Ok, I just ran a bunch of tests. Because I'll need to explain what I found to the furnace people as well, I put it all in chart form. (Plus, my job has me analyzing stuff all the time, so this is a bit normal for me to do.)



To measure gas flow rates, I went outside and watched my meter. The quarter foot needle spun fast, so I watched it spin and timed it for 1 minute. On my power bill, it gives a direct ratio for BTU per cubit feet. Here in Utah, we have high quality natural gas, so it's 909 BTU per ft^3.

For what I saw, the downstairs furnace appears to be stuck in the first stage. But it is definitely consuming natural gas at a decent rate. The upstairs furnace seemed to go a bit beyond what it says it should burn in BTUs for stage two.

I'll have to try forcing the downstairs furnace to run on high fire. But I'll need the furnace people to work with me on that (I can't void any warranties.) Overall though, it seems that I'm losing a lot of heat into the slab.

The upstairs furnace heated quickly. So quickly, it didn't appear to radiate into the walls and furniture. I just had hot air.

I turned off the thermostat, and let the room stabilize back down to around 74. Comparing the BTUs used, it seems the downstairs furnace needed to use about twice as much gas as the upstairs furnace to heat a floor. Ugh.

Based oon your spec, the first stage is 42 K BTU/hr and it looks like your right on the money.

Which means, the second stage isn't kicking in. Because you should have measured 60K BTU/hr.

Not sure what you mean by vent temperature range. Does this mean discharge temp measured at about the same time? If it does mean that, then your heating the slab.

20/20 Hindsight says the ducts should have been insulated.

Job well done: Furnace is NOT running in the second stage (or it doesn't have enough gas supply. Doubt the latter.)

I'm not too happy with the contractor who doesn't know how to hook up a second stage.

The R value of concrete is not good at all: R-value (insulation) - Wikipedia, the free encyclopedia

It's interesting what these guys recommend for ducts in concrete. 1/4" thick. http://www.bart.gov/docs/STD_SPEC/BF...%2031%2000.pdf

Section 4.5.2 Guide for Concrete Floor and Slab ... - Google Books

reccomends watertight joints and sand.

Not a whole lot of difference in thermal conductivity. Thermal Conductivity of some common Materials

Thanks for the comments.

When I talked about temperature range of the vents, I was summarizing the temperatures I was seeing coming out of the 6 vents in the basement.

When temperatures peaked, one vent was giving out 113 degree air, the next as 107 degree air, the next vent was at 67 degrees, then 83, then 99, and the last vent was 95 degrees. So the range was from 113 to 67 degrees.

What I'm gathering is that air is leaving the furnace at least at 113 degrees (the hottest vent had that temp). By the time it travels to the vents, it cools significantly. One vent cools all the way down to 67 degrees! My guess on the path is that the cement duct is perhaps only 30-40 feet from source to destination. Wow, valuable lesson why ducts should be insulated.

I just realized a better test I could have ran. I'm worried about the variables of heat loss, and variable of the entire floor capturing and holding heat (solid objects and air). For example, the basement has lost about 7 degrees in 5 hours time. It may just be that the basement furnace very slowly heats the downstairs, and during this time, heat is radiated outside while the furnace kicks new heat into the floor. But overall, it uses BTUs efficiently.

So to factor in heat loss, I should give the experiment a total time to run, say, 8 hours. I should increase the temperature of a floor 10 degrees and maintain that temperature until the 8 hours is up. Then do the same for the upper floor, maintaining the temp increase for the same total amount of time. I might just find out that the downstairs furnace is up to the task. My only problem is that the gas meter doesn't have a good way of measuring cubic foot usage for a several hour time span. It tends to only work well for a month. Maybe my furnace company has some gauge that would help me figure this out...

Remember, that the furnace typically has a heat rise, somewhat modified by relative humidity. It will raise the temp of the air coming in by x degrees.

I have my doubts that insulating ducts in cement would really help. The themal conductivity isn't that great for cement . The long run probably has more of an influence.

For temperature uiformity, you may have to resort to zoning with register or duct dampers. For fun, you could get thermostats that can be adjusted everywhere.

The 30% difference between high and low fire will knock off 30% of the time to raise it 10 degrees.

7 degrees in 5 hours probably means the furnace is going to be running more. If it's running more with the duct variation, then temperature uniformity may be really bad.

A 67 degree discharge temp isn't going to heat air very well.

The furnaces ability to maintain the space, is going to depend largely on the surrounding temperatures, BUT the ground is fairly uniform. You might have some direct exposure to the outside.

You have a lot of area in the ductwort, but the thermal conductivity isn't great, but 3 ducts in parallel is probably worse than a single duct. Heat Loss Through Enclosure Walls Equations and Calculations - Engineers Edge

Calculating Heat Loss - Features - Process Heating

Do you know what the ducts are surrounded in and how thick they are?

You need to look at steady state performance once you get the high fire thing to work. i.e room temperature variation after say 24 hours.

