Initial Experiences with Plenum Heat Exchangers

[rickh1001]

I just completed installing the last of 3 plenum heat exchangers, into oil hot air furnaces that serviced our converted barn/home (two furnaces - one at each end), and a smaller farmhouse 50 ft away. Although the weather has just started turning colder (it is about 30F today), it just seems to me they are not putting out the heat equivalent to the oil burners they are replacing. I am using 16X18" units for the two furnaces in the barn. These are "rated" at about 129,000 btu/h. I measured the air temp coming up at the register at about 140F, when the boiler system was heated to about 175F. As oil burners, I used to put in a 0.7 gal/min nozzle, or about 97,000 btu/h, and they would run for fairly short periods then shut off. However, the heat exchangers seem to run forever, even with only a 30F outdoor temp, as though they are not really putting out that much heat. I think they won't be able to handle it when it hit 0F or below. I certainly don't believe they are putting out more heat than the oil burners did. I haven't measured the exact supply/return temps on each of the zones yet - but both lines are very hot. I am running 1" copper lines, and have should at least 8 gal/min as calculated by several approaches. The supply and return line temps don't feel that different (this is very unscientific at this point, and I don't expect my hands to accurately detect 20F differentials), but it seems the exchangers are not extracting as much heat as they should, rather than it being a supply problem. I feel I am not getting anywhere near the same heat out of the heat exchangers that I did with the oil burners, that theoretically should be putting out less heat.

I know this is a complex question, but in general, what experiences have others had on using these plenum heat exchangers of this general size?

There is a downside to using plenum heat exchangers that I am starting to see. Because they seem to run much longer, the fan electricity is going to add up over the winter. Eventually, I want to replace them with baseboard and floor radiant heat, which will be much more efficient electricity-wise. These units allowed me to get the system going quickly, and retain a backup heat source. However, as I am watching their performance, I am not impressed so far.

Does this match up with others, or did I do something wrong on the install?

 

[Tony H]

I noticed the same with my system running one heat ex 20x19 . The output air is not as warm as with the gas and it runs longer. I turned the speed on the pump to high and this seemed to help some , the temp out the outlet water is much cooler than the input on mine. Since this is my first year I am also wondering how it will work when the temps get way colder

 

[pybyr]

I haven't yet got my water-air HX running, but before I bought it, I looked into them a lot, and found that a lot of the specs are dependent not only on water temp and flow but also air flow, and it seemed to me that some of the specifications used to market them (at least the ones marketed on ebay and/ or outdoor boiler supply sites) tend to assume parameters that seemed over-optimistic to me.

That's why I ended up going with a big 4-row coil that I ordered from Nationwide Coils after Ross there helped me by modelling it under a variety of temps and flows (lots of the ones generally marketed seem to be 2-row coils)

 

[pybyr]

one other question/ suggestion to check- are you guys certain that you have these hooked up to run in counterflow- that is, the hot water goes in the part of the coil nearest where the air exits, and cool water comes out nearest where cold air enters?

that'll keep max temp differential between the two media (air and water) at all points, which is a much more efficient way to transfer maximum heat than the other way 'round

 

[Tony H]

pybyr - Let me get this straight it makes a difference to the heat exchange temp gain based on the direction of flow thru the exchanger ? Mine is installed with the hot entering the lower side closest to the air input and cooler water leaves at the top where the heated air exits. This appears to be backwards from your example.
The water is much cooler leaving the exchanger so I know it's transfering alot of its heat but how much am I losing because it's flowing the wrong direction ?
It can be changed but it won't be fun to take apart all that stuff again but is it worth it in your opinion ?
Another fix I have considered would be to add another coil located downstream to tramsfer more heat .
Thanks
Tony

 

[rickh1001]

pybyr,

Great idea on bringing the supply in at the top and the return out the bottom, to maximize heat differentials. I am kicking myself for not thinking of it that way. Instead, I brought the supply into the bottom, and out the top, thinking only of forcing any air out more easily. However, what you say makes perfect sense. Looks like I have some re-soldering ahead of me!

Overall though, I am still getting the sense that the heat exchangers I have won't be enough for -20 deg temps. I am starting to think about tapping off the supply lines to some baseboard units, while keeping the plenum exchangers, to get more of the heat out of the various zones.

