More winter, more problems

[Giantsean]

Hi All,

Sorry to bring up the exact same conversation every single winter, but I am having some problems getting my downstairs warm now that the weather has turned. It appears that about 20 degrees outside temp and I start to have problems. Add wind and it's worse.

I have a Navien CH-240 feeding a TAM7 air handler w/ a 42K BTU (noted, ideal conditions) hydro coil, which is dedicated to the 1st floor and split into two zones. The maximum distance from the AH to the farthest register is about 50ft. I have done a LOT of work buttoning up the downstairs - replaced windows, applied insulation where there was none, air sealed, but I just cannot get the 1st floor past 65 on a cold day. I have tried tuning the TAM for maximum airflow, but it almost seems like the problem is that it is lukewarm air by the time it comes out of the registers. It makes me wonder if heat is actually being removed from this coil or whether there is some weird ducting problem elsewhere.

I also tried upping the supply temp on the Navien to 180. Just as last year, I can get it to peak in the high 170's but it kicks off fast and throttles back to the 160 area. Some have told me it is because return temp is too high. I have unplugged the ODR and left the remote to try to dial in the supply temp, but it still seems to do whatever it wants.

At this point I almost don't even know where to begin. I really don't understand why it's so difficult to get this place warm!

Thank you for any advice

 

[Tom Sawyer]

if I had to guess, I'd say the ductwork is not properly sized or installed

 

[Sponsor]

 

[Standardairconditioner]

Blower fan c.f.m. in your furnance might be inadequate.

A "inline fan duct" or "inline booster fan" might help.

 

[Giantsean]

Thanks guys. So in practical terms, is the problem that the warm air is cooling down too much before it gets to the registers, or it is simply leaking out of poorly sealed ducts, or is it sucking in colder air somewhere along the way... or all of the above? (I feel a single "Yes" coming on )

 

[Standardairconditioner]

Regardless what you will read on the internet, we need to see it on-site to make an accurate assessment.

I mean, you or a professional, will have to INSPECT each and EVERY bit of duct work in your entire house, from the furnace all the way to the rooms.

I know if I did that in person, I would definitely find the problem 100%.

How to you expect that to happen from a forum thread?

 

[Dana]

What Tom Sawyer said. Even if the duct design is correct, if it it's flex duct it's harder to make the call, since the duct impedance when it isn't fully stretched to length is pretty high, and varies quite a bit.

Zoning ducted systems with duct dampers is also fraught with complications.

 

[Jadnashua]

What you have not said is what the heat load analysis calls for. Get yourself a quality temperature probe and see what the temperature of the air is right after the heat exchanger, then what it is when it gets to the registers. Running the fan faster means less time to absorb heat as it passes by the heat exchanger. Too slow, and you cannot get that warmed air to the registers before it cools off. Any leaks along the way, and it gets tougher. The duct sizing can only carry a certain amount of air, and therefore heat...that may be way off.

 

[Giantsean]

I don't think these guys know what a heat load calculation IS. Kidding aside (and I'm half serious), I have done a few calcs from a few sources for just the 1st floor and came up with around 45K-50K BTU. The coil is supposedly good for 46 or so depending on various conditions.

At any rate, I am going to have it looked at by a pro. There are probably a dozen things wrong with the install that are having a cumulative effect.

One final question - I have been told (even by some of you here) that because the design of the Trane/AS hydro coil (cross flow I think was the term used) that the supply MUST be plumbed to the correct inlet. That much is understood. What I'd like to ask is whether having it backwards as it is now is having a significant enough impact that I will get instant bang for buck by fixing it now. Like will I see a substantial difference, or will it just be slight?

 

[Dana]

The term you're looking for is "counter flow" heat exchanger. To get the specified heat transfer out of the thing the water flow cannot be swapped.

With a counter flow heat exchanger the incoming cool air hits the coil at the out-going cooler-water end, and leaves the heat exchanger at the warm water side. That way the much-cooler air still extracts heat from the cooler water side of the coil. If you swap it around all of the heat exchange is happening at that first layer of coil, since the warmer air further into the coil can't extract much heat from the cooler water.

