the hottest part of the summer and the coldest part of the winter. It is not always convenient to get it fixed for several days. When we build our retirement home, it will have two smaller units, even if it does cost more up front and is overkill.

 

You have a 4,000 square foot house in Arkansas and it only has one central air-conditioner?!?!