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Old 04-04-2012, 01:39 PM   #1
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Semi-DIY Central air?


Not just an out of the box question, there's a little more to it:

We are thinking about adding Central air, we got 3 estimates which were all very good, but at least 2 of them state I either need to get my own electrician out there to hook it up or have them bring theirs. I emailed guy # 3, but i have this funny feeling he will also tell me the same thing.

Now ... I have a very good relationship with my electrician and he gives me very good prices because I always use him, so that is definitely who i'll call.

So the DIY'er in me is now thinking ... "if I need to bring my own electrician, what else can i do?"

I have forced air heat, so the ductwork is already in place.

PO had the furnace replaced and for some reason had a coil added that just sits there. All 3 companies verified the coil can be used.

So ... is it a horrible idea to pour a concrete pad myself, buy a 2.5 ton compressor, put it on the pad, call my electrician to hook it up and THEN calling an HVAC company to come do the piping?

I'm just thinking out loud here, feel free to poke holes in my idea. Worst case I'll have the HVAC company do the whole thing except of course the electric. Boogie woogie woogie.

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Old 04-04-2012, 01:42 PM   #2
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Semi-DIY Central air?


Duct work first. In cooling mode the air moves much faster than in heating. You are going to need to upsize your duct work to accomodate or you'll have serious issues.

It's called a Manual D.

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Old 04-04-2012, 01:44 PM   #3
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Semi-DIY Central air?


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Originally Posted by Doc Holliday View Post
Duct work first. In cooling mode the air moves much faster than in heating. You are going to need to upsize your duct work to accomodate or you'll have serious issues.

It's called a Manual D.
Interesting. All 3 guys who looked at it said the ductwork was accurate
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Old 04-04-2012, 01:45 PM   #4
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Semi-DIY Central air?


Down here in Texas we can do all the electric but up in the northern states code requires that an electrican to not only run power to the system but hook up the system as well, both high and low voltage.
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Old 04-04-2012, 01:45 PM   #5
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Semi-DIY Central air?


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Interesting. All 3 guys who looked at it said the ductwork was accurate
Have you ever had cooling before? If not and the ducts were only sized for heating than it's not.
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Old 04-04-2012, 01:49 PM   #6
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Semi-DIY Central air?


If your duct work is too small the system is going to have a high static pressure and low air flow across the coil. Refrigerant will not evaporate properly, send liquid back to the compressor and kill it.

All the while your air temperature will not be correct, too warm.
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Old 04-04-2012, 01:49 PM   #7
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Have you ever had cooling before? If not and the ducts were only sized for heating than it's not.
As far as I know there's never been cooling. I'm almost certain the PO just added the coil to the furnace to make it easier on herself or future owners to add it.

I find it very strange that 3 out of 3 stated my ductwork was accurate.
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Old 04-04-2012, 01:51 PM   #8
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Semi-DIY Central air?


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If your duc work is too small the system is going to have a high static pressure and low air flow across the coil. Refrigerant will not evaporate properly, send liquid back to the compressor and kill it.
OK - reason enough to have someone do the whole thing right there.
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Old 04-04-2012, 01:59 PM   #9
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Semi-DIY Central air?


Yup. While it may not be rocket science it is very technical. One application has to perfectly exist with another to have a completely and properly running hvac system.
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Old 04-04-2012, 01:59 PM   #10
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Semi-DIY Central air?


There are a couple of variables involved.

If you're the homeowner you can pull permits and do the electrical, not necessarily the actual hookup, as some municipalities will not allow you to physically put the unit in place minus a license.

Some municipalities will allow homeowner to do the ductwork. Of course everything has to be to code and if they come to beleive you are incapable, they can compel you to hire a professional to finish.

Just have good plans drawn out and speak with your licensing/inspection department. That's about it.

-------------------------------------------------------------------------------------

This part is questionable. I have read some municipalities may allow homeowners to work with units that have the newer R410a gas. It isn't EPA mandated certification required, see HERE.

As such, you could possibly do a full install which may apply to geothermal systems as well. You would most likely void any manufacturer warranty though, as they require system integrity/certification by licensed personel. You may can do some but would need to know what certified folks must perform to apply for warranty.

Good luck
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Old 04-04-2012, 02:08 PM   #11
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Semi-DIY Central air?


Ductwork is going to add a whole different level of complexity to this job. One that I'm not ready to deal with and pay for this year. If the ductwork is really undersized for a/c then this isn't happening for a while if ever. The ease of the job (existing ductwork, coil already there etc.) is what made us consider it.

I'm still blown away by the fact that 3 companies all stated the ductwork was accurate. Unless it really WAS put in place with a/c in mind. I know it was replaced in the 50's or 60's, at least that's what my heating guy says.
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Old 04-04-2012, 02:12 PM   #12
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Semi-DIY Central air?


http://www.johnrwhite.net/three%20st...t%20sizing.htm
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Old 04-04-2012, 02:21 PM   #13
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Semi-DIY Central air?


Sure looks like it's close to rocket science. This is for heat pump systems but it'll still give you a good idea.

http://efficientcomfort.net/Rules_an...ct_Systems.pdf
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Old 04-04-2012, 02:42 PM   #14
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Semi-DIY Central air?


Quote:
Originally Posted by CoconutPete View Post
Ductwork is going to add a whole different level of complexity to this job. One that I'm not ready to deal with and pay for this year. If the ductwork is really undersized for a/c then this isn't happening for a while if ever. The ease of the job (existing ductwork, coil already there etc.) is what made us consider it.

I'm still blown away by the fact that 3 companies all stated the ductwork was accurate. Unless it really WAS put in place with a/c in mind. I know it was replaced in the 50's or 60's, at least that's what my heating guy says.

You can always call all three contractors back and ask them how they came to the conclusion that the duct work was correct to accomodate cooling. Ask about the differences in cfm for heating and cooling, about Manual D's. You may get lucky and it turns out the duct work was installed in anticipation of cooling.
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Old 04-04-2012, 03:16 PM   #15
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Semi-DIY Central air?


Quote:
Originally Posted by Doc Holliday View Post
Down here in Texas we can do all the electric but up in the northern states code requires that an electrican to not only run power to the system but hook up the system as well, both high and low voltage.
Strange. I can run my own electric. And don't need to be an electrician. Maybe your thinking of Washington state.

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