Backpressure on domestic heating/cooling systems

[ggarnier]

I am familiar with the concept that it is risky to close too many room outlets in a house in an attempt to concentrate heating/cooling effects where the people are.

Now that I am living in a house with a seriously under-sized furnace/AC, I'd really like to be able to do this, but without risking expensive equipment.

Is it practical to instrument the output plenum for backpressure (and a good benchmark number for a safe limit) as a means of determining when one more closed outlet is one too many?

Thanks.

 

[KenRad]

The problem with reducing airflow in a conventional home cooling system is that you run the risk of freeze-up of the evaporator coil due to a reduction in heat transfer. On the heating side, there are other risks of equipment damage depending on the type of heating employed.

When you say "instrument the outlet plenum," I assume you mean taking a static pressure reading on the supply air duct. If you know the pressure/flow characteristics of your evaporator fan (ie, if you have the fan curve or chart), then you can correlate supply pressure at the fan discharge to flow, and determine at what pressure you drop below the recommended minimum flow. Most manufacturers will tell you to maintain at least 300 CFM per ton of cooling (typical values are 400-450 CFM/ton). This number should work as a minimum flow for the heating mode, but it would be a good idea to check that with the manufacturer to be sure.

If I were you and I didn't have the fan information, I would shut a few vents and see what happens. I wouldn't shut any more than about 25% of the total number in the house. If you don't get the expected results, then your best bet is to look into increasing the capacity of your system or adding supplemental heat/cooling units (like window air conditioning).

 

[Goorah]

Ggarnier:

Sounds like you're a "hands on" guy; so why not devise a "poor man's" VAV system .... sort of like the many constant speed fan bypass vav systems on the market?

Don't have much time; but I think you'll understand: if not ask more questions here.

If there's areas you don't care to heat or cool, shut them off entirely.

BUT

Get a hold of your furnace/AC's fan curve and see what max static is at minimum allowable airflow to prevent icing or overheating; whichever is higher. Then buy or make from scratch a supply to return bypass (8" round duct?) with a pressure relief damper and a flow balance damper... maybe doesn't even have to be that complicated... just a weighted "flapper "damper?


Ideally you will monitor supply backpressure and limit it to the max at earlier mentioned min airflow with this bypass damper.

If there are only 2 seasonal configurations, you could just do it with a fixed damper in the bypass: Set it up for cooling in summer and htg in winter.

If you decide to go this route, let me know and we can walk thru the steps to follow.
Buddha's advice 2,500 yrs ago (I'm Christian!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."

 

[KenRad]

Goorah:

A supply-to-return bypass could be used to keep the fan operating in a stable region of its curve, but how effective would it be in preventing coil icing or overheating, since your delta T across the unit drops as you begin bypassing flow? I believe the manufacturer's minimum flow requirements are based on more than just fan performance (ie, with a reasonable delta T in mind), and that with bypassing flow you can easily reach a bypass flowrate that will give you problems with freeze up or overheating.

If ggarnier doesn't have a problem with hands-on, maybe he could just cut in a few new supply air registers in the rooms he wants to condition, fed from the branches that he wants to cut off.

 

[Goorah]

KenRad: Thanks for joining in the discussion.

What I'm suggesting in NOT bypassing the unit or coil; but the supply duct system. So flow over the coil is constant with this "run around loop".

It looks like this: >>>>>R[ unit]S >>>> to supply
|<< / <<|

where the / is the fixed or modulating bypass valve.

Do you see? Buddha's advice 2,500 yrs ago (I'm Christian!!???!!): &quot;After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it.&quot;

 

[KenRad]

Goorah:

I believe I understand what you are trying to accomplish -- keeping a constant flow through the unit (across the coil, heating elements, furnace) by bringing some of the supply air back into the return (please let me know if I'm still not getting it).

What I am saying is that if you bypass too much flow, your &quot;mixed air&quot; temperature (average of room return and bypass flow) can change enough to cause the problems that you are trying to prevent. Think about an extreme case, where we shut off 80% of the supply air flow at the registers in the heating mode (ridiculous I know; just trying to make a point). Most of the supply air would be brought right back into the return to keep your static where it needs to be. The mixed air temperature would approach your supply temperature, and you could easily reach a temperature limit. I realize that you're only talking about bypassing a small fraction of the total flow, but I think that his ductwork labor would be better spent in redirecting branch lines to the occupied space, depending on the distance involved.

