Water to Air HX Designs - specs vs performance

[MNBobcat]

Hi Guys,

Some of the water to air heat exchangers have 1 inch inlets and others, like the one Central Boiler offers, have 3/4 inch inlets.

Does the inlet size and internal tubing size have an impact on the BTUs and efficiency? The HX that CB has that I'm looking at is rated at 140,000 BTUs. I don't know if that should be the only thing I need worry about or if I need to pay attention to things like the tubing size? National Coil can make me up a HX but its 3 times the price. It does have a 1 inch inlet though. Just not sure its worth the cost.

 

[MNBobcat]

Never mind. I called National Coil. I thought I'd post the answer in case anyone else has the question. The inlet size doesn't matter. The only thing you need to worry about is the BTU rating.

 

[stee6043]

I suspect they meant to say "the difference between 3/4" and 1" is negligible". Certainly, size matters to the extent that it determines flow characteristics in your system. You can't pump 200,000 btu through 3/4" pipe....

I seem to remember 3/4" pipe being rated at less than 100k. But if you're only stopping down to 3/4" for the HX perhaps you'll be okay. I'm not sure how the pro's would really respond to this if they weren't trying to sell to you.

My run-of-the mill 18x18 water to air HX has 1" inlet/outlets. I believe it was rated at 80,000 or 100,000btu and was sub $300....

 

[jebatty]

While I might be inclined to agree that the inlet/outlet side might not be overly material, I also think I'm looking through some swiss cheese. The btuh rating must depend upon a specified cfm of air passing through the exchanger to extract the heat from the water, the temperature of the incoming air and the temp of the outgoing air, the delta-T between inlet and outlet temp of the water, and gpm's of water moving through the exchanger, and then the gpm's impact the pressure drop in psi (more gpm's = more pressure drop), which then relates to added pump head in the circulation loop and the ability of the circulator to move the specified gpm's at the pump head of the loop.

I guess I would ask the mfr/supplier to provide for the btuh rating the specs for cfm, delta-T for air temp, delta-T for water temp, gpm and pressure drop. Then determine whether you can meet those specs to get the btuh you think you need.

 

[MNBobcat]

Ayuh,... Not the question, but still on topic,...
If you're Handy enough,....
Automotive radiators are an Option,.... a Cheap option...
It comes down to the Space you have to work with,+ plumbin' it in...

Hi Bondo.

I know some guys have gone that route and it worked well for them. Its a good tip. I may do that in the garage. My house is 7,000 square feet though and I need to get as many BTUs out of the coil as I can. With the manufactured coils they have spec sheets that are helpful in figuring out the best option.

Thanks for the suggestion.

 

[MNBobcat]

While I might be inclined to agree that the inlet/outlet side might not be overly material, I also think I'm looking through some swiss cheese. The btuh rating must depend upon a specified cfm of air passing through the exchanger to extract the heat from the water, the temperature of the incoming air and the temp of the outgoing air, the delta-T between inlet and outlet temp of the water, and gpm's of water moving through the exchanger, and then the gpm's impact the pressure drop in psi (more gpm's = more pressure drop), which then relates to added pump head in the circulation loop and the ability of the circulator to move the specified gpm's at the pump head of the loop.

I guess I would ask the mfr/supplier to provide for the btuh rating the specs for cfm, delta-T for air temp, delta-T for water temp, gpm and pressure drop. Then determine whether you can meet those specs to get the btuh you think you need.

Hi JeBatty,


Well said and a good point. I did have central boiler fax me a spec sheet on the HX earlier today. My furnace puts out 1,570 CFM in the cooling stage which is the fan speed I'll set up to run after I install the HX. I won't know for sure the GPM I'll get but it looks like anything over 5 should put me in good shape. *crossing my fingers.

I've heard Delta-T before but don't know what it means.

 

[jebatty]

Delta-T is change (delta) in temperature, one side to the other. For water, (inlet temp - outlet temp) x 500 x gpm = btu. Therefore, if you're moving 5 gpm, and your water delta-T is 20F (common for hydronic systems; 180F in and 160F out, for baseboard, or 120F in and 100F out, for radiant floor, for example), btu = 50,000.

