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Options for hydronic air handler control: properly coordinate ECM blower motor and variable-speed condenser compressor

x_david_x | Posted in Mechanicals on


First post. Not sure if this is best in Mechanicals or Energy Efficiency. Please move as you see fit.

High level: Replacing condenser unit and air conditioner coil. Since the work is being done, now is the time to upgrade the air handler mechanicals and controls as well. I need help understanding options for properly coordinating an ECM infinitely variable blower motor with a multi-stage/variable speed condenser compressor. And this needs to work with my hydronic heat too (currently air, but will be adding radiant at some point).

Details: Current system is comprised of a modulating condenser boiler (Buderus GB142-60) with a closed loop to an indirect water heater for domestic hot water. The home heating utilizes the remnants of an older all-in-one system air handler with a heating coil, and the original control board for that system (Lennox CompleteHeat AM30). The A coil sits above this air handler. The blower motor is capable of multiple speeds, but I believe when the system was retrofitted to work with the new boiler, it was reduced to single speed duty.

There are 3 zones, managed by mechanical duct zone dampers. Technically, there is a 4th, AC-only zone in the house, but that is managed independently (a smaller AC unit and air handler in the attic to cool an upstairs easier). As mentioned above, I do intend to add infloor radiant in the future. This might end up being a mix of under existing floors and in a concrete slab in newly renovated areas. Due to unknowns/variables of the future, the number future of radiant zones is also an unknown. If I had to guess, I would say anywhere from 2 to 6 radiant zones. The radiant zones don’t necessarily need to be managed by the same controller system.

The old AC uses R22. The new will obviously use R-410a and require a new A coil. The air handler has a number of water pipes in front of the part of the plenum with the A coil, which will increase labor on swapping out the A coil.

Since all this work is being put into a new AC system, I’ve decided the time is right to upgrade the hydronic air handler system at the same time. I’m looking at all components and looking at options to take advantage of the current technology out there. Specifically, I’d like to go with a variable speed ECM blower motor and a variable or 2-stage AC condenser unit. I realize ROI might be weak on some of these systems, but I’m looking more at the comfort factor.

Where I am struggling is in a control board that will drive an infinitely variable ECM blower motor in support of the mod-con boiler to a hydronic heat coil, as well as interfacing properly with a variable speed condenser compressor. In a perfect world, this would be a third party control board so that I’m not locked into a particular vendor’s system. It would just be a normal, 24 volt system and I’d be free to mix and match other, third party thermostats.

What I seem to be finding is that when the vendors went to variable speed (more than 2 speed) condensers, they decided to lock customers into their electronics, forcing control board and thermostats to be from the same vendor as the condenser. From the little research I’ve done, it seems that using 3rd party thermostats with these systems, for example, reduces a variable stage condenser to use only 2 stages and may limit other benefits of the system. At that point, it makes no sense to buy a variable speed unit if it is going to be restricted to 2 stages.

Side note: I’m sure some people would decide to not buy the variable speed condenser because of this. I sure hope the vendors reconsider the path they are taking.

One last question I have regards infinitely variable speed ECM blower motors. I say “infinitely variable,” because it appears the term “variable” usually means a fixed number of speeds (much the way variable for compressors does). The motors may be capable of infinite speed adjustments, but they are being restricted to set speeds. In such a use case, will the ECM motors still independently adjust to air pressure variations, or will they be forced to stay at the speed set by the control board?

Products: Here is a list of products I’ve jotted down. I have no loyalty to any brand. I will say that due to problems with the Lennox product, I probably won’t be buying more from them. That doesn’t mean that they don’t have good systems, but I’m still not doing it. 🙂

Condenser products:

  • Variable speed Bryant (189BNV)
  • Variable speed Carrier (24VNA9)
  • 2 stage Goodman (DSXC18)

Hydronic air handler products:

  • Ecologix (EZT – Zone Comfort Tall System)
  • First Company (possibly model VMB-277 Series (Variable Speed), but their website is confusing)
  • Summeraire (TC3550E)
  • Lennox CBWMV — as noted above, there would have to be a darn good reason to go with it

3rd party controller products:

  • Tekmar (tN2 House Control 402)
  • Aprilaire (Aprilaire Model 6504 Zoned Temperature Control)
  • Honeywell (model??)
  • Others?

