Hi, its Rat again. I'll be putting up a few topics, looking for any assistance and ideas I can get.

In this episode, I am installing a propane furnace (next year to add an A/C or air source heat pump) and complete ductwork system in my new house. No contractors brought in, just me .

So, details on the house: Its a 1344 sqft rambler style home. 28x48. I have a brand new 60k Goodman furnace sitting in the house, will be installing the system in a few weeks time. The only part of the HVAC that was done for me was the cold air returns stubbed in. Nothing else done.

Would love advice on designing the ductwork system. It needs to be ducted cold air returns up to the furnace intake. No 'under door' cold air returns allowed per county regulations.

Here is a blueprint of the upstairs.
Attached File

On a basement?

You say the returns are already run. Where are they? Ceiling or floor?

Planning to put the supply runs in the ceiling or floors?

Is the basement (with the stairs running down it looks like there is a basement) finished space?  

How well insulated is the house?  Are the basement walls insulated?  

If the basement is going to be conditioned space, given your MN location, I suspect your 60k furnace will be too small.  If the house is super insulated...well maybe.  But, I'm guessing if you're doing this on a tight budget, it's probably not super insulated.  If it has to heat the basement too, then you're heating 2688 sq ft (assuming its a full basement).  I would think you would need at least a 100k furnace to heat the entire space and even that might be a little on the small side given your frozen north location.  I bet a lot of contractors would recommend something in the 110,000k range.  Also, is it a 80% unit or 95%+ unit?  A 60k 80% unit puts out less heat than a 60k 95%+ unit.  

You also want to know now if you'll be installing a heat pump or just AC later.  A heat pump uses larger supply ducts than a heat/AC only system because the heat pump needs to move more volume in order for it to work correctly.

Where will the furnace itself be installed?

Airflow on a HVAC system is very important, I would go to a local supply house and get recommendations for someone to do it right the first time. Not saying you're not capable. 

Hiring out is not an option. No money for that.

I got quotes from 3 different contractors for the job, just not possible to pay that much.

I did buy the furnace size they had quoted me. It's a 96% model.

I'm plenty capable, but I know there are tricks to the trade, be was hoping to get a few of them.

It's a brand new house, very well insulated. Will have a full finished basement. I'll get a pic where the cold air returns are stubbed in upstairs soon. Basement is a blank slate, just built it. I built the basement with ICF forms.

ICF basement is good for insulation value.

What is the insulation type and R value of the main floor walls?

What is the insulation value of the ceiling?

Can't help without more info.

Walls are 2x6, fiberglass insulation. Can't find spec on R value used, I'm sure it standard code.
Attic is R48+ blown insulation.
All Anderson insulated windows/doors.

Its brand new construction, and MN has pretty aggressive codes on energy efficiency. Its tight and well insulated.

I put red boxes where the cold air returns are. Attached File

I put yellow boxes where the walls are downstairs. The long wall is load bearing, as is the small wall by the stairs.

The basement will have a carbon copy of the upstairs bathroom directly above. Simplified plumbing.
Attached File

That level of insulation is marginal for your area.  Meets code I'm sure but just because something meets code doesn't mean it's the best it can be.  If you have fiberglass batts in the walls then you've probably got R19.  Once you calculate in the thermal bridging losses of the studs, your R19 value wall is actually quite a bit less effective R value.  For your super cold area, I would want R60 in the attic.  

With those R values, I definitely fear your furnace is undersized.  I built a 2450 sq ft house last year.  That sq ft size includes the basement.  We have a dual fuel heat pump/propane furnace.  Our furnace is a 95% efficient unit.  It's a 90k BTU furnace.  Also, I live in a warmer climate than you (we can see 0* but never for long) and the house has much more R value insulation than yours.  I think your unit will be struggling to keep the house comfortable once the temps really dip down into the sub zero range.  

Are your air returns near the floor or near the ceiling?  Ideally, if your returns are in the floor the ducts should be in the ceiling.  If the returns are in or near the ceilings, then the supply should be in the floors.  

