HVAC Unit Sizing for 2,600 SF?

[WranglerBowman]

Hi, I have a 2,600 SF home that is 2 levels (1,300 each, full basement and 1st floor) with an older oil furnace and 1997 York 230V 26.1 amp AC condensor, all ducted, that I want to replace with an all electric energy star rated unit.  I'm in the mid-Atlantic region, mild winters.  Based on what I've read it looks like I will need a 5 ton unit, but I also want to be sure I'd qualify for the federal tax credits. Not sure if there's a SEER rating you have to meet, as i've read both.  I would like to DIY this as much as I can but don't mind hiring to charge the unit.  Can you all great people help me find a unit at a good price based on the above?  Thanks. 

[index]

You don't have nearly enough information. What size and efficiency is your Oil Furnace (BTUs)? what size is your AC? How old is your house? The best way I know to calculate your needs is look at your usage on a cold day and on a hot day using a smart thermostat. For example-

If your coldest day last year was 0F and your furnace is 80% efficient, 100k BTU and ran 10 hours - you need 800K BTU in 24 hours from a heat pump to maintain temperature. That is 33k BTU per hour so a 3 ton (36k BTU) unit could keep up. Its a little more coplicated though because you have to look at the heating curves and make sure the unit runs at 0F and determine the rating. Some 3 ton heat pumps may only put out 18k BTUs at 0 which means you need heat strips or other auxiliary heating.

You can do the same thing for summer.

The issue with heat pumps is when you have unbalanced heating/cooling loads like you need 2 tons in the summer and 3 tons in the winter. You end up short cycling in the summer, have poor humidity control and the unit will wear out a lot faster. You can solve this (if your summer to winter needs are not too drastic) by buying an inverter unit.

The tax credit requires a minimum seer rating (cooling) and HSPF (heating) depending on where you are located. Google is your friend. If you really need a 5 ton unit, you are going to have a hard time finding one that qualifies because efficiency goes down as the unit size increases. You will probably need to do a dual zone.

Have you ever installed an AC before? You have to do sheet metal work, electrical, plumbing, and flaring/brazing. You are going to have a hell of a time finding someone to charge the unit for you as well, so count on spending $500 on the necessary equipment (vacuum pump, micron gauge, various hoses, valve puller etc.) For reference, I've installed 4 mini-splits, a ducted mini-split, and a furnace and I've been putting off installing a central unit because I know it is going to be a PITA. Alpine Air sells decent high efficiency units to the public and states where they qualify for tax credits - look there.

[WranglerBowman]

@index Thanks for what you provided so far.  To help clarify the house was built in 1977, so not the best insulation or construction.  Current oil furnace (which I want to replace with electric) is 100k btu's at 85% efficiency.  I don't know what size the ac unit as I don't see that anywhere on the condenser.  Only tag on the unit lists "350PSIG High Side, 300PSIG Low Side, 208/230V 1PH 60HZ 19.8RLA 97.0 LRA".  I'm in USDA Planting Zone 7b.  I don't have a smart thermostat but we typically keep the house at 78F in the summer (hot days the AC runs non stop from sun up to midnight, and in the winter it's set to 67F, but the oil furnace rarely runs (less than 40 gals a year) because we primarily heat with a wood stove.

Good to know about the inverter and demand between summer and winter, this is a learning experience for me.  Civil Engineer by trade but also do electrical, plumbing, metal work (welding/brazing), etc. for myself, so handy capable, but this is out of my experience realm.  Planning to stay away from dual zone just because the house already has duct work that's not separated.  I hate to hire people because if I do I get a quote for $9k-$15k for a unit and installation, only to find out that the unit is by itself $4,500 and they just charged me $7k for a days worth of work for 2 people to do a half azz job... 

[GilesMM]

I would get 3 free estimates from qualified techs in order to help size and spec the equipment.  Insist they do a proper manual J calculation which takes all variables into account.  If the balk, get a different tech.   Ask them all your questions.  Are you going to be going your own refrigerant work?  Have you considered a heat pump? Does your jurisdiction allow DIY on HVAC?


https://www.servicetitan.com/tools/hvac-load-calculator

[WranglerBowman]

@GilesMM I think I will get estimates and see what the equipment sizes come back with.  I'll look at the Manual J Calc and get familiar with it.  I was NOT planning to do refrigerant work.  Heat pump is what I'm planning on and our County will allow DIY.

[GilesMM]

@GilesMM I think I will get estimates and see what the equipment sizes come back with.  I'll look at the Manual J Calc and get familiar with it.  I was NOT planning to do refrigerant work.  Heat pump is what I'm planning on and our County will allow DIY.

You are planning on self-installing a heat pump that does not use any refrigerant?  Or do you mean you are planning on an electric heat-strip type furnace?

[index]

@GilesMM you are not going to find a HVAC company that does a true load calc as part of a free estimate. @WranglerBowman Best to pay $500 and get a real load calc done or buy an ecobee and push this off until next year once you have data to size your system.

