Heating coil size / diagnostic

tc60045

Guys,

I have a situation and I'd like to expose my diagnostic to see if I am finally starting to think like hydronic pro. Emphasis on starting...

I have an inadequate heat supply from an air handler that covers the 1st floor of our house. It consists of:
- a 20" x 30" heating coil in my air handler. https://www.firstco.com/documents/ProductDocuments/hwc209.pdf
- connected via a janky 1" copper to 1" pex to 3/4" copper to 3/4" ID header on the coils (10' away so ****, I know)
- supplied via a Grundfos UP 26-64 F
- as one of 4 zones on the secondary off a 200K BTU boiler (nominal; 160 MBH)
- [edit: @ 22 PSI]

Inadequate = not warm enough given Chicago winter and best efforts to improve insulation / mitigate heat losses and sufficient "layer up" feedback to wife / children. Manual J and square footage questions - you can ask, but I'll refer to "inadequate." Demand for heat is not met by supply.

I kept water supply temp at 140 last winter and this air handler ran the entire months of Jan and Feb, never satisfying the call for heat on our first floor. The fact that our bedrooms on 2fl / 3fl were warm (via a separate handler / coil / supply line / pump on same secondary) seemed to me to prove that I was rate limited somewhere on the supply side of this 1st floor handler.

Where is my rate limiting factor?

This coil set has some moderate head, with the instructions PDF not giving data beyond 9 GPM (7.5 ft of head). 9 GPM at deltaT of 20 gives me 90K BTU. The PDF says that higher supply temperature will give me more BTU, but my hydronic mind says that is only possible with higher delta T, not a higher supply temperature per se. Thus, I'm going to declare these coils maxed at 90K BTU.

Looking backwards from there, 3/4" copper and 1" pex, respectively, have max GPM of 6.5 and 7.5. Very short run from boiler to unit (10 feet away), but let's assume we are rate limited in the pipe at 6.5 GPM. That gives me 65K BTU. That feels right, but I don't have enough experience to justify my gut on that.

Then there is the Grundfos. The Grundfos literature says that at 7.5 feed of head, it will try to pump at 22 GPM. I'm not sure that is happening through the 3/4" pipe, though. But I can't declare the pump as the rate limiter at all.

So my analysis would be that the limiter in this system is first the 3/4" supply and second, the head of the coil set.

Thus,If I can plumb a big fat 1 1/4 pipe the whopping 10 feet to the handler, and find a new coil set that can handle a 1 1/4" connection, I will have eliminated my first rate limiter. I need to make sure the internals of the coil set still work -- big tubing, in parallel, inside the coils, so that the head stays under, say 12, I'll be able to keep using my Grundfos pump.

Am I on the right track?

Most gratefully, TC

hot_rod

Any idea what then heat load is for the space? Start with that, see if the coil output matches.

Then, of course, can the current ducting move the required cfm and are the rooms supplied with adequate runs from the ducting.

Could be the air side is the constipation.

tc60045

Rod, thanks, as always for your thoughts!

Unit is a Carrier F4VP with a 3/4 HP blower and 5 ton Puron compressor. Top speed is 1575 CFM heating, 1750 cooling.

I used MBH = CFM * 1.08 * AT_Rise / 1000 and see that at 1575 CFM and 65,000 MBH, we'd see our AT enter at 70 degrees and leave at 108. I would say that registers were blowing nearby (different from leaving temperature, I know) at around 110, so that seems close.

At that CFM, I would definitely be unable to get north of 120 MBH, as that would be 70 + 70 = 140 = entering water temperature!

Ducting - my guy looked at it yesterday and thought I was a little return limited, but was OK. Couple of balancing issues but nothing dramatic. I didn't follow his duct sizing math as he did his thing...

Zman

I am thinking you have a lot of math but not much data.

Unless there is some resistance in the water side that you haven't though of, you should have plenty of flow. There is a chance the circ is not working correctly but I doubt it. How is the supply/return delta?

I think the air side is the likely culprit. Are all the coils clean?
I would suggest measuring out all the ducts and looking up the specs on the coils and fans. You can check that just like a hydronic circuit.

Sending hotter water to the coils will give you more BTUs.

aircooled81

Calculate what the coil is actually doing btu-wise, if you know how much air is passing through the coil, you can do this math. then you will know how many btu's you are doing, here's my idea...
if you do not own a manometer, buy one, borrow one or make one. I included how to make one below, and how to use it.
- Turn the fan on, measure the pres diff between entering and leaving side of coil. .16/.19/.22 drop will give you the CFM value or atleast rough estimate to match on that chart.
- Determine flow with the rest of the info.
If you know that entering air volume use the chart, you know your entering water temp is 140 - so get that cfm and line it up in the 140 column.
- you have 4 options for btu's per hour, so this is how you determine that.
- Measure the water temp entering and leaving the coil, we'll call this the delta T, take that number and dump it into this occasion.
- BTUH = 500 x WDelta T x GPM
- now you need this GPM, but you're not sure how to get it right, well the good news is you can solve for GPM by solving for BTUH. so take a measurement of the air entering the coil, and leaving the coil. we'll call that the ADelta T (Air - Water), but lets use it in this equation. BTUH = 1.08 x CFM X ADelta T.
- now you know the CFM cause you solved for it above with pressure drop. plug it into the equation and boom, now you have the BTUH.
- take the BTUH and plug it into BTUH = 500 x GPM X WDelta T
now you have your flow(GPM) to solve for and can determine if this GPM is the resistance problem.