 

[applewood]

I'm planning to go coil in plenum also so I'm wondering which 4 coil unit you ordered and how expensive they are? May decide to go directly to under floor pex.

 

[Medman]

I also am only going to be using a coil in the plenum of my furnace. It is my only option because I have a slab-on-grade house with exterior walls of concrete block. (Original structure is from 1960 and was built as a cottage.) All exterior walls have 2x6 insulated walls sistered outside the structure, but inside access for piping, etc. is limited. It is a downflow configuration with all supply ducts run in the slab with all returns in the attic space (all new and insulated as of last week).
I purchased a new air handler with a variable speed DC fan to replace the old furnace, partly because I was concerned about the cost of running the fan all the time. I plan on running the fan at low speed all the time, with the speed increasing when the thermostat calls for heat. Hot water will only flow through the coil when the t-stat calls for heat as well. This hopefully will keep the ducts in the slab warm and keep the returns in the attic warm.
The slab is over two feet thick and under the furnace room it is over four feet thick - I had to jackhammer a new hole for the supply plenum to locate it in a more convenient spot. This mass will absorb a lot of heat. I hope it holds the heat as well.

Firing should happen this week, but no later that Dec. 1. (fingers crossed!)

 

[jgrimm]

The charts I have seen for water to air heat exchangers show the "rated" outputs at the highest airflow, gpm and water temp along with the lowest entering air temp. Yours is probably rated the same. For example, 1400 CFM, 12 gpm, 205 deg water, and 55 deg air. None of these numbers are realistic, your HX dealer should know this. My real life experience with a 140btu rated HX is closer to 71k calculated from 7 GPM and ~20deg water temperature drop.

Do you know what your GPM is?

Changing to counter flow will help. When you do that add a thermometer on the inlet and outlet.

 

[pybyr]

pybyr - Let me get this straight it makes a difference to the heat exchange temp gain based on the direction of flow thru the exchanger ? Mine is installed with the hot entering the lower side closest to the air input and cooler water leaves at the top where the heated air exits. This appears to be backwards from your example.
The water is much cooler leaving the exchanger so I know it's transfering alot of its heat but how much am I losing because it's flowing the wrong direction ?
It can be changed but it won't be fun to take apart all that stuff again but is it worth it in your opinion ?
Another fix I have considered would be to add another coil located downstream to tramsfer more heat .
Thanks
Tony

any exchange process (heat, chemical, etc.) is rendered a lot more efficient if you can maintain counterflow (your kidneys work on this principle)--

here's an explanation

http://en.wikipedia.org/wiki/Countercurrent_exchange

in the liquid to air heat exchanger setting, counterflow means that the coldest air hits the coldest portion of the coil, but, based on that difference, still picks up some heat.

as that partly-warmed air moves through the fins, it heads to warmer fins & tubes, where it picks up still more heat along the way.

without counterflow, your cold air picks up whatever heat it can pick up in a single pass by the hottest coil, and then it's literally downhill all the rest of the way through the exchanger

 

[pybyr]

I'm planning to go coil in plenum also so I'm wondering which 4 coil unit you ordered and how expensive they are? May decide to go directly to under floor pex.

if you go with the coil, call Ross at "Nationwide Coils," and have your data on needed BTU outputs, water temperatures and air CFMS handy. he can then work with you to figure out what most closely meets your needs and at what cost, and then he'll work directly with the manufacturer to have a coil made, fast, to not only fit your thermal parameters, but also your duct sizes and available space

 

[mtfallsmikey]

Depending on entering hot water temp, etc. 130-140 deg. outlet air temp. is about right. An oil/gas furnace (especially older ones) has leaving air temps close to 190 deg.

 

[rickh1001]

pybyr

I have the same question as TonyH, and I wondering if during your research, you came up with any numbers regarding the relative efficiencies of the overall heat transfer, if you plumb it counterflow or "normal"? Do you think it will it be a 5% type of improvement, or 50%?

It won't be all that hard to re-do the piping near each HX, but I just convinced myself a few days ago that I was all done, the air is out of the system and everything is starting to purr. Hate to shut everything down and redo the plumbing unless it is worth it.