 

[pybyr]

I second all of jebatty's points. When I was shopping around for a water to air HX, I found that the specs of most units on ebay or most vendors made a lot of- to my mind- overly optimistic assumptions about rates of water and air flow, entering water temp (180 is common to assume in fossil-fired hydronics, but with wood, and especially if you contemplate ever using storage, you'll want to be able to extract a useful BTU output even at lower water temperatures).

There are also some "rules of thumb" about the pipe size needed to move a certain BTU range; I'd look for a HX that uses ones no smaller than those rules of thumb for the BTUs you are aiming for. In concept, some restrictions can be tolerated for short distances of pipe (which I suppose you could look at the HX as), but better flow = better heat movement and exchange, and less back pressure (lets you use a smaller pump, or a lower speed on a 3 speed pump- and thus less electricity- to get the job done).

My own view- when spending the kind of $ that an overall new wood fired hydronic (or hydronic-air hybrid) system costs, the only thing that'd be more regrettable than "going a bit wild" on HX sizing and specs is discovering that you've paid hundreds of dollars for an HX that gives sub-par performance.

My water-air HX is a monstrosity-- but it does exactly what was intended, which was to start making lots of really warm air the moment that it gets a flow of even what would be considered "low temp" water in comparison to normal hydronic assumptions for a water-air HX. Between the HX and my Econoburn, this previously hard to heat nearly 200 yr old farmhouse is more comfortable than ever before.
https://www.hearth.com/talk/threads/25090/

 

[stee6043]

Pybyr, your HX is an absolute beast and I am jealous. I think my furnace would collapse from the weight! Cool setup....

 

[dogwood]

Stee. I was so jealous of and inspired by Trevor's (Pybyr)'s monster w/a hx, I called his supplier, who pulled up his design and made me one to fit my furnace. I just had to have it.

Mike

 

[stee6043]

Stee. I was so jealous of and inspired by Trevor's (Pybyr)'s monster w/a hx, I called his supplier, who pulled up his design and made me one to fit my furnace. I just had to have it.

Mike

If I didn't already have a run-of-the-mill HX....and a serious desire to go radiant in-floor asap....I'd probably look into one as well. This board is bad news to my savings account in so many ways...

 

[mole]

Trevor,
It sounds like there's a lot of people here with heat exchanger envy. What's important isn't the size, its that it makes your wife happy.

 

[pybyr]

Thanks for the kind remarks -- I'd originally been thinking that the air/ water HX would probably be an interim arrangement pending eventual radiant tubing -- but in implementation, I am so pleased with the results that the motivation for radiant is a lot less than before. I know all of the good reasons that radiant is excellent- and to my mind it's the only way to go if doing new construction or extensive renovation -- but I have to say that a low speed/ high CFM blower with a big HX can deliver a lot better performance and comfort than forced warm air is generally given credit for being able to do.

 

[dogwood]

I have been thinking of going with a hybrid system if my w/a hx-furnace combo doesn't put out enough heat. Trouble is my current forced air furnace does not put out a lot of cfms. It maxes out at 1320 cfms at 0.3 static pressure. This is an average static pressure off the furnace's fan data chart. Dont know what mine is in reality. The highest it might be is 1420 cfms, the lowest 1180 cfms off the same chart at various static pressures from 0.1 to 0.6, Lately I've been thinking if I installed radiant panels in some of the rooms, and closed those room's registers, the heat load the w/a hx had to produce would be reduced enough not to have to pull the existing funace out and put in a new blower.

If I am luckier, and get the superlative results Trevor's gotten, I will get enough heat through the hx so that that won't be necessay. 3000+ square feet is a lot of space to heat with 10 ft. ceilings. We shall see. I can run a lot of gpms through the hx to compensate for the lower cfms, but this I would think would deplete my storage too rapidly. Nationwide Coils figured at the low end, approaching 140-150 degrees water temp and 12 gpm, I'd be getting 80,000 btus. Delta T is 18 degrees. Fluid pressure drop is listed as 14.2, but I don't know what this means. If you could explain that Jebatty I appreciate it. I don't recall the high end output with 180 degrees water but it might have been somewhere between 135-145,000 btus. I'm hoping to run considerably less than 12 gpms and may come out all right.