ECM blowers

  • ???

I’m assuming the boiler can’t be dynamically adjusted by the system and that I’ll simply be adjusting the temperature that it will hold (for house heat) and relying on the modulation of the blower air speed and duration to account for higher or lower demand. If there is a product that dynamically/magically adjusts this particular boiler (GB142), I’m all ears.

Although I listed hydronic air handler products and 3rd party controllers, I don’t really know if they will accomplish my goal of properly integrating components in this system.

I appreciate any help people can provide!


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  1. GBA Editor
    Martin Holladay | | #1

    Can anyone say "ductless minisplit"?

    Your question is a textbook example of why complicated, site-built heating and cooling systems have more maintenance and comminsioning problems than "appliance in a box" approaches like ductless minisplits. (Ductless minisplits are engineered at the factory, not in a client's basement.)

    To answer your question: There are reasons that ECM blowers must be hooked up to electronic controls provided by the manufacturer of the ECM blower. Otherwise, things get complicated fast.

    I advise you to choose an optimized single-speed blower with a cfm rating that matches your needs.

    The more complex your system, the harder it will be to troubleshoot future problems.

  2. Expert Member
    Dana Dorsett | | #2

    The boiler will modulate it's firing rate based on the return water temp, so if the blower is running at a lower speed it'll throttle back to the extent that it can. The GB142-60 is a pretty gia-normous boiler relative to the heat loads of most houses smaller than 12-15,000 square feet, and it takes a pretty big air handler & coil to deliver even the MINIMUM-fire output of 63,000 BTU/hr (nearly 2x the heat load of my house @ 0F), and that may or may not be reasonably sized for the right-sized cooling coil for your house. Modulation ranges are not infinite.

    Before you can make reasonable or optimal equipment choices you need to nail down the heating & cooling loads accurately. If you have functioning cooling system, you can measure the duty cycle of the condenser on a hot day to narrow in on the cooling load.

    If you have some mid to late winter gas bills, you can nail down the heating load comparing fuel use against heating degree days:

    Or, you could hire a RESNET rater or engineer (and NOT a run of the mill HVAC contractor) to run a careful but aggressive Manual-J load calculation (and probably save more money on raw equipment costs by down-sizing than the professional calculation costs.)

    I suspect the existing equipment is woefully oversized for your loads, and you may be better off taking a different approach.

  3. x_david_x | | #3

    Hi Dana,

    Thanks for your constructive response.

    I understand that the boiler will modulate to hold the temperature for the loop going to the hydronic coil. What I was getting at is that an optimal solution would involve the boiler modulation and the ECM fan modulation working together, with knowledge of delta T and knowledge of the boiler's efficiency curve to find the best combination, regardless of external variables. I realize that's a pipe dream (pun half intended), and probably not worth it, but I figured I'd ask just the same.

    That wasn't my main question though. My main area I'm soliciting help for is: I need help understanding options for properly coordinating an ECM infinitely variable blower motor with a multi-stage/variable speed condenser compressor. And this needs to work with my hydronic heat too (currently air, but will be adding radiant at some point).

    I'm afraid that I'm going to be stuck using the AC vendor's electronics and that is what I'm trying to avoid, because that's the same situation I find myself in currently, where those electronics are very difficult to replace. In a perfect world, there would be standards that would allow for any number of 3rd parties to develop controls that could work to optimize this system. It seems when they went from 2 stage to N stage compressors, the vendors decided to go down the proprietary route. I'm trying to find out if there are 3rd party controls that can interface with these proprietary systems.

    My alternative option is to go with a 2 stage compressor (that doesn't use proprietary communication to the control board) and use 3rd party controls/air handler/ECM motor/etc. I really like the idea of the compressor being able to run low a lot of the time and run one notch up slightly less of the time, etc., as opposed to just low and high speeds. Low and high would still be a lot better than just high, but now is the time to optimize this part of the HVAC system.

