AC unit size mismatch

When your system was working did you ever have trouble cooling your house on the hottest days of the year?

When your house was cooled down on 80 to 85 degree days did the house feel clammy?

What size is your HVAC unit and what size is he suggesting?

two other questions, How many square foot are you cooling and how tight is your home?

Pebbles--

Hi, neighbor to my east. Answers to your questions:

1. No trouble keeping it cool, although the unit ran a lot when outdoor temperatures were over 100ºF. It failed (leak, low refrigerant charge) a couple months after we moved in and we have been keeping temperatures tolerable with two 115V window units since then.

2. Home is about 1150-sq.ft., typical late 1960's construction with minimal insulation in walls and ceiling, mostly aluminum-framed single-glazed windows. Full basement with one end (25-ft wide, south) exposed. Good shade from trees in afternoons. Major orientation is east or west. Windows are being replaced with 2x or 3x-glazed low-E vinyl units, 40% done now and rest probably about a year from now.

3. This winter I will be reinsulating the standard stud wall cavities with cellulose blown in to pack them tight, so they will have a net R-value about 13-14. I will also be reinsulating the attic to about R-45 or R-50, with air chutes on the edges for soffit-to-ridge ventilation. At that time I will also be sealing air leaks around standard light fixtures, and any other penetrations into the attic space (no recessed lights).

4. There have been no problems with high indoor humidity (summer or any other time) or anything related to that. There are no significant signs of water leakage in the basement, although there have been occasional times when a heavy rain after a prolonged dry period brought perhaps a liter of water into one corner for a day or two.

5. The present furnace is natural gas 80% efficient, rated 82,000 BtuH input, and has some trouble on the coldest days. With the pending increased insulation I intend to use a 92% efficient 60,000 BtuH input unit. The present condensing unit is 2.75-ton (33,000 BtuH cooling), with a matching evaporator coil.

6. The suggested ideal replacement coil size is 2-ton, hence the mismatch question in my post.

7. Basement walls are poured concrete, no interior insulation. Exposed south one has significant crack in one corner, because it was continuous with the adjoining retaining wall for the side yard. I will be sawing them apart and replacing that south wall sometime, with insulated concrete form (ICF) construction. At that time I will probably also be adding some appropriate form of passive solar heating.

Anything else?--JMM

Probably is NOT a good idea to "improve upon" a design of a matched system that was originally designed by experts. If using a VFD to match load sizes was a great thing, they'd have all been doing it years ago.

Sounds like your home could run either with a 2 ton or a 2.5 but based upon other comments, a 2.5 is smarter choice, if for no other reason than you have more air-flow to utilize.

Having a reliable heating/cooling system is important to not just you but others in your house. Sounds like the 60 mbh furnace might be a little small too. Make sure you have 1000 cfm available!!!

Slightly undersized cooling capacity is generally better than OVERSIZED but that's subjective. People always complain when they don't have "enough" when they need it or want it.

To answer previous questions, mismatching is not good business period. You can often "get away with" undersizing outdoor by 1/2 ton vs. the indoor but NEVER oversize outdoor with respect to indoor. Your outdoor HAS to have enough capacity to make the indoor coil reach dewpoint or you will NOT do any "air conditioning".

Also, varying the voltage to the compressor winding may have unrealized affects such as: will the compressor windings last with corrupted AC sine waves? or if its VFD, how will the capacitor circuit behave? ( If it doesn't continue to do its share of the work, the main windings will overwork.) Next: it would be an experiment ( that your wife will NOT like) determining the correct place to monitor for modulation. You'll be monkeying with superheat, subcooling, etc. If you slow it down too far, you won't reach dewpoint without varying the indoor blower too, and you might cause oil migration within the refrigerant circuits to cease, which will surely kill the compressor.

Moral to the story....don't go down that path. Just put in a new matched system and make everyone happy

Do all you mentioned during the winter.

In the spring, get a load calc done and

replace both units to proper size.

It is essential to have enough compressor and condensing capacity to chill the evaporator coil to 40F to get proper de-humidification. Air flow is then set so that coil temp results in about 58F air temp, which when it warms up to 75F, is right at 50% RH. Here in Houston where I live, humidity is almost 50% of the total cooling load. Even a 'tight' house (really well sealed) has on average 4 air changes per hour!!.