Just an update (in case people run into this thread months down the road and wanted to see the result). We've had the furnace folks over a few times. Here's what we've found:

* All tests showed the furnace is working as it should, our vents beneath the slab are just dumb.
* Forcing the furnace into the second stage helped very little. It decreased the time from about 5 hours to 3.5 hours to heat the room up 10 degrees.
* We also tried slowing down the air to its maximum, thus making the air hotter. But that only made the air slightly hotter. It still took over 3 hours to heat the room up. That one cold supply never got above 70 degrees (meaning, we were losing over 60 degrees to travel from the furnace to the supply!)

So, our furnace is useless. Their warranty doesn't cover bad vents. And our slab can't be ripped up and fixed. We tried to salvage the counterflow furnace by lifting the furnace up, and then running the air back up into the ceiling. But there's no room anywhere for these extra vents. So our solution is:

* I'm going to tap into the upstairs furnace vents, so we have one furnace for the whole house.
* I'm going to spend my Christmas vacation time running supplies and cold air returns to every room in the basement. We'll have to sheetrock, mud, tape, and repaint everything when we're done.
* They're going to try and sell our counterflow furnace for us. We paid about $2900 for it. We'll probably have to sell it for $1200 to make it competitive with the 80%. But it may take months to sell it, as counterflows aren't common. They're looking to see if it can be turned on it's side, and if so, it should be an easier sell.

So if any of you want a brand new counterflow furnace, let me know :P

What a mess. You might be able to install some electric baseboard heaters easier.
Will your other furnace you are going to tap have the heat capacity and the blower capacity to run the entire show? Think before you act!!

They did some calculations, two people said it would be OK.

I wouldn't mind a double check. The main furnace kicks into its second stage about 5 minutes in, and is rated to burn at 92,000 BTU (but from what I measured on the gas meter, it seemed to go up to 100,000 BTU). The entire sq ft will now be 2600 off that one furnace.

The main trunk line heading towards the east of the house will feed 10 supplies total (into the 4 bedrooms and 2 bathrooms). I hope a trunkline can handle that.

Brad:

Sorry your "professional installation" wet very bad. You seemed to tried to have it all together. As HVAC1000 has said in the past, the equipment is only as good as the installation.

Thank you for the feedback.

Manual D, Manual J

PS: I fixed a typo in post #25 for you: You had said increased the time. 5 to 3.5 is a decrease.

Sorry to keep bothering you all, but this forum keeps coming back as the most helpful place I find :)

I want to force the company to give me all my money back. But I think I might need some good documentation.

I had another furnace company over to give me their opinion. He pointed out something I hadn't considered. The CFM on my supply ducts are terribly inadequate for what the furnace needs. If I'm running three 4 or 5 inch ducts for my supply ducts, then that's only rated for about roughly 150 CFM. But my furnace is now jumpered to run at 1000 CFM most of the time!

Therefore, the installation was bad. It's like they're trying to drive a semi under a 4 foot tall bridge. The installer should have instantly seen that supply lines are totally inadequate, and not installed the furnace it in the first place. I shouldn't have to pay for the labor, and I shouldn't have to pay for a furnace I can't use.

Now what I'm looking for is some kind of official manual that backs me up. Something that a small claims court would agree is authoritative. My furnace manual does say it burns 650 CFM on low fire an 1000 CFM on hire fire. But it doesn't mention a thing on how big the supply ducts should be.

So here's where I'm picking your brains. What's the best way for me to make the case that three small duct lines are so inadequate that the installer/company failed in their job? Are there city codes, or state regulations, or government websites that explain it? I'm in the dark here on this one...

I would first check with your local building department. I am sure the furnace company took out a permit.

Use your local rules/codes whenever possible since they are already approved by the powers to be.

On a furnace install the codes usually say to follow the manufactures install instructions for enforcement but on duct install it is usually a code like the CABO code or the states mechanical code.

A citys and states might use a different code or year of issue code book since the codes are on 3 year cycles and many jurisdictions do not adopt a code cycle right away.

Some stuff to munch on: Procedures For HVAC System Design and Installation

Energy Savers: Sizing Heating and Cooling Systems

WarmAir.net - Duct Design and Dynamics

So, actually what this all means is that there are two calculations needed to adequately do duct design.

One is called MANUAL J. This basically determines the heat loss for the space(s).

The other is called MANUAL D. This is the reference for doing the duct design. Manual J must be performed first.

The right company would have software to do the Manual D and Manual J calculations and it would not take long to do.

These guys: HVAC Software, HVAC-Calc for Heat Loss, Heat Load Calculations Have a homeowner version of manual J for about $50

Having the results done by a PE (Professional Engineer) and possibly paying him to be an expert witness, then you probably have the cat in the bag.

Here is an example of the software:

Wrightsoft: HVAC Software - Product Overview

As an idea, call the software mfr and ask if there is anyone using their software near you and use it as a starting point.

I would agree, that it does seem a bit too small and I'll bet that the static pressure is too high too.