Also, I have no idea of the cfm for the air handler on the furnace. All three furnaces I am using are similar systems, which are normal sized household furnaces rated at (I think) about 110,000 btu. I can try to track down the manuals or go online, but does anyone know the ballpark cfm figures for such household furnaces? All three zones are running 8-10 gal/min, and the storage tank temp runs anywhere from 160 - 180F.

I can say that a seat of the pants estimate of the btu's I am getting out of the HX is that they are only putting out 50-60% of what the oil burners did - even though I was using only a 0.7 gal/min nozzle in each one. So I would guess I am seeing 40-50,000 btu max from each of the HX's, just guessing from the way the house warms up as compared to the oil burners. I am going to need to get more heat than that out of them, for the really cold weather.

 

[pybyr]

pybyr

I have the same question as TonyH, and I wondering if during your research, you came up with any numbers regarding the relative efficiencies of the overall heat transfer, if you plumb it counterflow or "normal"? Do you think it will it be a 5% type of improvement, or 50%?

It won't be all that hard to re-do the piping near each HX, but I just convinced myself a few days ago that I was all done, the air is out of the system and everything is starting to purr. Hate to shut everything down and redo the plumbing unless it is worth it.

Also, I have no idea of the cfm for the air handler on the furnace. All three furnaces I am using are similar systems, which are normal sized household furnaces rated at (I think) about 110,000 btu. I can try to track down the manuals or go online, but does anyone know the ballpark cfm figures for such household furnaces? All three zones are running 8-10 gal/min, and the storage tank temp runs anywhere from 160 - 180F.

I can say that a seat of the pants estimate of the btu's I am getting out of the HX is that they are only putting out 50-60% of what the oil burners did - even though I was using only a 0.7 gal/min nozzle in each one. So I would guess I am seeing 40-50,000 btu max from each of the HX's, just guessing from the way the house warms up as compared to the oil burners. I am going to need to get more heat than that out of them, for the really cold weather.

others around here have the actual formal technical horsepower (whether by education, experience, or both)- that I don't- to tell you _how_ big a difference counterflow makes. I do know that it is always used in formal design of exchange circuits

and think of it this way- I don't know if you have a 2 row coil or a 4 row coil, but if you have a 2-row coil in non-counterflow, then once the air has passed the first row of tubes, the second can't add any serious heat, because the fluid there is cooler than the coil that the air has already been through- so you're losing a substantial chunk of the coil's effective area. the same effect would be even more extreme if you have a 4 row coil, because only one out of 4 rows of coils is really able to do its best to heat the air passing through.

if you get the make and model of the furnace and then call the furnace manufacturer, it should not be a big deal for them to look up and tell you the CFMs; that's what I did.

While you are at it, also find out from the furnace manufacturer how much back pressure the blower can take- because, especially if you need to back up and order a new coil or coils, you'll want to be able to take that into consideration, too, so that your coil is not choking off the blower's ability to move air. Or, you may want to even try to measure what sort of air pressure you have on either side of your existing coil, because if it is somehow restricting the air flow your blower is able to achieve, then that'll also be crippling your net heat output.

Most of the coil vendors gave me "huh" responses to efforts to identify or account for such variables;

Ross at Nationwide was able to work with me to come up with a coil (24x24x 4 rows of 1/2 tube) that should move a heckuva lot of BTUs even when my water from storage starts getting on the cooler end of its range, and while introducing very little back pressure for my relatively slow moving 1600cfm blower in my ThermoPride. And it was delivered quickly - sooner than they even promised.

 

[Donl]

Damn! Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info. I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.

Don

 

[pybyr]

Damn! Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info. I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.

Don

my first reaction after reading your post was that if you have everything running really well, then why mess with success, regardless of what's optimum in theory- but then again, if your system is new, and hasn't been through the coldest times yet, you may want to switch your coilto counterflow before then

 

[pybyr]

The charts I have seen for water to air heat exchangers show the "rated" outputs at the highest airflow, gpm and water temp along with the lowest entering air temp. Yours is probably rated the same. For example, 1400 CFM, 12 gpm, 205 deg water, and 55 deg air. None of these numbers are realistic, your HX dealer should know this. My real life experience with a 140btu rated HX is closer to 71k calculated from 7 GPM and ~20deg water temperature drop.

Do you know what your GPM is?