My several heat loss calculations clustered around the 80,000 btu mark with one as high as 102,000 btus, which is what I designed for to be safe. My current furnace puts out 71,000 btu's and we're always chilly. However it does cycle on and off for unknown reasons, whereas air through the hx can be run constantly. Got a Grundfos SuperBrute 15-58 3-speed pump to push the water through so I wll have at least three speeds to play around with to get the flow right. I did get one inch inlets and outlets on my hx, but now wish I'd gone with the 1.25 as Trevor did. Missed a beat on that one. If anyone has any idea if my hx setup is workable please chime in. You might alleviate or increase my worries on this score until I actually get it in and operating. Good luck with yours MNBobcat.

Mike

 

[pybyr]

Trevor,
It sounds like there's a lot of people here with heat exchanger envy. What's important isn't the size, its that it makes your wife happy.

I never seemed to be able to keep my wife happy, but that's a different story, and, thankfully, well in the past.

My girlfriend is very happy with the results that my heat exchanger yields.

 

[pybyr]

I have been thinking of going with a hybrid system if my w/a hx-furnace combo doesn't put out enough heat. Trouble is my current forced air furnace does not put out a lot of cfms. It maxes out at 1320 cfms at 0.3 static pressure. This is an average static pressure off the furnace's fan data chart. Dont know what mine is in reality. The highest it might be is 1420 cfms, the lowest 1180 cfms off the same chart at various static pressures from 0.1 to 0.6, Lately I've been thinking if I installed radiant panels in some of the rooms, and closed those room's registers, the heat load the w/a hx had to produce would be reduced enough not to have to pull the existing funace out and put in a new blower.

If I am luckier, and get the superlative results Trevor's gotten, I will get enough heat through the hx so that that won't be necessay. 3000+ square feet is a lot of space to heat with 10 ft. ceilings. We shall see. I can run a lot of gpms through the hx to compensate for the lower cfms, but this I would think would deplete my storage too rapidly. Nationwide Coils figured at the low end, approaching 140-150 degrees water temp and 12 gpm, I'd be getting 80,000 btus. Delta T is 18 degrees. Fluid pressure drop is listed as 14.2, but I don't know what this means. If you could explain that Jebatty I appreciate it. I don't recall the high end output with 180 degrees water but it might have been somewhere between 135-145,000 btus. I'm hoping to run considerably less than 12 gpms and may come out all right.

My several heat loss calculations clustered around the 80,000 btu mark with one as high as 102,000 btus, which is what I designed for to be safe. My current furnace puts out 71,000 btu's and we're always chilly. However it does cycle on and off for unknown reasons, whereas air through the hx can be run constantly. Got a Grundfos SuperBrute 15-58 3-speed pump to push the water through so I wll have at least three speeds to play around with to get the flow right. I did get one inch inlets and outlets on my hx, but now wish I'd gone with the 1.25 as Trevor did. Missed a beat on that one. If anyone has any idea if my hx setup is workable please chime in. You might alleviate or increase my worries on this score until I actually get it in and operating. Good luck with yours MNBobcat.

Mike

One of the reasons that I went with the "anything worth doing is worth overdoing" HX is that it not only can readily transfer heat from the water to the air, it also creates the lowest possible resistance to the airflow. My furnace blower is rated, if I recall correctly, at 1600 cfm- so Mike, with your lower heat load, you may be fine.

One other thing you might consider, Mike, is taking a close look at your air system's ducting- to see what you can do to make sure that air is flowing readily, to the right places, with minimum resistance, and balanced supply/ return. Also, _plenty_ of return-- it's amazing how many systems you see with lots of registers but only a few dinky returns going into an undersized return trunk (furnace blowers don't do well when they have to pull hard to get return air). Also, that registers are on the outside walls and returns near the center of the house (yep, common sense, but again, not always what you find). Make sure that the joints of the ducts are a good fit- and if not, get some caulk and close them up, so that you're not sending conditioned air willy nilly out holes in the ducts into un-heated spaces (lots of little voids add up to a lot of air loss). When I bought this house, the preexisting ducts were a hodge podge that violated _every_ one of the above tenets- but I've gradually corrected them all, and each step yielded noticeable improvements. No furnace or HX is going to do well if it's fighting against those things.