    Another alternative is to go with the variable speed compressor, but use 3rd party electronics with it and use it in a 2 stage manner, until the 3rd party controls get smart enough to work with the variable speed compressor. Pretty darn tough to gauge whether that would ever happen. More likely is (probably/hopefully) that the industry will adopt new communication standards and vendors will feel pressured into supporting those standards with new products (not the products on the market today).

    My crystal ball says that the systems themselves are getting complex enough that analog signals over copper wires aren't going to cut it anymore. They need to switch to computer networking communication and get into the Internet of Things. We're just not quite there yet. We have smart thermostats that can bridge the gap between the modern world and the old 24 volt world, but that bridge appears to have some road blocks being put in place.

    Regarding the size of the boiler, I can certainly do those calculations and report back. However, I think you'll be surprised by the results. The house square footage isn't as big as those you mention, but it has a vaulted ceiling and a LOT of windows. Additionally, it is in zone 4a and humidity, although not the deep south, isn't non-existent either. I didn't build it. I bought it and I'm just trying to make some reasonable choices along the way to reduce consumption. Replacing a 5 year old boiler probably isn't going to happen. :-)

    I don't have a working AC in order to do the duty cycle calculations. Trust me, I wish the AC was working this week! The big AC unit died at the beginning of this week. We have a smaller AC unit with a blower in the attic (top of the vaulted ceiling) that can cool the upstairs and I had to pull out a big window unit and a bunch of fans to use in the meantime to barely keep up with the demand on the main level. That can't be used for any type of meaningful calculations.

    The benefit of the modulating boiler, multi-speed compressor, and multi-speed/infinitely variable speed ECM blower is that they can, in theory, make up for some amount of imperfection in sizing and relative sizing (optimal air handler for heat vs optimal air handler for cooling). I realize the modulation ranges aren't infinite, but it is better than no modulation.

    I'll report back when I run the heat load numbers.

    In the meantime, if anyone has any equipment ideas on integrating hydronic heating with a variable speed cooling unit, I'm interested in what others know/have used. Not sizing ideas, but integration ideas.


  4. Expert Member
    Dana Dorsett | | #4

    Heat load per square foot ratios tend to go down with size of the house. In zone 4A a typical 2500 square foot 2-story house with 2x4 framed house with 350-400 square feet of clear glass double panes with 1250' of unheated but tight basement will come in around 30,000 BTU/hr @ +15F. If it has 800 square feet of clear glass double panes (more than a 30% glazing/floor ratio, twice the national average) it'll be a bit over 40,000 BTU/hr @ +15F.

    A 4000' house like that might actually bring the modulation level of the boiler off of it's minimum modulated level, but even an 8000' house like that would still have HUGE margin with that boiler.

    Run the fuel-use load calculation and we'll see where it really lives.

    The air handler has to be able to deliver at least the 63KBTU/hr of boiler output any time it's running in order for the boiler to modulate. Whatever speed on the ECM blower is is the hard minimum- any slower and it'll short-cycle the boiler. Given that 63 KBTU/hr is likely to be well above your average midwinter load, and even at or above the 99% design load, any time or energy spent trying to get either the air handler OR the boiler to modulate is wasted, since it can't actually track the load with modulation. If you were starting with a right-sized boiler there might be a reason to optimize the modulation of each, but if you're headed toward radiant heating anyway, the time and money is far better spent optimizing the radiant design.

    Run an aggressive Manual-J and a Manual-S for the cooling, and spec the air handler for the cooling not the heating.

    Since you won't be replacing the boiler, if you'll still be using a hydronic coil, size the coil to be able to cover the design day heat with 135-140F EWT, and run the boiler fixed temp (no outdoor reset), with the air handler set to the lowest speed that doesn't cause the boiler to cycle.

    You may want to review this bit on condensing boilers before diving into the radiant heating project too. Sometimes it's cheaper to replace an oversized boiler (even a brand new one), than it is to install enough radiation on each zone to keep the boiler from short cycling itself into an early grave on zone calls. If you're keeping the oversized boiler, you'll probably need to install something like 50 gallons or more of buffer tank to make low-mass radiant work even with decent sized zones.