Changing to counter flow will help. When you do that add a thermometer on the inlet and outlet.

that's one of the beauties of dealing with some folks who are really pros at this- not just people making a sales pitch based on a best-case-scenario-that-never-really-happens set of assumptions-

and Ross at Nationwide Coils http://www.nationwidecoils.com/ (who is basically a distributor for Precision Coils http://www.precision-coils.com/ , who also took some time on the phone with me, but do not sell direct to end users) is someone who can and will will help you model the real variables that are likely to actually represent your particular system.

In the part of the trade that Nationwide/ Precision serve, "sort of works" means out of business, so they need to know how to get it right.

My only surprise, pleasantly, was that Precision was glad to speak with me on the phone, and after I found that they did not sell to end users, and asked who was a "rep" who'd be good at dealing with a DIY-er like me, pointed me to Ross, and he was all of the above

 

[Donl]

Damn! Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info. I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.

Don

my first reaction after reading your post was that if you have everything running really well, then why mess with success, regardless of what's optimum in theory- but then again, if your system is new, and hasn't been through the coldest times yet, you may want to switch your coilto counterflow before then

I could leave it as is, but that's not me. My curiosity alone needs to know how much a difference it will make. Even though everything is running ok, as I learn, I will make changes. That's the fun of it! Reversing the flow in the HX may result in faster house warm ups and less run time of the blower motor. That's a good thing.

Don

 

[pybyr]

Damn! Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info. I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.

Don

my first reaction after reading your post was that if you have everything running really well, then why mess with success, regardless of what's optimum in theory- but then again, if your system is new, and hasn't been through the coldest times yet, you may want to switch your coilto counterflow before then

I could leave it as is, but that's not me. My curiosity alone needs to know how much a difference it will make. Even though everything is running ok, as I learn, I will make changes. That's the fun of it! Reversing the flow in the HX may result in faster house warm ups and less run time of the blower motor. That's a good thing.

Don

glad to know that I'm not the only one that ticks that way

 

[Tony H]

Damn! Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info. I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.

Don

my first reaction after reading your post was that if you have everything running really well, then why mess with success, regardless of what's optimum in theory- but then again, if your system is new, and hasn't been through the coldest times yet, you may want to switch your coilto counterflow before then

I could leave it as is, but that's not me. My curiosity alone needs to know how much a difference it will make. Even though everything is running ok, as I learn, I will make changes. That's the fun of it! Reversing the flow in the HX may result in faster house warm ups and less run time of the blower motor. That's a good thing.

Don

glad to know that I'm not the only one that ticks that way

I for one am glad both you guys think that way, I am in a if it's working fine don't F it up mode. I am very interested in what happens on your system Don L . Are you able to measure input and output water temps before and after ? That would give a rough idea of the increase in heat transfered by changing the flow direction. I know there are alot of new systems coming online including mine and this seems like a good thing to know.

 

[Donl]

Damn! Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info. I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.

Don

my first reaction after reading your post was that if you have everything running really well, then why mess with success, regardless of what's optimum in theory- but then again, if your system is new, and hasn't been through the coldest times yet, you may want to switch your coilto counterflow before then

I could leave it as is, but that's not me. My curiosity alone needs to know how much a difference it will make. Even though everything is running ok, as I learn, I will make changes. That's the fun of it! Reversing the flow in the HX may result in faster house warm ups and less run time of the blower motor. That's a good thing.

Don

glad to know that I'm not the only one that ticks that way

I for one am glad both you guys think that way, I am in a if it's working fine don't F it up mode. I am very interested in what happens on your system Don L . Are you able to measure input and output water temps before and after ? That would give a rough idea of the increase in heat transfered by changing the flow direction. I know there are alot of new systems coming online including mine and this seems like a good thing to know.

I'll try to take a few measurements before and after to see if we can see any meaningful change. My worry is that there are so many variables involved that it will be difficult to get the before and after parameters just right. I think the best indication of improvement will show up in higher plenum air temperatures. I'll let you know what happens. I hope to do this tomorrow sometime.