 

[canyon]

Just a note for those that might not know, the stated BTU ratings on standard air handlers are for steam! You really do need to get the chart for heating water temp and flow rate to size appropriately. I found out the hard way some years ago!

 

[jebatty]

Nationwide Coils figured at the low end, approaching 140-150 degrees water temp and 12 gpm, I’d be getting 80,000 btus. Delta T is 18 degrees. Fluid pressure drop is listed as 14.2, but I don’t know what this means. If you could explain that Jebatty I appreciate it.

If pressure drop is 14.2 psi at 12 gpm, that equates to 32.7 feet of pump head at the same gpm, plus other pump head in your piping scheme (1 psi = 2.3 feet of pump head in round numbers). The pump curve for a 15-58 shows that it won't move that much water at 12 gpm. If my calculation is correct, then at 6 gpm pump head would be 9.7 feet, which is just about what a 15-58 will move on M. Switching it to H would make little difference. Pump head for other piping still needs to be added. So it appears that even on H a 15-58 likely will move about 6 gpm or less. That also means that btu's will 1/2 or less of 80,000, other factors unchanged.

Calculating pump head at various flows is a bit complicated. Roughly, if at X gpm pump head = A, then at gpm > X = X+Y, pump head is [(X+Y)/X]^1.75 x A. Example: gpm = 6 and pump head = 9.7 feet. Double the flow to 12 gpm. [(6+6)/6]^1.75 = 3.36; and 9.7 feet of head x 3.36 = 32.6 feet of head at 12 gpm. (Anyone else knowing more about this, please feel free to correct me or provide an easier way or easier explanation to deal with flow and head calculations.)

 

[Duetech]

MNBobcat,
My 20-20 hx has 1" ports and with water temps down to 120f (occurances with bridging and getting home too late to feed the coals and sleeping in and putting too much frozen wood in) the blower runs and runs. 140f is actually the lowest I can go in"cold" weather and not just have a large air circulator. 3/4" inlets would make the heat transfer critical and probably force my oil furnace to kick in. Even if you only go "run of the mill" you will probably be happier with the 1" ports. My boiler max's out at 178f so if you can improve on that you will see more of the optimum btu design for the heat echanger. In the real world everything we see will probably be less than what the max rating is any how so a little over sizing can compensate for a little under performance.

 

[Donl]

MNBobcat,
the blower runs and runs.

I put a strap on aquastat to my HX so that the fan will only run if the water temp is at the minimum level. This works well, and saves useless running of the blower motor.

 

[dogwood]

Thanks for your advice Trevor and Jebatty. Looks like back to the drawing board to refigure what sized pump will be needed to service that w/a hx. Didn't realize or understand how much head the hx was adding to that pipe run. Guess I now have a spare brand new Grundfos 15-58. Jebatty, I just went and looked at the hx spec sheet again. The pressure drop is listed in feet of water as opposed to psi. This should add only 14.2 feet of head to my original calculations as opposed to the 32.7 feet. If the 15-58 would have adequately serviced that line without 14.2 feet of extra head figured in, what sized 3- speed Grundfos pump do you think I would need to upgrade to to accomodate the amount of extra head calculated? I'll go find my original calculations for that line and see what I come up with adding that number in?

Trevor, appreciate your advice on the ducting. I'm actually in the middle of reconfiguring my main trunk now and re-taping all the joints with metallic duct tape. I had to pull out a length of plenum I had added years ago to service an upstairs addition. Didn't know you had to reduce the plenum's width ever so many feet (12-15' or so, if I remember) to maintain the proper air pressure in the ductwork as you get farther from the furnace. I am replacing that long section of plenum with 6' flex duct going to the individual registers.