What is ideal is a variable speed evaporator fan so you can use all the cooling capacity of the compressor and condensing coil. The SEER ratings are somewhat misleading, since they use 85F air temp as the condensing air. Down here, 100-105 is not uncommon and the EER goes way down to 6-8 from 14. Extra condenser capacity (either more fan Hp or larger coils) pays off here. Often the only difference in the condenser between a 30,000 BTU and 36,000 unit is fan power--no more area, just move more air (of course the compressor is bigger. )

That all said, a 'matched' system with thermostatic expansion valve gives the overall highest efficiency. Then upgrade the condenser fan if you have lots of >90F days and high humidity too.

I understand your reluctance to replace a good condensing unit. The problem that you may run into with an oversized condensing unit is that the compressor is going to try to pump more refrigerant than the capillary tubes on your evaporator are allowing through. This will result in dropping the evaporating temperature below freezing and you will end up with an ice block above your furnace. This same thing can happen to a matched system under the "right" conditions. Using a TXV valve on the new evaporator may take care of this problem, but it is more money now and possible maintenance issues in the future.

I would just try using the old condensing unit as is. If you ice up the evaporator too often, then adjust the capacity of the condensing unit. You can do that by controlling the speed of the compressor (which may or may not shorten it's life), control the speed of the outdoor fan, or overcharge the unit with refrigerant. Since you're an electrical guy, I'm guessing that you're going with options 1 or 2. If so, put your transducer on the suction side to control the evaporator emperature to 40 F or so.

Have fun!

Sorry, but I think you are "way out to lunch" on that advice but, he can do it anyway to an old condenser and let us know how you (jmueller) made out, but please don't monkey with your new equipment.

As far as arbitrarily listing 40 degrees, that's not a benchmark, its DEWPOINT you need to reach, AND maintain an acceptable superheat and subcooling reading!

Yes, oversizing a condenser may freeze a coil but a much more annoying thing is the coil might actually sing and howl from over-driving the orifice.

One thing people forget, the LAST "guy" in the refrigeration circuit who needs to be cooled is the COMPRESSOR WINDINGS, and when anything goes wrong, slight loss of gas etc., the windings are the FIRST to feel the loss of capacity and start to run hot, long before the coil or you (the occupants), do.

I don't think I'm that far out to lunch Sniccus. I will admit that I'm not much of a refrigeration systems guy, however I understand coils and psychrometrics. My thought is that if the condenser unit is good right now and the only reason to replace it is to match it to the evaporator, then why not try to control it's capacity if needed. The only thing that you can hurt is the compressor and he is being told to replace it anyway. Well, besides the cost of the condensing unit, I suppose there is the cost of an extra service call. But, if it were my system, I'd take the chance that the compressor is robust enough to handle the off-design conditions and try to get it to run a few more years.

My apologies to you sir! I believe I was taking exception to the bulk of Keith Bower's post above yours. Your comments were much more accurate and sensible. Sorry again !

I love testing, altering things to learn more on how they work. But its risky business playing with Mom's air conditioner you know....hehe...

The best way to play around with his VFD's or what ever electrical wizardry he wants to experiment with, is get a small window A/C, strip the metal cabinet, screw some plexiglass on it, solder on more access fittings, cobble in an expansion valve, put variable speed on both fans ( need an AC with 2 fan motors), put some temp monitors in key places and then play all he wants and compare readings to standardized AC theory...now that would be fun.

But you were dead-on with one thing, I think he's just trying to limp this thing along awhile longer by forcing it to comply with available conditions. Still isn't a good idea but I would be curious on how it works for him. Surely the efficiency will drop and the overall btu performance will fall but, its fun learning by hands-on!!

Find someone with a 2 ton CU that needs a 2-1/2 ton CU and swap.

Sounds like it is a good time for you to consider a geothermal heat pump instal for your heating/cooling/hot water needs...will provide you with better comfort, and save monthly operating costs...as well as doing your part for the environment.

Proper sizing is still critical of course.

KGB

Yes, Guest

They are significantly more energy efficient, and I have friends with them as well as having priced them for new construction. Considering this is an urban core lot, the ground coil would have to be in wells bored in the front yard. There is a lot of limestone underlying here, so probably 2/3 of the bore length will be through various strata, with some of that being pretty high quality stone.

Therefore the installation cost would be high. The real estate values for my neighborhood would not support the cost of a geothermal heat pump. Since natural gas is a locally sourced commodity and its carbon footprint is a lot lower than coal for electrical generation, gas for heating and electric for cooling is a reasonable approach. The extra money saved can be put into other forms of energy efficiency & environmental protection.

Thanks--John M.