 

[KenRad]

Goorah:
Ok, conscience has compelled me to reveal my ulterior motives in disagreeing with you….years ago, I approached a Trane applications engineer about something I wanted to do with a single stage packaged unit to achieve a lower supply air temperature for an industrial process. I wanted to install a supply-to-return bypass (sound familiar?), which would lower my mixed air temperature and hence lower the supply air temp. By his reaction, you would have thought that I proposed to remodel his house using asbestos-lined ductwork. Anyway, when I saw that you had 25 yrs at Trane, I had to disagree with something you said.

(PS - I installed the bypass anyway, and it worked like a champ.)

 

[ggarnier]

Thanks for all the info on this - it's precisely because I'm not an HVAC person (although ME by training, and a little hands-on) that I addressed my inquiry to this forum.

What I may have overlooked is a more direct solution, ironically close at hand: direct measurement of an appropriate temperature in the furnace with one of the infrared thermometers (non-contact) made by my employer. I haven't looked at my furnace to see how practical this would be, but it seems likely that I could find a clear sight line, and the thermometer can be programmed with high and low alarms. This unfortunately would keep me in the feedback loop, but at least I could get an idea about the effect of closing a particular set of registers.

I guess I'd actually need two IR thermometers, one looking at the AC evaporator, and the other at the firebox (pardon my ignorance of HVAC terminology). Can anyone give me safe limit temps to avoid freeze-up or burn-through?

Thanks.

 

[imac]

Yikes! Seriously undersized cooling & heating system? Your knees are knocking in winter and you are sweating buckets in the summer?

My best advice is to changeout the whole system now and avoid all the costs associated with monkeying around with a basically useless system. An undersized cooling unit is doomed to failure because it cannot control the latent load (why else would you have cooling?) and the refrigeration compressor is constantly exposed to high load conditions caused by excessively superheated vapor. If the system is overcharged to reduce superheat, capacity is further reduced and compressor damage due to liquid floodback it likely. You can't fool mother nature this time.

Get yourself a two-stage gas furnace (if natural gas is available to you, otherwise consider a heat pump) with a variable speed blower motor. Then, if you like, experiment with a zoning system. I'd recommened a two zone system in a older home unless you plan to make signifigant changes to the air distribution system. Honeywell, California Economizer, Aprilaire, and others manufacture add-on residential zoning systems.

I would not spend a penny on the existing HVAC system. Borrow the money to get something good. Variable speed motor technology is well advanced and exceptionally reliable. Variable speed motors use far less energy (about 80% less than a PSC motor) and have the added benefit of helping to improve your comfort! That's what it's all about in the end.

Hope this helps.

 

[drfox]

imac has good advice. Don't throw good money into a bad situation.
If you are unsure of what you have, simply measure the ducting, draw out a floor plan and use the handy dandy 'ductulator' and get the facts on what you have to work with.
If the equipment is undersized, experience says the ducting is also.
Undersized equipment, by the way, can work very well in zoned applications.
The absolute worse action and all too common is to upsize equipment and not consider the air distribution.
No matter what the test results say, it looks like you have some work to do. Good luck!

 

[superheat]

I too say replace with the right size. Close off dampers until you lose superheat. That means you do not have enough airflow. Get a superheat charge to tell what the superheat is suposed to be. If you are going to install a larger system, You will replace condensing unit, evaporator and line set. If you tear up the system you have now, you are planning on replacing it aren't you. The acid burn-out will not be a problem, because you will replace the whole system. There will be no acid contamination after you change everything.
Undersized systems tend to last longer in my experience, unless it gets real hot inside the house. They stay on longer. The worst thing that happens to most systems is STARTING them. Since the system is running all the time the relative humidity will be low. Undersized system do a good job with latent heat removal, because they stay on all the time.

 

[Goorah]

Howdy All:

When we're working for someone else, we're required to &quot;do it right&quot;; but in a pinch on my own home system, I usually opt for the baling twine and scotch tape! ... Thatis until I sell the home. Since I live out of the city limits, I'm usually not subject to code inspections (SSHHHHHHH!!! ... did I say THAT?!) But I do it &quot;safe&quot; (U.L & AGA &quot;approved&quot; of course!)For a customer, I'd NEVER sign off on it though.

To answer the question about the RA approaching the supply: That's exactly right; and with a modulating bypass (but this one is fixed seasonally) the unit would control to RA temp and would go to low fire and then shut down as the damper closed more. So there would be no limits violated. I would also install an over ride upper limit and lower limit to prevent (depending on the mode) overheating of the furnace or too low a suction and coil frosting in cooling.

This could all be setup with the t-stat sensor in the RA airstream as well; with an &quot;offset&quot; for the difference between space temp and RA stream temp.



Buddha's advice 2,500 yrs ago (I'm Christian!!???!!): &quot;After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it.&quot;