The furnace is off the kitchen, about a third of the way in from the end the house. My current main return is fairly large and right next to the furnace. It's as big as the return opening on the furnace and located in a short hallway adjacent to the furnace on the first floor and open to the rest of the house. I have a smaller, and probably inadequate, 12' x 12"return servicing the upstairs which is about 1000 square feet or so in size. It runs through the 3.5 x 15" wall cavity into the main return below it. Overall the house has 21 registers and just those two returns. I installed all the registers are on the outside walls upstairs and downstairs on the ceiling close to the outside walls. Should there be more satellite returns scattered through the house rather than one or two centralized returns?

Since heat rises, the upstairs is always warmer than downstairs. However, the end room above the garage upstairs is always cold. Do you think ducting a return all the way to that room would help the air flow to that room? Its about 60 feet from the upstairs return register. I wonder how, or if, it would draw return air that far away.

On another note, have you insulated your plenum and ductwork. mine has insulation inside the main trunk line, but I thought it might be advantageous to put more insulation on the outside. Any suggestions on how to go about doing that on rectangular ductwork? Do people just lay pink fibergalss insulation on the ductwork or wrap it around somehow? Any suggestions would be appreciated.

Thanks again for both your thoughtful input.

Mike

 

[pybyr]

I'm not a pro, but my understanding is that placement of returns will depend a lot on whether you keep doors open, and how many partitions a house has. If you keep doors open and there isn't a lot of stuff in the way to resist the airflow, a couple of central returns may work OK. On the other hand, having a return upstairs (perhaps you can sneak it through the back of a first floor closet?) and at the far end of the house may help achieve more even temperatures, which may result in less need for heat input.

I haven't insulated my ducts yet, though perhaps I should (but there are other things on my house I still need to tighten up that are higher priority). I'd avoid fiberglass- rodents tend to either carry it off or nest in it, There are probably materials made for duct insulation- Google around.

 

[dogwood]

The more I think about this the more I realize I am confused by the terminology being used on the hx spec sheet. The spec sheet says exactly this: "Fluid pressure drop (ft. of water) = 14.2". Is there a direct relationship of this figure to head loss, or some way to derive from that number what the head loss may be. Doesn't make sense to me that one hx can add 14.2 feet of head, but who knows. There is also a number for fluid velocity (ft/s) = 5.11, if that's any help to figure this out. Anybody know the relation between the two, or maybe a ballpark figure much head a good sized w/a hx might add?

Mike

 

[jebatty]

“Fluid pressure drop (ft. of water) = 14.2”.

14.2" x .036 = 0.51 psi x 2.3 = 1.2 feet of head at fluid velocity = 5.11 ft/s.

fluid velocity (ft/s) = 5.11

I'm assuming 3/4" pipe:

Known: Pipe I.D. (inches), Fluid Velocity (feet/second)
Unknowns: Liquid Flow Rate (GPM)
Flow Rate (GPM) = 2.447994 x (pipe I.D.)2 x Fluid Velocity

gpm = 2.45 x (0.75)2 x 5.11 = 7 gpm

Assume delta-T=20, then at 7 gpm, btu = 7 x 20 x 500 = 70,000 btu

Does this help?

BTW, I got all of these calculation from Google. For example, "convert in of water to psi" gave me the first answer. And "gpm, fluid velocity" gave me the second answer. And the quick formula for btu is "gpm x delta-T x 500."

 

[dogwood]

Jim, I have 1.5 inch pipe right up to the 1.0 inch intake on the hx. I know the required flow is 12 gpm at 150 degrees water to get the minimally needed btu output. I just need to know how much head the hx is adding to the system to figure which pump to get. If I understand your first calculation, is 1.2 feet of head what my hx exchanger adding to the total head for that pipe line. That low figure may hopefully even make my UPS 15-58FC workable once I recalculate the total head for this line, which I will do immediately.

I appreciate your patience in explaining this. I've been googling this subject all afternoon without doping it out. Its not easy being an amateur in this field. Thanks again.

P.S. If 1.2 feet of head is all that is added to the line, the Grundfos should be able to do the job on high with a total of only 2.726 of head and flow of 12 gpm. Hope I understood your first calculation correctly. Please let me know.

Mike