Don

 

[rickh1001]

You know, I have been thinking a bit about this whole plenum HX approach. It was the quickest and easiest way to get the system going. Now I am starting to see some of the shortcomings of this approach. First, the good side. My wife has not hassled me for a week or more, since we actually have sensible heat, and I temporarily no longer have to endure the terrible fate that having taken out a home equity loan, that in fact the new system will not work. It works, and we have burned no oil for many weeks now. The flip side is, that come mid-January, when it is -20F and the winds are blowing, that those meager little HX's are going to have us all wrapped in blankets. As they (used to) say, no one gets fired for buying IBM. At the end of the day, the new system has to work. I think the plenum HX approach gets one started, but by far isn't appearing to be the best long term solution. I am hoping the dog eats the electric bill for running the blower motors so long, before we get it. Plus, it is almost impossible to think about running the system in a prolonged power outage, without a generator, and the air handler has to be hooked up to the backup, rather than just a circulator.

So I am thinking about alternatives, and Dan Holohan's love of radiant heating comes to mind. For our converted barn, we have major volume, with 13 ft ceilings in the 35 ft half of it, and conventional 8 ft ceilings in the other half/bedroom area. Everything I have read says that convective heat is worst in tall ceilings. Radiative heat is the key here. Ideally, I would put in a second floor on top of the existing one with radiant heat underneath - but things have been a little busy, and I am still waiting for the numerous blisters to clear up after soldering so many joints on the main heating system. So forget a new floor for a few years. But Holohan got me thinking, that I have plenty of wall space and ceiling space available. Instead of this somewhat ridiculous little HX stuffed into the plenum of the oil burner, why not put up expansive panels of radiant heat surfaces? It makes no difference if they are on the floor, the walls or the ceilings.

As I have been thinking the problem over, radiant heating is inherently much more suitable for our wood heat systems. We all seem to struggle to maintain even 170F, yet radiant systems work with temps of <100F. So why struggle with an inefficient approach, of trying to heat to 180F or so, then extract the heat out with a water/air HX? As I look around the rooms, forgetting the floors, there is all kinds of space on the walls and ceilings to put in attractive panels, that could radiate more than enough heat for each room. For example, right now looking to my right, we have a map/bulletin board roughly 4X6 feet. I could make that instead a 4X6 ft radiative panel, and still put the corkboard and map over it. As Holahan noted, it doesn't matter if the heat source is on the floor, the walls or the ceilings - it is the same heat. Heat doesn't rise - it transmits via radiation, convention and conduction. Hot air rises (both politically and thermodynamically unfortunately). So I am thinking rather than put up with the problems of the HX, I will start to figure how to put radiant heat surfaces into the room. Eventually, I will use the same zone plumbing to feed these, and eliminate the plenum HX and its power-hungry fan, and be able to heat the whole house with only a UPS to feed the EKO and the zone circs.

The Hx's were an OK first step to get the system up and running - but I don't see them as a viable long term heat solution.

 

[pybyr]

You know, I have been thinking a bit about this whole plenum HX approach. It was the quickest and easiest way to get the system going. Now I am starting to see some of the shortcomings of this approach. First, the good side. My wife has not hassled me for a week or more, since we actually have sensible heat, and I temporarily no longer have to endure the terrible fate that having taken out a home equity loan, that in fact the new system will not work. It works, and we have burned no oil for many weeks now. The flip side is, that come mid-January, when it is -20F and the winds are blowing, that those meager little HX's are going to have us all wrapped in blankets. As they (used to) say, no one gets fired for buying IBM. At the end of the day, the new system has to work. I think the plenum HX approach gets one started, but by far isn't appearing to be the best long term solution. I am hoping the dog eats the electric bill for running the blower motors so long, before we get it. Plus, it is almost impossible to think about running the system in a prolonged power outage, without a generator, and the air handler has to be hooked up to the backup, rather than just a circulator.

So I am thinking about alternatives, and Dan Holohan's love of radiant heating comes to mind. For our converted barn, we have major volume, with 13 ft ceilings in the 35 ft half of it, and conventional 8 ft ceilings in the other half/bedroom area. Everything I have read says that convective heat is worst in tall ceilings. Radiative heat is the key here. Ideally, I would put in a second floor on top of the existing one with radiant heat underneath - but things have been a little busy, and I am still waiting for the numerous blisters to clear up after soldering so many joints on the main heating system. So forget a new floor for a few years. But Holohan got me thinking, that I have plenty of wall space and ceiling space available. Instead of this somewhat ridiculous little HX stuffed into the plenum of the oil burner, why not put up expansive panels of radiant heat surfaces? It makes no difference if they are on the floor, the walls or the ceilings.

