Author Topic: How hard to DIY install a furnace?  (Read 403 times)

Jon Bon

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How hard to DIY install a furnace?
« on: January 31, 2019, 05:16:31 AM »
As  I am dealing with these terrible horrible very bad temperatures. It makes me think a few of my furnaces might be ticking time bombs. They are probably > 20 years old and had a rough life (renters and poor maintenance)

So it makes me think if they make it until spring/summer and I have a few nice days of 75 degree weather I might give it a shot myself.

I am pretty confident in dealing with the electrical and the gas piping. I have experience with both and I am confident I can do those correctly and safely. I've worked with sheet metal and pvc before as well. A few questions for anyone has attempted this.

1. Wiring - do you just plug it in? Do you have to mess with the motherboard or any type of wiring set up (outside of hooking up to the 120V)
2. AC - what do you do with the AC coils?
3. Plenum - do you just connect the furnace to AC by fabricating a box out of sheet metal?

What other wisdom can you leave me?


405programmer

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Re: How hard to DIY install a furnace?
« Reply #1 on: January 31, 2019, 06:22:51 AM »
Disclaimer, I was an HVAC apprentice for 2 summers in college when my grandfather still ran an HVAC company. So I'm very far from an expert but I do remember a thing or two about HVAC.
 
If your AC system is also 20 years old you might consider replacing the whole system if you replace the central air handler. In order to take the AC coil out of the central air handler you'll need to pump down refrigerant using a vacuum pump and then cut the copper lines near the coil. The coil then just slides out from the air handler. The problem comes in reassembly. Some coolant is always lost in the pumping down / cutting the lines so you'd need to add some more after it's all hooked up. That old style coolant R22 is not allow to be manufactured anymore so the prices continue to rise as supply decreases. That means a couple pounds of the R22 coolant could be 50% the cost of a new 140a type coolant system.

Okay that's likely an exaggeration but definitely price stuff like that out before you start chopping on your central air handler. The easiest thing you could replace if you're worried about reliability are the heating element and blower motor. Usually, though not always, you can replace the heating element and the blower motor (fan motor) without removing your A-coil. Then again replacing something like a fan motor when the current one is working fine is a waste. What is your end goal of the furnace upgrade? If efficiency, upgrade it all at once and get a more efficient AC and furnace. If peace of mind, stash some money in your high interest savings account and have a couple of local HVAC contractors in your contact list so you can find help when it finally dies on the coldest / hottest day of the year.

To your detailed questions:
1 Wiring: You'll need to wire the thermostat, might involve pulling 5 wire thermostat bundle instead of 3 or 4 wire bundles. Just depends on model. Usually the 120V connection is hard wired to a dedicated breaker. Some furnaces have plugs though. Depends on local codes.
2 AC: Likely need to cut copper lines to remove. Depends on space available. If you have room on both sides in an attic then you might be able to gently bend the copper tubing and set the A coil next to the air handler. If you cut, you must pump first. BTW trying to bend 20 year old copper pressurized with horrible-for-the-environment toxic gas is a 100% bad idea. It might work though.
3 plenum: try to get as close as you can to the existing plenum's size opening. It's usually impossible for a perfect fit. You'll make an expander / reducer out of sheet metal. Duck bill pliers and tin snips make the job easier but there are many ways to get this part done. Usually in a tight space so requires some creativity. Some seal with foil tape but I'd suggest sheet metal screws and seal seams with duct mastic.

Again if you're replacing a furnace that has the Air conditioner coil inside the air handler as one unit, it's often easier to replace the whole mess together. If you size up or size down your old A coil might not fit. Air follows the path of least resistance so if there's a gap next to your A coil in the new handler box, most of the air is just going to run right past the coil and not cool down efficiently. Full HVAC replacement is definitely hard DIY work because of the combination of systems involved. It's possible to get an understanding of electrical, gas, refrigerant, plumbing (drain lines) but here is a case where they all come together usually in an attic with limited space. So it's not like adding a ceiling fan where you flip a breaker, mount a box and twist some wires. This is something that takes much more planning.