The negative to putting supply runs in the attic is the conditioned air losses to the cold attic space.  It would be much better to run the ducts in the conditioned space of the basement if you can.

I love the dual fuel heat pump/propane furnace and highly recommend it but I also live in a warmer climate than you.  The heap pump probably supplies about 75% of my heating requirements.  In your climate, I doubt the heat pump would be used much at all.

Don't know what to say about the furnace size, 3 different HVAC guys gave a quote for that size furnace.

The cold air returns are near the floor, and the registers MUST be in the floor. Per code, I'm not allowed to run them through the attic.

First, do a room by room load calculation. I'm sure someone had to do a heat loss calculation to get your permits (this is what gives you the furnace size), but this will tell you how much airflow (CFM) you need to each room. It will also tell you what size ac you need.

  DO NOT put in a bigger furnace because someone on the internet thinks it's too small. Where I am (WI) we have a design temperature of -10* and 60k would probably be too big for that house, but go by your heat loss calculations.

  As far as tips on ductwork, keep it straight. I would try to locate the furnace in the middle of those returns and where you can run the supply and return straight down the length of the house. Use a media air cleaner (I like Honeywell) on the side of the furnace and run your return duct off of that so you don't have to mess with a filter rack.

  You are going to have to do some research on duct sizing, or better yet, hire someone to design it for you. I've seen far too many diyers (and "Pros") go to Home Depot and buy what they think they need and end up with a pile of shit that doesn't work and a cracked heat exchanger. There are plenty of "rule of thumbs", but you kinda need to know how they apply or your system won't work.

Good luck!

Did they do a Manual J or just spitball a number? I think 60k would be fine for just the main floor but you'll need to send heat to your basement too.

Assuming you are putting the furnace in the utility room at one end of the house, I would run a single trunk down the center of the house and branch off the trunk to the various supply points. The returns on the floor are far from ideal but sounds like you are stuck with it.

You'll have to size the trunk correctly. It HAS to drop in size as you get farther from the unit.

I'd probably put two in the upper left bedroom, one in each of the two lower bedrooms, two in the living room, two in the dining room, one in the kitchen under the sink, one in each bathroom, and one in the utility room. Then put 4 in the big room in the basement, one in the utility room, one in the basement bathroom, two in the upper right room. You'll want to send more heat to the basement in the winter and less AC in the summer.

There is a site online that you provide them the info on your house and they will do a Manual J and a duct design for you. It wasn't too pricey and would be totally worth the money spent....even being on a tight budget.

https://www.webrepshvac.com/category.jhtm?cid=2990

One of the contractors took a whole ton of numbers, window sizes/count, any wind breaks, etc, etc. The house wasn't here yet so I sent him copies of the blueprints. Knew there would be a finished basement (had the blueprint). Came up with a 60k furnace. The other two more/less spitballed the furnace size, but both also recommended a 60k furnace. Since all 3 agreed, I went out and bought a 60k furnace.

Their service is $189, and I think that is worth the cost over me guessing. It all has to work out in the end (tested by the building inspector), so that cost is cheaper than me screwing up even once.

Thanks!

The basement really doesn't add much heat loss over a slab on grade house unless there is a lot of exposed walls, windows, patio doors, etc. He also has insulated concrete forms which will help quite a bit. I bet his basement only adds 5,000 btu or so to the total heat loss of the house. Here in south eastern Wisconsin, you will routinely see 60k btu furnaces in new houses that are 2,000 square feet,  (4,000 with the basement) and they have no problem heating at or below our -10* design temperature.

Good plan!

I had them do a Manual J for me and I was happy with what they gave me. Thought it was pretty reasonable. I didn't use their duct design service but their Manual J I purchased from them was spot on. They called me with questions and a couple days later I had a completed calculation.

I also used some online Manual J calculator and I came up with the same size recommendation as they did. However, I didn't trust my result since I'm no expert.

I'm interested in what numbers they give you.