Solution to the problem, possibly

I have been following the discussion on system sizing on another thread for 10 pages worth. In it, several of our most knowledgeable posters bring up differing opinions. Teddy Bear argues for sizing for peak load, and of course including a dehumidifier in the system to deal with the consequent humidity problems.

Dash said "Sizing for peak load ,will likely oversize every time,then you will need a dehumidifier".

It occurs to me that the problem requires *two* AC sizes, and that might explain some of the dissent. You must size for decently high runtimes, to do otherwise will practically guarantee low energy efficiency and poor humidity removal. This is the Manual J philosophy as I understand it.

But... there is a peak load condition which many people object to, where Manual J might lack the sheer power to 1) cool down temperatures quickly, or 2) cope with an unusually high number of people in a party, or other legitimate concerns. (Of course poor installation design, leaks or craftsmanship may *also* destroy peak power, I am assuming the installation does not have these problems.)

It seems the solution which satisfies all these legitimate concerns, is some form of a two-stage system. The 1st stage has the responsibility for high runtimes and good humidity removal, and the 2nd stage is just there for the peak load conditions. Of course it is costly, but performance-wise is this not the solution to the sizing problem?

Regards -- P.Student

P.S. This does not eliminate the objection Teddy Bear states, that no AC system on earth can control humidity in mild weather when it is not running. I'm just trying to find an objective solution (however costly) to the argument about AC sizing. Clearly there are a lot of less costly solutions to be applied in many situations, too.

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It seems the solution which satisfies all these legitimate concerns, is some form of a two-stage system. The 1st stage has the responsibility for high runtimes and good humidity removal, and the 2nd stage is just there for the peak load conditions. Of course it is costly, but performance-wise is this not the solution to the sizing problem?

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You're on the right track when it comes to designing for penultimate residential comfort systems. Two stage w/variable speed blower, TXV on evap, all rooms receiving proper cfm's at peak demand, humidistat incorporated into thermostat, and, if you're going all the way, returns in all rooms except kitchens and baths, all air handlers and ductwork are in the conditioned space, out of the attic.

For the rest of us working slobs that likely can't spring for such an elaborate rig, size by Manual J, install a TXV, and do a top drawer installation on everything. Make sure ductwork is superbly sealed, whether it be in attic or not. And make sure system can breath, indoors and out. As a bonus, spurn fast food for a month and spring for a 8321 VisionPro with humidistat.
It'll probably cover the bases for most of the conditions one might throw at it.

Oversizing with a two speed/stage compressor is fine for the reasons stated ,have a few of them.


However it makes the humidity control much less effective.Say Man J is a 3 ton,and you install a 4 ton,low on the 3 ton ,was 1.5 tons,with the 4 it's 2 tons on low,so the dehumidification won't be as effective.It gets worse if you go to a 5 ton.Still it's better then a 3 ton single speed.

With Copelands two stage,low is 70% capacity ,so it's even worse,but still viable.

Why does the Infinity and Thermidistat ,zoning systems,in our home, maintain <55% RH all the time in Florida ,and why does Teddy say it's not so.

I suspect a lot of jobs Teddy talks about, had other problems,oversized,duct leaks,envelope leaks ,etc.,that a dehumidifier solved,leaving the main cause unattended to.

We have Aerosealed numerous homes,where duct leakage was the source of high humidity,or low capacity.

>>Why does the Infinity and Thermidistat ,zoning systems,in our home,
>>maintain <55% RH all the time in Florida ,and why does Teddy say it's not so.

Dash, could it be that the overcooling is not as objectionable as one might suspect? (anyone would have to assume the installation in your home is very correctly done). Are the periods of no-cooling, wet weather possibly much shorter than the 1-week scenario Teddy refers to? Another ideas, is Teddy shooting for a lower RH, perhaps 45% rather than 55%?

It does seem true that a dedicated dehumidifier is a "fix" that can cover a certain number of other sins. In the case of peak-load sizing, that is explicit.

In theory, the two-machine concept could have a 2nd single-stage AC dedicated solely to the difference between peak load and Manual J -- of course no same person I know of would design a home that way. But I am thinking it would work.

