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Discussion Starter · #1 ·
I'm putting a new down flow air handler in my house and am trying to get the wiring done asap. Originally had a 10kw heat kit which had a set of two breakers and electrician said would require a 6/2 run. Now I'm thinking of bumping up to 15kw or 20kw. If I go 20kw would I likely just do a second 6/2 run from the panel and the heat kit would have 4 breakers? Also wondering if standard procedure is to run NM-B through crawl space to junction box on joist under unit then come up through floor with THHN in conduit and into furnace?
 

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Code violation to run NM along the under side of a joist. Its not protected when you do that.


Are you increasing the size of all of your duct work to use a 20 KW strip heater? Why are you up sizing the heat kit, did you add on to the house?


For 20 KW, it would be 2-6/2 wire runs, and 2 60amp double pole breakers.
 
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Upsizing is not an upgrade especially when keeping the duct sizes the same.

The smaller heat kits need less airflow -> it could get very noisy with 15 or 20 kw and fan speed set accordingly; worst case, there's insufficient airflow and they cycle on limit.

More capacity makes cycles shorter which reduces comfort.

A real upgrade would be putting a heatpump so you use a lot less strip heat.
 
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Discussion Starter · #4 ·
The house after addition is going to total about 2100sqft which according to the BTU calculators desires around a 20kw heat kit. I am also putting in a 5 ton heat pump which will do most of the heating so maybe a 10kw strip is enough to bridge the gap when heat pump is in the defrost mode? My hvac service guy thinks not and that I should go with a 15kw minimum. As far as the wire goes in crawlspace, I thought larger size wire like 6/2 could in fact be run across the bottom of joists but I'll investigate that further. I'll also check into the duct sizing that was designed by a friend of mine that does commercial hvac to see if it's adequate for larger heat kit.
 

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What calculator did you use? Don't size by sq ft, you need a load calculation if it hasn't been done.

What kind of climate climate are you in?

The heat strips need to be sized for the entire heating load in case heatpump breaks down.

The 5 ton heatpump may be really lousy for dehumidifying and needs very large air ducts to get proper airflow.

When an addition is done either the entire duct system gets re-designed from the ground up (so the trunk lines are properly sized for the branches connected) or the addition gets it's own source of heat.

Mini-split heatpumps can fit the bill when the addition is too small for a conventional system and or it's impractical to runs ducts. They maintain heating capacity better in cold weather than most central heatpumps but even using them, it's advisable to put some baseboard heaters in just in case.
 

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Discussion Starter · #6 ·
What calculator did you use? Don't size by sq ft, you need a load calculation if it hasn't been done.

I did sqft times 40 to get BTUs, then divided by 3400 to convert to KWs.

What kind of climate climate are you in?

Climate zone 3.

Also have a nice Lopi wood stove as backup and to take the edge off on coldest of days.

As I mentioned, the ductwork has been redesigned.
 

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Doing it that way, would come out to 25KWs of strip heat. Doubt you need anything over 15 KW.


How big was the house before you did the addition.
 
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I did sqft times 40 to get BTUs, then divided by 3400 to convert to KWs.
Rules of thumb don't work and result in oversizing.

Probably don't need more than 15kw but depends on design temp and house construction.

Heat strips can be staged and regardless of what size installed, it's a good idea to use a 2-stage thermostat and keep the full kit off unless really required.

If you go heatpump, a higher end stat will be needed to do that and stage the backup. But well worth the extra money.

As I mentioned, the ductwork has been redesigned.
Not necessarily properly - need to know actual btu's per room.
 

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Discussion Starter · #10 ·
Rules of thumb don't work and result in oversizing.

Probably don't need more than 15kw but depends on design temp and house construction.

Heat strips can be staged and regardless of what size installed, it's a good idea to use a 2-stage thermostat and keep the full kit off unless really required.

If you go heatpump, a higher end stat will be needed to do that and stage the backup. But well worth the extra money.



Not necessarily properly - need to know actual btu's per room.

Good to know about the staging option, thanks! I'll quiz my buddy who helped me design the system about BTUs per room this weekend when we finally drop in the plenum!
 

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The design of the duct system dictates how much capacity can be put in. (really, it should be done the other way around with the capacity and bTus of each room known before doing the duct system)

In a very dry climate it's okay to oversize a heatpump to reduce use of strip heat. But 5 tons is a stretch - maybe 3 or 4.

Oregon had dry mild summers with mild winters, right?
 

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Discussion Starter · #13 ·
Then it didn't go from needing 10 KW to needing 20 KW.


Probably don't need 5 ton of cooling either.
We had an oil furnace before starting the remodel which I'm missing already so never had an electric furnace of any kind. Guy at the HVAC store originally threw a 10kw on the order and my friend didn't notice but agrees now that's a bit undersized for 2000sqft. Also talked to my HVAC service guy who's father owned the business before him and had done the installation of the oil furnace on this house for previous owners waaaaaaaaay back in the 20th century:wink2:and he definitely thinks I should be running a 20kw kit.

As far as the 5 ton goes it's not for the cooling except for maybe a couple of weeks each summer but for the benefit of a high HSPF. However, I believe you sir are right and that 5 ton is oversized for this house (another oversight on my friend's part) and I'm in the process of trying to change the order to a 4 ton.

I'm not complaining and know I should have taken the time to do more research or any for that matter on the front end but the ball is rolling and though its best to learn from other's mistakes, that's not really how it happens.:vs_smirk:
 

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Just keep in mind, someone that has a lot of experience doesn’t necessarily mean that they are right in their advice.
It could mean they’ve been doing it wrong for a really long time.
Without a load calculation you don’t really know what you need.
 

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Also talked to my HVAC service guy who's father owned the business before him and had done the installation of the oil furnace on this house for previous owners waaaaaaaaay back in the 20th century
Doesn't matter what he says. 20kw is a huge amount of heat.

Equipment is not undersized unless it can't keep up in the coldest weather you get, set up correctly and running continuously.

Did your 10kw system have trouble keeping up?

If yes, have you done everything within your means to reduce heat loss?

The way to size is with a load calc.

If a house of this size Oregon (which doesn't seem to get very cold) needs more than 15kw, it's really poorly insulated or has very high ceilings, or tons of glass.




As far as the 5 ton goes it's not for the cooling except for maybe a couple of weeks each summer but for the benefit of a high HSPF. However, I believe you sir are right and that 5 ton is oversized for this house (another oversight on my friend's part) and I'm in the process of trying to change the order to a 4 ton.
You have to verify your duct system can handle the 1600 cfm a 4 ton unit needs.

As a frame of reference, 20kw may be okay with 1200 cfm. (actually manufacturers recommendations vary)

The load calculation is good for sizing heatpumps because you can determine the balance point, below which strip heat is required and decide what makes sense.

You need to make sure the components you're using are approved matches and ideally use a thermostat that supports staging backup heat, and locking out strip heat when it's too warm.

Really, this isn't very diy friendly - i with knowledge of the theory but not being an installer wouldn't do this myself. It's not worth the risks and hassle; plus all of the equipment you have to buy to do the job right can really cut into your savings.
 
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Sorry to say. But its starting to sound more and more like your on the road to calamity. So are you sure you want to risk not having a working heating system in the winter.
 

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I live in climate zone 5 in a house that is 100+ years old, limited insulation, original single pane windows and even I don't need 40 btu/sq ft. That is from a proper heat loss calculation and calculating usage through heating degree days.

You have people with knowledge of all the wrong ways to do it leading you down a bad path that will most likely result in disappointment.
 
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