As I have been thinking the problem over, radiant heating is inherently much more suitable for our wood heat systems. We all seem to struggle to maintain even 170F, yet radiant systems work with temps of <100F. So why struggle with an inefficient approach, of trying to heat to 180F or so, then extract the heat out with a water/air HX? As I look around the rooms, forgetting the floors, there is all kinds of space on the walls and ceilings to put in attractive panels, that could radiate more than enough heat for each room. For example, right now looking to my right, we have a map/bulletin board roughly 4X6 feet. I could make that instead a 4X6 ft radiative panel, and still put the corkboard and map over it. As Holahan noted, it doesn't matter if the heat source is on the floor, the walls or the ceilings - it is the same heat. Heat doesn't rise - it transmits via radiation, convention and conduction. Hot air rises (both politically and thermodynamically unfortunately). So I am thinking rather than put up with the problems of the HX, I will start to figure how to put radiant heat surfaces into the room. Eventually, I will use the same zone plumbing to feed these, and eliminate the plenum HX and its power-hungry fan, and be able to heat the whole house with only a UPS to feed the EKO and the zone circs.

The Hx's were an OK first step to get the system up and running - but I don't see them as a viable long term heat solution.

none of us ever said that air was an ideal way of transferring a lot of heat to a large cubic voume in a cold climate... I knew that even before I dropped the better part of a grand on a ridiculously big water-air HX.

but if forced air is what you've got (like me) and if you don't have a bottomless wallet to change the whole system while you shift to wood gasification+storage, then a properly sized water-->air HX is not a stupid thing to consider. and, to my mind, better to "blow the dough" once, and hopefully only once, on something that'll work really well than to try to cheap out on something that is affordable up front but ineffective in the long run

and/ or if, like me, you have existing floors that you want to keep (in my case, 180 year old random width old growth spruce planking 2+ inches thick) that are incompatible with meeting your whole heat load with radiant, then air can be part of the solution

between figuring out my ancient house that was scarily close to falling in on itself when I bought it, and looking at "state of the art" new houses that are so tight that they need air+air HX units to maintain indoor air quality while they heat with radiant floors that are slow to respond to changes in desired temperature, I increasingly think that a "hybrid" approach--- radiant for the "base load" of keeping the chill out, plus a timed air system (which can respond faster than radiant panels) would be the cat's meow.

I suppose, as usual, that the Europeans are ahead of us on this, as their wall-hung panel radiators combine radiant and convection.

 

[mtfallsmikey]

A 110K BTU furnace would probably be rated in the ballpark of 1200-1400 cfm, sans AC/hydrocoils, etc. Duct sizing can be critical here too. As well as the GPM you are running thru the coil. I agree...my HX was a stopgap measure as well, the original plan was to do BB with TRV's, but cost and time came into play. I'm also looking at redoing my floors next year, only want to pull the base trim once.

 

[Medman]

Radiant wall panels would be a good approach for me as well, especially considering the amount of accessible wall space I have. However, I would have to run the supply/return piping for the zones in the attic, and the thought of spending any more time up there (after a week of replacing ducts and insulating) makes my back hurt. Maybe in a year...or two...

I am interested to see how well the new air handler/backup electric furnace works. With my old electric furnace, 70 k BTUs of electric heat could not maintain 68 degrees in January, even though my heat loss calcs come in under 50 k BTUs worst case. When I got in the attic to add some additional returns, I found that at some point in the past one of the renovation contractors had crushed a section of the return trunk. The damage was not visible because the duct was covered in insulation, however the seams were split and the return was basically pulling in cold attic air. The furnace was trying to heat this below freezing air and could not maintain temp.
Now that I have replaced all the return ducts and sealed all joints tight, the new furnace should be able to keep the temp. up using only electric. The cost for electric will still be the same, but the temp should be higher. With the plenum coil installed, I should be able to move the required 50k BTUs into the house without issue.
Also, the new air handler has a variable speed DC fan, which uses about 1/10 the electricity of the old single speed AC blower. This should help reduce costs too. The bonus is that I can adjust the speed to account for higher backpressure from the coil. I am using a differential air pressure gauge to measure the pressure on the ducts, but I haven't connected it yet.