In common homes with 2 AC systems, the load seems assymetrical more often than not. I wonder, would there be any way to size lean and use the lower-load AC in place of a dedicated peak load unit? Just for those few hours each month when load approaches peak. I can forsee that would create overcooling in part of the house but perhaps there would be a way to mitigate that. Just possibly, the homeowner would accept a degree of overcooling just as he might accept overcooling to reduce humidity. IF these musings are valid in theory, then it looks like a problem of controls since so many homes already have two systems.

If a problem can be solved with a computer instead of one of those old round Honeywell thermostats, there is potential for progress in this way. I'm not claiming anything one can do today, just exploring how controls might evolve.

Best wishes -- P.Student

Overcooling to dehum,is not a probelm ,IMHO,we set it from 77 to 79,so lowest it overools,is 74.When I get up in the morning,in mild weather,it's often at 74 with 52 to 53RH,feels the same to me as 77/78 and <50 RH.

I feel a slight tug on my chain! True, many of the homes we fix with dehumidification have misc. defects. Most defects are difficult to fix. Others are the best with spray foam insulation and all ducts in the occupied space. The best of these home have minor defects. Several a/c contractors in high humidity climates use supplemental dehumidification with most of their installs.
I am investigating building a new home in FL. I want to comply with the FL building/ASHRAE code ventilation requirements of 60 cfm of mechanical fresh air. I want to be able to maintain <50%RH while ventilating/infiltration during all weather conditions including tropical storms or ten days of cool rainy weather. I do not want to take a chance on mold or dust mites in my new home. My biggest concern is during the wet cool weather 65^F-75^F. How does an a/c only system maintain <50%RH with latent moisture load of 2,100 btus/hr. from ventilation, 2,000 latent from 4 occupants breathing and activities plus and moisture diffusing into the home from outside? How much over-cooling to deal with +4,000 btus/hr. of latent load. Or do we take our chances on not having wet cool weather of any duration?
I attened a presentation from national a/c mfg. explaining the limitations of an experimental 2 speed a/c with 80% reheat at maintaining. This was tricked to the max. and still had weather conditions when it was unable to maintain <50%RH with typical Houston weather with over cooling less than 3^F.

The consensus was that supplemental dehumidification was currently the "gold standard" to avoid high indoor humidity during "mother nature's inevitable wetness". The combination of a adequately sized high SEER straight a/c and a whole house dehumidifier is competitive and easier to set-up and service over the life of the system. I do not have a problem with a/c contractors selling the system of their choice and waiting for humidity problems before adding dehumidification. My point has been more to the customer's being sold <50%RH with "mother nature's inevitable wetness". Many of homes we fix have recently purchased new a/cs, many new 2 speed, to solve high humidity problems. Research continues for the better varible a/c and lower cost high eff. whole house dehumidifier. Short term, the hi-eff. dehu remove 5 pints per KW with simple, low cost investment, and provides total control regardless temp or a/c load. Future? Wait and see!

Only a slight tug on your chain, Teddy! I have a lot of respect for what you are saying and believe you have the confidence and integrity not to be too bothered. I do enjoy your posts and have learned much from them.

Best wishes -- P.Student

P. Student,

Okay man, you got the engineering side of my brain jumping with this subject matter. I'm not an HVAC engineer/designer but am a technician that mulls over this stuff a lot so what I'm thinking about here would be subject to the realities of thermodynamics, feasibility, and practical application. I will state right off the bat that I'm thinking out loud more than from experience with this idea, since my experience with this idea is for commercial applications.

For the record, we're speaking as if money is no object, as mentioned in your initial post. And the object being the penultimate residential system that can answer any demand placed upon it, from peak sensible heat removal under maximum heat load conditions to average comfort needs to cool but muggy day dehumidification needs to economizer abilities when outdoor temperature and humidity could simulate conditioned air to sensible heat addition in cold weather. A true blue year round residential climate control system.

My thoughts? A residential chilled water system using the likes of a Multi-Aqua chiller ( http://www.multiaqua.com ). This system would consist of an air cooled water chiller unit that would send chilled water to multiple air handlers throughout the house, each sized for the heat removal needs for each zone. The chiller would cycle to maintain chilled water temperature setpoint vs. cycling to maintain room temperature setpoint. A form of capacity control would be included so the chiller need not operate at full capacity under lighter load conditions.