My advice? Go down to the HVAC specialty store or start calling around. There's probably someone who can help you DIY this as a "moonlighter". You will be 100% responsible to pull permits yourself and accept all liability yourself but you can pay the moonlighter to use his/her specialty tools and experience to make the whole process easier. Unless you have a large shop already there are going to be some new tools you'll need.
« Last Edit: January 31, 2019, 06:33:58 AM by 405programmer »

Jon Bon

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Re: How hard to DIY install a furnace?
« Reply #2 on: January 31, 2019, 06:31:08 AM »
@405programmer  Thanks this is awesome stuff.

Ok so everything you said sounds pretty dang reasonable. I think that I can do this without touching the AC. As I dont want to mess with refrigerant and running an empty AC unit can ruin it because if you are dumb and let all the stuff leak out!

I think all the furnaces in my area have the A-coil above the furnace. I have seen a few videos where they strap the AC unit to the joists with hangers, then they are able to remove the furnace from below. Does this sound feasible?

In terms of replacement v repair I have a GREAT hvac guy. But even he gets super busy in the middle of winter. I tend to buy shitty old houses with shitty old furnaces of unknown age, reliability, and maintenance. So for peace of mind it makes sense to replace.

My thought process on this is I can wait til it breaks (which I do with most things) or be proactive. I just really dont have to have a no heat situation in a rental. I can put up with space heaters and 24 hours of 50 degrees in my own house but id rather not have to do that to my tenants. The cost of my HVAC guy would be 2500, im thinking I could get the furance for $800 and maybe another $200 in parts. Feels like good savings to me?






bw_94

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Re: How hard to DIY install a furnace?
« Reply #3 on: January 31, 2019, 06:35:00 AM »
Jon Bon, I am also thinking of doing the same come spring time. Furnaces seem to be a little more than $800 depending on efficiency and all, too.

Another thing to note, if you are replacing an 80% efficient furnace with a high efficiency one, you will need to run new PVC vent/combustion air. And if you're replacing a high efficiency with an 80%, you will not be able to re-use the existing PVC vent/combustion air piping.

Let me know how it goes for you! Good luck.

405programmer

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Re: How hard to DIY install a furnace?
« Reply #4 on: January 31, 2019, 06:40:06 AM »
Dang! 2500 for just a furnace install? Usually the contractor price is roughly parts X2.

Yes I do remember doing some rigging to suspend coils / plenums from the joists while we switched out other parts. My job was usually re-wiring the fan motor because those haven't changed much in 20 years (basically match the colors).

For timing I'd give yourself closer to a week the very first time you do this and I bet from then one you could do the job in under a weekend ;)

Good luck!

Jon Bon

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Re: How hard to DIY install a furnace?
« Reply #5 on: January 31, 2019, 06:47:09 AM »
Dang! 2500 for just a furnace install? Usually the contractor price is roughly parts X2.

Yes I do remember doing some rigging to suspend coils / plenums from the joists while we switched out other parts. My job was usually re-wiring the fan motor because those haven't changed much in 20 years (basically match the colors).

For timing I'd give yourself closer to a week the very first time you do this and I bet from then one you could do the job in under a weekend ;)

Good luck!

Lol yeah like I said hes a good dude. Not a super high end unit. But enough for 20 years of relatively worry free operation.

Yup, I have a 90, gonna replace with a 90. Buddy has an 80 who wants to DIY as well, so we will do the PVC out the side of the house for that.

Ill check back in the spring when I give this the old college try!


Papa bear

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Re: How hard to DIY install a furnace?
« Reply #6 on: January 31, 2019, 06:58:18 AM »
I’ll help you with those furnaces if you help me with my mini splits.


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Jon Bon

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Re: How hard to DIY install a furnace?
« Reply #7 on: January 31, 2019, 07:00:52 AM »
Iíll help you with those furnaces if you help me with my mini splits.


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DEAL!