The effect of that error isn't a problem if the coil is 3x oversized for the the load, but if you really need to get something north of 40K out of a 46K coil it HAS to be plumbed correctly to get there- it's fundamental!

You would have a measurable improvement if you fixed that particular screw up right now. Whether it would be enough of a change to fix the comfort problem isn't entirely certain, but you will at least have eliminated one known impediment.

 

[Giantsean]

Thanks exactly the answer I was looking for. Thanks for your continued help Dana!

 

[Giantsean]

Okay sorry I lied when I said last question

The supply and returns come off the loop as 3/4" copper, then switch to 3/4" PEX before going back to 3/4 copper to the 3/4 inlet of the coil. As long as I'm reversing these lines, is it worth changing the PEX (or even the copper) to 1" for max flow? The PEX takes at least 3 90's getting to the inlet, as you can see from the pics

 

[Jadnashua]

I'd probably just keep it copper all of the way. You will see some flow restriction in the elbows and ID of the pex verses the copper. There are charts that can tell you the flow restriction from the length of the tubing and the installed fittings. Whether it will be enough to be a problem, you'd have to run the numbers.

 

[Tom Sawyer]

Yep, ditch the PEX, the restrictions killing you.

 

[Giantsean]

Should I go up to 1" or is that overkill being that the coil inlets are 3/4?

The max flow measured in the coil spec that Dana posted above is 9GPM (needed to get into the 40,000 range). Is that possible w/ 3/4 copper runs and will elbows create significant restrictions?

 

[Tom Sawyer]

Every elbow equals 10 feet of pipe.

 

[Giantsean]

I'd better find a GPM calculator... my biggest question though is can you get 9GPM out of a 3/4 copper pipe?

Or do I need to go to 1"...

 

[Jadnashua]

The copper tubing organization says you should not exceed 8fps flow rate using cold water (5fps for hot) for extended time in copper. You can fairly easily figure the volume of the pipe and then calculate how many gallons that is at that velocity.

 

[Dana]

So why not just do it?

3/4" copper contains ~0.025 gallons per foot. 5 feet per second is then 0.025 x 5 fps= 0.125 gallons per second, or ( x 60=) 7.5 gpm

3/4" PEX is more like 0.020 gallons per foot...

So if you really NEED the max flow through the coil (and I'm not convinced that you do), you'd be better off upgrading the plumbing size. But it's dead-simple to correct the direction of flow first using the existing plumbing to see if that gets you there without raising the flow rate. Even with the high head of the PEX ells it's not as if you have a gazillion miles of equivalent feet of PEX between the Navien and the air handler in the picture.

 

[WorthFlorida]

Hi All,
Sorry to bring up the exact same conversation every single winter, but I am having some problems getting my downstairs warm now that the weather has turned. It appears that about 20 degrees outside temp and I start to have problems. Add wind and it's worse.
....

I doubt there is any problem with your heating system other than the cold air return, home insulation, and/or windows & doors might be leaking air? With a two story home the cold air return MUST be near the ceiling of the second floor for both heating and cooling. We all know that heat rises and when your outside temp drops the heating system will run for longer periods and if you have a smart system that will automatically increase the temperature of the heated air or hot water determined by the outside temp. The hotter heated air will then rise faster. If you are not pulling the excess heat off the second floor it will always be much warmer. Another factor is insulation. As heat escapes from the home, cold air obviously will drop to the first floor. As you stated 'Add wind.." might suggest that there is enough air leaks in the home to further cause the heating system to run longer and harder. Some homes the second floor cold air returns are at floor level but they still need to be up high on the wall. If you have cold air returns on both floors, and if possible, block off some of the return on the first floor but be careful that you do not starve the furnace of air flow. Having owned four different two story homes, the second floor will always be warmer no matter how good the duct work is.

I had a home a 2 story Illinois and it had no Tyvek rap, therefore, the house was pretty drafty during windy days. When my son developed allergies I had installed a electronic air cleaner whereby the fan had to run 24/7 during the heating season. This circulated the air and made the difference in temp between the first and second floor more comfortable. The electric bill increase was hardly noticeable. Three cold air returns near the ceiling in each bedroom and one at floor level on the first floor. So try running your fan 24 hrs a day during your cold snaps and see if there is any improvements.