During peak loads, with all zones calling for cooling, the chiller may run as constantly as a direct expansion (DX) split system or package system would. The latter systems, however, would require compressors and air handlers for each zone vs. the chilled water loop requiring only one or two compressors within one unit for multiple air handlers.

During milder weather that still requires comfort cooling, the chiller would cycle more as chilled water setpoint would be easier to maintain (contrary to commercial chillers which commonly do not cycle but rather reduce capacity of the compressor according to heat load demand). If the residential chiller also had a form of capacity control, such as two stage compressors, it would run more economically during milder conditions to achieve the work. It would also have the capacity should peak load conditions resurface, thereby giving the system more flexibility than a DX system (without staging or capacity control).

During cool but muggy conditions, such as wet, rainy days, the chilled water system could be called upon to dehumidify, with reheat added downstream of the cooling coil to avoid overcooling the zone(s). Reheat energy could come from solar, hot water loop, electric heat strips, or warm water from a desuperheater heat exchanger on the compressor discharge line.

During conditions where the outdoor air conditions would approach parameters similar to indoor conditioned air under higher heat demands, an economizer system could introduce outdoor air to minimize chiller use when indoor heat loads still exist but outdoor air could accomplish comfort cooling.

And of course for heating purposes the same air handlers could accomplish this, either as the chiller also being a heat pump and heating the water loop, or an additional coil for hot water heat, or heat strips, or solar assisted heat, or...

Additionally, I would design each air handler to be in the conditioned space, in a ceiling furdown, for example, with all tightly sealed ductwork also in the conditioned space and out of the attic, concealed by furdowns or chases. This would minimize capacity losses through ductwork and air handling equipment significantly.
The house itself would use radiant barrier roof decking and have above average insulation and tightness of construction. This dovetails with the thinking that a house and its climate control system should be designed as complimentary components rather than the far more adversarial approach taken with most residential applications (for example, placing cooling equipment in baking hot attics, among many other points of departure from a complimentary process).

In the end my musings realistically may be a pipe dream (pardon the pun) but, modified for a given application, seems to have merit for larger residential applications requiring more than one zone and would offer the entire range of climate control for all expected conditions. And, with money being no object in this form of hypothesis, it's fun to dream up a penultimate climate control sytsem.

Anyway, thanks for spurring my HVAC design side on your thread. :cool:

I am over-awed by your description of the penultimate home AC system! Glad to read about it and if anyone else has criticisms, that is always fascinating. Besides cost of course.

I might tentatively add if you were to go this far, it might be possible to build in a cool-storage facility. For example build a great big ice lump and use that to contribute to peak load cooling. If you get time-of-day pricing from your electric utility, the price difference between peak and off-peak electricity can be pretty large.

You might not believe me, but I thought I was getting *just* outside the realm of practical cost with what I described. While a whole other AC system just for peak load is too much $$$, this closely approximates a 2-stage AC which many people already buy. It seems to tell me a 2-stage AC is both a theoretical and practical answer to optimizing both peak load and low-load needs. If you think I am wrong please tell me.

And if there is any merit to the fuzzy idea that a common 2-unit house system could be made to show some of the benefits, that would be interesting to pursue. On the other hand, to really use both would imply some kind of zoning (or at least more ductwork) and too much complexity would kill the idea for practical purposes.

Thanks -- P.Student

P.S. And I have to wonder if the man who replaced a 2.5-ton upstairs system with 5.0 ton, might have a *practical* application for some unusual ductwork. In his case the problem is unique and maybe complicated, but much of the cost is already sunk.

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I might tentatively add if you were to go this far, it might be possible to build in a cool-storage facility. For example build a great big ice lump and use that to contribute to peak load cooling. If you get time-of-day pricing from your electric utility, the price difference between peak and off-peak electricity can be pretty large.

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You may already know many commercial applications use a cool storage system whereby the chillers create an ice bank at night (off-peak energy usage). The chiller then assumes a standby mode during the day if the load allows and comes online if the ice bank can't meet building load.

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It seems to tell me a 2-stage AC is both a theoretical and practical answer to optimizing both peak load and low-load needs. If you think I am wrong please tell me.