  5. x_david_x | | #5

    Hi Dana,

    Thanks again. I've been tied up with other things, but do plan on running the numbers.

    One reason I don't think it is grossly oversized is that I don't see a short-cycling problem. Now I'm not sitting next to the boiler 24x7 in the heating months, but I did spend some time with it after it was installed and I just don't remember that being a problem. And that was in late November, not the coldest time of the year here. I think the key here is that it is an atypical house (unfortunately, from the heating/cooling perspective).

    I know where you are going with this. It does no good to have a modulating boiler if the minimum firing rate is higher than required for the load. In that case it is no different than a non-modulating system. On, over heat, off, cool, on, over heat, etc. I just don't think that is happening here. It might not be optimal in the beginning and ending parts of the heating season, but those times of year aren't going to suck the heat out of the house as fast either, so it will delay the time between thermostat calls for heat. But like I said, I'll run the numbers. :-)

    Radiant is a whole other discussion. I'll touch on that when I report back with numbers.

    I haven't gotten around to buying and installing an outdoor reset on the boiler yet. It is just running fixed temp right now. It is on my list though. I did wonder about the complexities of having outdoor reset with radiant and hydronic air, but it has been a while since I pondered it.

    I'll read the article you linked to. I'm guessing it might bring up the same concerns for zone size as you are bringing up for the hydronic -- namely that you don't want your zones too small or it will cause short cycling? I'll give it a read and thanks for pointing me toward it.

    It has been a while since I've thought about the specifics of radiant, but 140 seems high to me. I'm guessing you are saying that with a big enough loop and a slightly higher input temperature, the boiler again can stay above its minimum firing point and not short cycle? I'll have to look more closely at it before I do anything with radiant, but I do have to be careful about temperatures, since a good area that I want to heat is pre-existing wood floors. I can't get the wood over the first part of the loop too hot. That may just be a function of loop placement, and the flow might be such that the whole length will be close to the same temp, but it is something I need to keep in mind. One room has 3 outside wall and one right next to it has 1.x outside walls, so I have to be cognizant of getting the best heat next to the walls before the inner part of the room, I believe. But like I said, it has been a long time since I thought about it and I do intend to put a lot of thought and research into it before doing anything.


  6. x_david_x | | #6

    Quick question, while I'm thinking of it. Any recommendations on software that can help with radiant loop layout, and general radiant design? I have to believe that those things exist in this day and age, but I haven't looked for them yet.

  7. Expert Member
    Dana Dorsett | | #7

    When matched to a similarly oversized hydro-air coil, even a ridiculously oversized boiler won't short cycle. The hydro-air coil can deliver the heat as fast as the boiler is shoving it in. But the duty cycle of the system as a whole is pretty low even on the coldest day of the year.

    What fixed output temp have you been running? What is the model number of the current air handler?

    With a right sized modulating system it'll run pretty continuously from Thankgiving until April Fools day, with a nearly 100% duty-cycle, modulating with load.

    An entering water temp of 135-140F with a return water temp of 115-120F is about right for a condensing hydro-air coil, since it'll sill be in the mid-90s for combustion efficiency . Much cooler than that risks register exit air temps at the far end of the duct system not sufficiently above body temp to avoid a wind-chill effect. With a ridiculously oversized coil you can feed it 125F air and still get 100F+ air at the far end of most systems, but it could make some rooms feel drafty.

    That said I run the hydro-air zone on my system with an EWT of 122-125F, and for the most part it's fine, but one bedroom register delivers a tepid ~95F air into that room. Even with 125F water the hydro-air zone (which covers a substantial part of the house) is good for 40-44,000 BTU/hr, which is above ~28-30K heat load for that zone (indeed, more than the whole house load at -5F, well below the 99% outside temperature bin at my house) but not 2x oversized, let alone 4-5x oversized. It was originally married to a 120,000 BTU/hr output cast iron beast, which I sent to the scrap yard 15 years into it's lifecycle. With 180F EWT the hydro air delivered the full 120K just fine without cycling the beastie-boiler, but running only a 25-30% duty cycle even at temps well below 0F (more than 3x, closer to 4x oversized for the load.)