[index]

@WranglerBowman the tax credit is for 30% up to $2k for a qualifying system. The only central inverter systems with flare fittings are from Mr. Cool and Blueridge (alpine) and of those, assuming you live south of PN and WV, only 2 ton systems and smaller qualify. Based on the size of your house, you probably need a 3 ton for heating capacity or a 2 to 2.5 ton for cooling. You could add heat strips to a 2 ton and qualify for the credit and probably make it work on cooling as long as you are ok with it having a little trouble keeping up on days in the high 90's. You probably won't use the heat strips much as you have the wood stove for auxilary heating, but you need them incase you are out of town to prevent a freeze situation. 

https://www.alpinehomeair.com/product/furnaces-heaters/ultra-efficient-ducted-heat-pump-air-handler-complete-systems/blueridge/bmah2420-kit

2T system kit is $3k
Heat strip $160
Return plenum $400
Electric wire, whip, conduit, breakers - $200
Sheet metal - $100
Tools (lineset bender and flare tool) - $150

Commissioning (Vac and release) - $500 on FB marketplace if you are lucky (no rebate). If you diy this part, you need about $600 worth of tools.

$4.5k or $3.3k after tax incentive - This is going to take you a couple days of research (YouTube university) and two full days to install. The system will not be exactly what you need. For that you need to wait a year with an Ecobee ($120) or pay $500 for a load calc. Assuming you need a 3 ton unit, you are looking at $5500 using the same math and no rebate. You will have no warranty and if you mess up a flare, you are looking at about $1.5K to get it fixed (reflared and recharged by a HVAC shop). 

Compare this to a 10k quote for a 3 Ton unit that will qualify for the $2k rebate (8k after rebate). So you are saving $2.5k roughly by spending 4 days learning and installing and you get a full warranty if something is messed up.   

If you had a couple of minisplit installs under your belt, I'd say go for it, but in your possition I'd see if you can get a qualifying unit installed for 10k or less. It's a lot of work and risk to save 2.5k.

The math works better for minisplits where HVAC companies charge $4k a pop for $1.5k units+materials + $600 in tools that are much easier to DIY. I.e. you save about $2k on your first install and max risk is having to replace half of a $1000 unit.   


 

[GilesMM]

@GilesMM you are not going to find a HVAC company that does a true load calc as part of a free estimate. @WranglerBowman Best to pay $500 and get a real load calc done or buy an ecobee and push this off until next year once you have data to size your system.

I've never had a problem requiring it as they can't probably estimate what equipment you need without.  Some will try to base it off the existing equipment or a rule of thumb.  The often upsize as they make more money off you and with a calc you have no means to refute them.

[index]

@GilesMM you are not going to find a HVAC company that does a true load calc as part of a free estimate. @WranglerBowman Best to pay $500 and get a real load calc done or buy an ecobee and push this off until next year once you have data to size your system.

I've never had a problem requiring it as they can't probably estimate what equipment you need without.  Some will try to base it off the existing equipment or a rule of thumb.  The often upsize as they make more money off you and with a calc you have no means to refute them.

Yeah a real manual J calc. is a couple hours of work which is why you are going to have to pay for it. The SF + windows and insulation estimates is not a real load calc. You can do much better with your own thermostat. Systems are chronically over capacity because nobody wants a pissed off customer when their house is 60F when there is a 1/10 year cold snap. Bigger is always better - Murica.

[Woodshark]

You can run your own manual J online with https://www.loadcalc.net/ or Coolcalc.com.

I've done it and it not too hard. Our basement system was oversized so when it died I had four HVAC companies come out for estimates. All four never did a load calc, just wanted to replace with the same size or larger. I finally did the calc myself and found I need one less than 1/2 the size of the original. I had the new smaller unit installed. It's been 14 months and it's perfect.

[JLee]

I'd highly recommend doing an energy efficiency consultation and do insulation / air sealing before sizing your HVAC equipment (plus saving costs / energy in the long run).

The combination of insulation/air sealing and going from a ~20yo boiler to a Navien direct vent combi boiler / water heater cut my winter natural gas use by about 30% (NJ).

[jessetayriver]

Hi, I have a 2,600 SF home that is 2 levels (1,300 each, full basement and 1st floor) with an older oil furnace and 1997 York 230V 26.1 amp AC condensor, all ducted, that I want to replace with an all electric energy star rated unit.  I'm in the mid-Atlantic region, mild winters.  Based on what I've read it looks like I will need a 5 ton unit, but I also want to be sure I'd qualify for the federal tax credits. Not sure if there's a SEER rating you have to meet, as i've read both.  I would like to DIY this as much as I can but don't mind hiring to charge the unit.  Can you all great people help me find a unit at a good price based on the above?  Thanks.

Hey I just posted about HVAC sizing here: https://forum.mrmoneymustache.com/do-it-yourself-forum!/sizing-of-replacement-furnaces-and-boilers/

In principle ASHPs should be sized to cooling and not heating and there's not really a great way to size cooling using historic data. This implication of this is that good ashp conversions are homes in which cooling and heating demand are roughly equivalent in btu/hr. Regardless, I often use the spreadsheet for ashp conversions in older homes to see if there's a big discrepancy between Man J and spreadsheet calculated heat demand.

In many years of working on HVAC design it's vastly more likely that an individual will oversize vs undersize. In most things built post ww2 I'll generally try to drive heat loss at design temp to <40kbtu/hr first via (in order) conditioning ducts in attics and foundation crawlspaces, air sealing attics & adding insulation to attics.