manometer is a device that measures air pressure, we calculate air pressure in inches. some clear vinyl tube with a sharpie marking out the inches, and making a u-bend we can calculate how far the water travels up or down when measured against the ducts air pressure. This measurement on both sides of the duct will have a drop in pressure, the chart you have shows CFM based on how the coils pressure drop was.
Frankly I'm real impressed with the amount of detail they provide in that submital allows you to solve such great equations with little initial data.

EBEBRATT-Ed

Raise your water temperature. 140 degrees will not cut it.
Why? Because your heating say 70 degree return air with 140 degree water. That's 70 difference between the coil supply and the air. Now run 200 degree water through the coil. Now you have 130 degree difference between the coil and the air. Which set up do you thing will deliver more btus??

I am not saying to do that

Do it right. Do a heat loss, find the gpm and temperature required, size the pipe and the coil and the circulator and find out what you need. Re engineering a job that doesn't work is more difficult than starting from scratch on a new job. If your stuck with the coil find out from the mfg how many cfm, gpm ,water temp and head you have to offset the heat loss you calculated (if it is capable of doing it) Then work on the pipe size and pump

Ironman

Simple question: are you sure the coil is not air bound?

tc60045

Ironman said:

Simple question: are you sure the coil is not air bound?

Great question - thx!. These coils are low point in the system with air purge in the attic on a similar set of coils. So 99.999999999% likely that the coil isn't air bound. I'll be sure to purge it properly before I test.

tc60045

Zman said:

I am thinking you have a lot of math but not much data.

Zman, you are 100% right -- and you made me bust a gut! We are in the season when I can easily do some testing on the 1st floor, so I'm going to get busy measuring with different water supply temps and see how things respond.

Zman said:

Unless there is some resistance in the water side that you haven't though of, you should have plenty of flow. There is a chance the circ is not working correctly but I doubt it. How is the supply/return delta?

I'm not sure that I have plenty of flow on the supply side, given that the system is stopped down to 3/4" for about 10' on both supply and return.

1. I will measure everything at different supply temps (140, 160, 180).
2. I think I will have my plumber re-plumb with 1 1/4 between secondary and coils. That will get stopped down to 7/8 (and lower) in the coils, but it removes a potential bottleneck and is the connection size I would need if I replaced the coils. It *should* give me a 50% lift in MBH if indeed the tubing was the bottleneck.
3. I will re-measure everything at different supply temps.

Zman said:

I think the air side is the likely culprit. Are all the coils clean?
I would suggest measuring out all the ducts and looking up the specs on the coils and fans. You can check that just like a hydronic circuit.

I will clean it all out to be sure and then do some measuring.

Zman said:

Sending hotter water to the coils will give you more BTUs.

Yes, but it will force me to add a mixing valve elsewhere.

Many thanks for your feedback!

tc60045

EBEBRATT-Ed said:

Raise your water temperature. 140 degrees will not cut it.
Why? Because your heating say 70 degree return air with 140 degree water. That's 70 difference between the coil supply and the air. Now run 200 degree water through the coil. Now you have 130 degree difference between the coil and the air. Which set up do you thing will deliver more btus??

First, Ed, thank you for your feedback -- you and others here have helped push me to collect some data, and I am grateful!

I understand: MBH=CFM*dT*1.08, where dT = difference between (average) temperature of water supplied and entering air temperature. Raise the supply 20 degrees, raise dT 20 degrees, raise MBH by a lot -- I totally get it.

But raising that temp means installing a mixing valve elsewhere, which I am trying to avoid. I may be willing to flex that now, so know that your feedback nudged me...

EBEBRATT-Ed said:

Do it right. Do a heat loss....

Manual-J on this house would cost me a small fortune, I am sure. I would rather spend $300 on re-plumbing the piping to see if I am supply-constrained, knowing that the $300 is money I would have to spend anyway to plumb the connection to a new coil set if the coils end up being the constraint.

EBEBRATT-Ed said:

...find the gpm and temperature required, size the pipe and the coil and the circulator and find out what you need. Re engineering a job that doesn't work is more difficult than starting from scratch on a new job. If your stuck with the coil find out from the mfg how many cfm, gpm ,water temp and head you have to offset the heat loss you calculated (if it is capable of doing it) Then work on the pipe size and pump

This is the part that I thought I did properly above. I'm sincere when I ask: what did I miss in my approach? I started with the pdf from the manufacturer and worked backwards to identify bottlenecks.

TC