    The lower the water temp, the bigger the hydro-air coil has to be to deliver the heat, and if you right-size the air handler for the AC you may need even higher water temps than 140F to keep it from cycling (or you may need to run the air handler at max speed). But that's OK- it's better to run at 88% combustion efficiency and not short-cycle the boiler than to oversize the AC just to accommodate an oversized boiler. If you design the future radiation system right you can run lower temps and hit in the 90s for combustion efficiency, even though it's only cycling on/off (at long enough cycles that it doesn't impact efficiency or boiler longevity, when done right.)

    You're far away from starting to figure out loop layouts etc. Start with getting a competent full-on room-by-room Manual-J, (sanity checked by the fuel use load calculation) from which the rest all flows. Then (as much fun as it is to play Jr. Hydronic Engineer), hire a competent designer to spec the radiation and every valve & pump on the system, bearing in mind system compatibility with the current boiler (assuming you're really keeping it.) DIY radiant designs tend to be more expensive up front, operate at lower efficiency (particularly on the pumping power end), and often require expensive fixes to dial it all in and have it work even adequately, let alone optimally. Even if you end up DIY-ing the installation & plumbing, let a true pro (and not just a plumber who hacks on hydronic systems) handle the design.

  8. x_david_x | | #8

    Hi Dana,

    My comment about 140 degrees was for future radiant. The hydro air loop EWT is set to 140 right now. It has been too long since I looked at it and I don't know the return temp. It has been on my list of things to do to optimize it.

    I'm using short cycle and low duty cycle in the same way I guess. The boiler isn't forced to turn off due to the blower not taking enough heat out of the loop. And although the duty cycle isn't continuous from Thanksgiving to April Fools' Day, I don't think it is 5 minutes on, 5 minutes off all the time.

    The current air handler model number is a mystery. As I mentioned before, it is part of an old Lennox CompleteHeat. That product had a heating module (kind of like a tankless water heater, except it did have a little tank of water in there). That was general model number HM30. The air handler with the hydronic coil (and A coil), fan, control board, etc. is an AM30. There are multiple versions of AM30s with different sized blower motors, different dimensions, different coil sizes, etc. The wiring diagram only says AM30, it doesn't have the more detailed model number on there. Based on the width of the squirrel cage, I believe it is probably one of these 3:

    AM30Q3/4 -70
    AM30Q3/4- 90
    AM30Q3/4 -105

    It is possible that it is an AM30Q4/5-120.

    Best guess is that it is an AM30Q3/4-105. That one has a heating capacity range of 53,500 − 142,600 BTUh. If it is the 120, then 68,000 − 163,700 BTUh.

    No guarantees on the model and stats though until it gets ripped apart and I can see the motor size and measure the blower better.

    I don't doubt at all that I may have set the hydro loop to 140 to get some good warm air out of the registers, given that some registers are far away from the blower. That's another reason I want to go with an ECM running lower and longer, allowing for a possibly lower loop temp. However, as you point out, I can't run it so low that the boiler is forced to go below its minimum firing point and has to shut off/on/off/on.

    I didn't realize that you had hydro air. What make/model?

    Regarding running the air handler at max speed, I may have mentioned it in my first post, but I do believe that is what it is doing now. I'm almost positive it is set to single speed, whereas when set up with the other half of the CompleteHeat, it was multispeed. The HVAC guy had to make some compromises in retrofitting the new boiler and the old air handler together and one of those was single speed for the blower and I believe it was set to high.

    Hmm...better to run a boiler at suboptimal burns than to oversize an AC. So you are saying better to have a smaller boiler that isn't operating at its lowest burn rate (hence not optimal efficiency) and then have a "right sized" AC, so that you can save on AC electricity consumption and that saving on that consumption will outweigh the losses of sub-optimal boiler consumption? I guess it depends on how many cooling days there are vs heating days, etc. Optimal (perfection) is probably not achievable with all these variables and competing needs no matter which way you slice it.