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You're on target. Two-stage cooling could be used in a wide variety of residential applications, and is perfect for high humidity locales. If a good deal of thought went into both the home's construction as well as its HVAC system, the second stage might only be needed during extreme weather conditions rather than anytime the outdoor temp goes over ninety degrees.

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And if there is any merit to the fuzzy idea that a common 2-unit house system could be made to show some of the benefits, that would be interesting to pursue. On the other hand, to really use both would imply some kind of zoning (or at least more ductwork) and too much complexity would kill the idea for practical purposes.

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I think a more basic version of my penultimate chilled water system in my last post would work well for two story houses or homes needing more than one zone. One condensing unit that handles two or more air handlers. You wouldn't have an army of condensers on the ground or multiple opportunities for refrigerant leaks (just water leaks if the piping wasn't done carefully) and the single condenser could have two stage compressors (or the more recent advances in scroll compressor technology whereby one compressor can operate at two capacity levels...Lennox is using this in their HS20 series units). Two or more zones using split systems would require air handlers with condensate drain piping, so substituting a DX coil with a chilled water coil, eliminating linesets to the condensers, and consolidating all the refrigeration equipment into a single cabinet....may or may not break even on cost comparison, or may be initially higher on the chiller system due to being less commonly used. I would be interested in how it would compare with operating costs over a DX system.

And for one thing, NOTHING cools and dehumidifies like a properly fed and balanced chilled water fan coil! The entire coil is cold when the gpm through and airflow over the coil is right...beats a DX coil with a piston all to pieces! I don't have any hard numbers but I'd even put a chiller fan coil over an expansion valve fed DX coil as far as cooling and dehumidification performance.

Still, kinda pricey but I'd love to see a Multiaqua system in action and test its performance over a period of time.

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Originally posted by shophound
d for one thing, NOTHING cools and dehumidifies like a properly fed and balanced chilled water fan coil! The entire coil is cold when the gpm through and airflow over the coil is right...beats a DX coil with a piston all to pieces! I don't have any hard numbers but I'd even put a chiller fan coil over an expansion valve fed DX coil as far as cooling and dehumidification performance.
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Consider that as chilled water absorbs heat, the temperature of the water rises. 1 btu raises 1 lb. of water on degree. Raising the temperature decreases latent removal dramaticaly. Refrigerants evaporate at a specific temperature enabling them to remove much more latent heat per ton of capacity. Also consider much lower btus per watt making ice compared to direct cooling. Also add standby loss of ice storage and pumping water plus investment and maintainence cost. And yet no ability remove only latent heat. Complicated, expensive, and inefficient but good for level loading. TB

Well Teddy.

I might be selling my first whole house dehumidifier.

Because of circumstances beyond our control, we'll be using it as a cure all.

If they work half as good as you claim, the customer will be happy.

Dash said:

...However it makes the humidity control much less effective.Say Man J is a 3 ton,and you install a 4 ton,
>>low on the 3 ton ,was 1.5 tons,with the 4 it's 2 tons on low,so the
>>dehumidification won't be as effective.It gets worse if you go to a 5 ton.
>>Still it's better then a 3 ton single speed.

>>With Copelands two stage,low is 70% capacity ,so it's even worse,but still viable.

So what you are saying is by trying to optimize the sensible BTU needs, my idea would be getting farther from optimum on humidity removal. Sigh.

Tell me guys if you think this is logical: Reluctantly I am moved to suggest this implies a THREE stage system (hate getting too far from present reality). The bottom stage might ideally be sized to run nearly constantly, and the middle would be sized to handle all *normal* requirements. Then the top stage would exist only to handle the peak load needs. Rats, I think this is a logical conclusion but all these components spell unreal expense I think.

Back when I suspected something could be done with off-the-shelf parts, costing little more than a "typical" top-of-the-line Trane or whatever, it seemed almost doable. Now it does not, unless someone has a clever brainstorm. At least I can see another dimension of things that beg for optimizing.

Best wishes -- P.Student

As a running background issue I always see that there is a choice between overcooling, and a dedicated dehumidifier during mild wet weather. The amount of time this occurs will vary with the region, but I am beginning to think in Houston that overcooling might not be that uncomfortable if designed well. OTOH a dedicated dehumidifier is a brute force solution, and (a cheap one) was after all the choice of Building America homes nearby.