    As much heat as we need to put out to keep this house warm, it is even more difficult to cool. I'm less worried about a variable speed AC being oversized, because it can modulate down even further than the boiler (25% vs 30%).

    Yes, ideally I'd like lower temps for the future radiant and low firing rate. I'm not doing layouts now for the radiant. I don't even know when that is going to happen. I've been putting it off for years. One other reason for a slightly bigger boiler is because my head load may change with the radiant. There is an unheated porch that may be remodeled and I may add heat to the detached garage (either infloor heat if I want to rip it all up, or probably more efficient (but not as comfortable), another hydro blower). In addition, 2 areas inside the house that I want to add radiant to (dining room adjoining living room, each a partial level down from the main level and from each other) are currently pretty cold in the winter unless we run a gas fireplace. So you can almost think of those as adding to the heat load too. This is where the vaulted ceiling is located (at least above the living room) and with them sitting lower than the rest of the house to begin with, force air does pretty much nothing to heat those rooms -- hence radiant to the rescue! I just need to find the time to do it. :-)

    I actually prefer your empirical approach to heat load calculation than all the theory based calculations in the world. A large part of my job is troubleshooting large computer systems and I'll take empirical (how it *does* work) vs theory (how it should work, given some set of predetermined variables that are usually not applicable) any day.

    Jr. Hydronic Engineer. JRHE. I think I'll make up some business cards. ;-) I'll definitely have help with the radiant.

    I ran the calcs based on your article and here are the results (rounded):

    Daily Base of 65:
    77,000 BTU/hr
    ASHRAE 1.4x sizing factor = 108,000 BTU/hr

    Daily Base of 60:
    80,000 BTU/hr
    ASHRAE 1.4x sizing factor = 112,000 BTU/hr

    Assuming I ran the calcs correctly, this does explain why I have a bigger than typical boiler. Yes, I could have probably gotten by with a GB142-45, but not the GB142-30 (well, not without risk of trouble once or three times in a century). However, as I mentioned earlier, I believe my heat load will increase with my radiant projects, so the extra capacity won't go to waste. I suppose it could be argued that those extra heat loads aren't "needed" in really cold weather and that we can be cold in the garage or cold in a couple rooms during record breaking cold and still live. Heck, we do have the gas fireplace that can be used on an as-needed basis if we were short on the boiler side of things.

    Given that I do have an indirect water heater, the extra BTUs do get put to use in ensuring it can keep up with the DHW demand too. Again, could people live with running out of hot water? Yes. I don't think the system is ultra, humongously, grossly oversized though, for this house.

    I will give you, that with the current loads, the GB142-45 would have had me running nearly continuously, at peak efficiency (assuming I optimized the hydronic air loop temp), from at least December 21st of 2015 to January 21st of 2016 (my billing cycle) this past winter. The average was 17 degrees. Oddly enough, the way the math works out, the 60 degree base and 65 degree base both come out to 16.5 degrees for that month (for the 142-45). That must be the exact cross over point between base days.

    That assumes that there weren't any huge swings up and down. In reality, in about the middle of January there was a cold snap (as there usually is), so it would have shut off some in late December, but not most of the later half of January. My 142-60 had a lower duty cycle than the 142-45 would have. When heat loads are added (and/or colder winters), that wouldn't reduce the duty cycle of the 142-45, but it would probably lower its efficiency somewhat and that could make the 142-60 more efficient overall.

    By the way, I found a copy/paste typo in your article. It reads, "At a balance point of 60°F there are only 40 F° heating degrees, and the implied load is 45 F° x 785 BTU/F-hr = ~31,400 BTU/hr." I believe it should read, "At a balance point of 60°F there are only 40 F° heating degrees, and the implied load is 40 F° x 785 BTU/F-hr = ~31,400 BTU/hr."

    Thanks again for the information. I'm curious about your system. Is the hydronic air system from the same vendor as your AC?


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