two basic questions

Brad White_91

Hey, that rhymes.... kinda catchy!

Agreed, ATTIC windows open as a default. You cannot over-ventilate an attic. Rain? Eh. Unless blowing laterally, I would not sweat it too much. Install drain shields (aluminum sheeting over the sill, pitched inside high, outside low.


Run time indicates to me another possibility, that the system may be over-sized. Not sure where you live or how close to design OD conditions you are, but if close to design your system wants to run nearly constantly. If it cycles now, it will only be worse when the weather lets up a bit. I do not consider your run time ratio to be efficient at all.

The problem with cycling is that you lose humidity control rapidly. Long, low and steady is the rule.

Ask the installer what the calculated heat gain figures were and how that relates to your installed equipment.

My personal rule of thumb is to undersize the system slightly. If I calculate a 3.5 ton load, I may install a 3-ton system. If a 5-ton, maybe install a 4-ton system. Why?

The hours of occurance where the full capacity is needed is so small you can live, especially at night when you will appreciate the humidity control. In mild weather it will cycle, sure, but not nearly as much.

There is a tendency sometimes to over-size and like boilers, this is a case where more is NOT better...

Yes, get a pro to check things out as Mike said, but within that see if you can run a lower indoor fan speed and if you have a "dehumidification mode". This over-rides temperature control and foucuses on humidity (the higher need of temperature or humidity) by running the system on a lower speed.

My $0.02,

Brad

J.M.

two basic questions

Happy 4th of July everyone.
Two basic questions from a still-learning homeowner. We've been running our central a/c system for approximately 5 days, and I've got two questions: (1) When should I open and close the attic windows? The air handler is in the attic, and I'm in the process of getting quotes on attic ventilation systems. In the meantime, should I always keep the windows open (when it's not raining), or keep them closed? (2) I've been timing cycles the last day. The system seems to run 5-7 minutes and go off for 9-14 minutes. How do I determine if this is operating efficiently?
Thanks,
JM

Empire_2

Under Construction ;-)

Since you are getting attic Ventilation and it will be temp controlled, I am assuming that it's HOT. OPEN for now is a good and safe response.

2). Have your system checked by a Professional. Way to short on and off may be in fact the units time to recover from a fault from a Pressure switch trip, or temp control, or a multiple # of scenario's. If you have the warewithall to think about the attic ventilation, Go the extra mile and make sure the HVAC is at it's best operating performance. Don't skimp on you indoor environment.

Mike T.

J.M.

Thanks & two quick follow-ups

Thanks Mike and Brad.
I wish you were in the Boston area.
I have a 4-ton system. The other two bidders on the project sized it at 3.5 and 4 respectively. Two follow-ups: (1) does it make any difference that this is a high-velocity, mini-duct system? might that be contributing? (2) If it is sized incorrectly (so it should be 3.5 tons), what do I do about that now that there's a 4-ton installed?

My contractor (who installed a Viesmann boiler as well as central air) has a great reputation, comes highly recommended, but has been a bear to work with. Nothing has worked as promised, and every time I call (e.g., heat not getting to baby's room, AC blowing warm air), they've implied it's something I've done. Then it turns out it's something they failed to check. On it goes with this short-cycle issue.

Thanks for your help.

JM

J.M.

suggestions of next steps?

Thanks Mike and Brad. I wish you were in the Boston area. I have a 4-ton system. The other two bidders on the project sized it at 3.5 and 4 respectively. Two follow-ups: (1) does it make any difference that this is a high-velocity, mini-duct system? might that be contributing? (2) If it is sized incorrectly (so it should be 3.5 tons), what do I do about that now that there's a 4-ton installed?

My contractor (who installed a Viesmann boiler as well as central air) has a great reputation, comes highly recommended, but has been a bear to work with. Nothing has worked as promised, and every time I call (e.g., heat not getting to baby's room, AC blowing warm air), they've implied it's something I've done. Then it turns out it's something they failed to check. On it goes with this short-cycle issue.

Thanks for your help.

JM

Mark_46

JM

With regard to sizing. Keep in mind High Velocity Mini Duct systems sacrifice between 10-20% of capacity right off the top because of the lower CFM at which they operate. So if you have a 4 ton condensor (and matching evaporator/air handler) it is between 43,200 and 38,400 BTU in a Hi-V configuration.

Mark_46

Hey Brad

Juicy info. I've had some of the things you mentioned on my mind as well.

Interesting, you say a system should almost want to run all the time! Surely a fair amount of on-time is needed to exchange the indoor air enough times and remove humidity and cool. But eventually the system must make temp otherwise we would need a small loan to pay our electricity bills ). Isn't running almost constantly, by some measure, inefficient?

I like to think of cycle times like this. On-time is relative to system capacity. Of course on-time cant be too short for the reasons above. Off-time is relative to the building's envelope ability to hold the temp. (eg insulation, tree shade, etc).

I need to better measure my cycle times but they are very dependent on outside temp and I need insulation in certain parts of the house. (thats the next project!) But in your opinion what should normal efficient cycle times be in minutes?

Also you mentioned dehumidification modes. I've been rolling something like this around in my head and have been wondering if there are thermostats that can control A/C based on the 2 requirements. Of course, sometimes a thermostat is satisfied as far as temp but the humidity level creeping upwards can be felt. But the system wont turn on so the set temp must be lowered to make it run. I guess there are t-stats that allow the setting of temp or humidity levels? Mine do not have that.

Brad White_9

JM

Actually I am in Boston. Where are you?

Brad

Brad White_9

I think you are on to it, Mark

Yes, it seems a bit of a conundrum at first, but not when you look at it more deeply.

Yes, longer run-times means larger energy bills, but short run times in a humid environment means not only less moisture removal but you are depressing the temperature while not removing the moisture... this positively drives UP the relative humidity.

If you were dealing only in sensible (temperature related) heat as if a heating system in reverse, I agree with you that your building envelope's ability to retain heat would govern and give your AC system a rest. In the desert southwest, this is often the case. (Well, ok not often, it runs just to be tolerable!)

You got it right that a system responding only to temperature will occassionally fall down on the dehumidification job. Only those systems that have a second stage responding to RH will over-ride temperature. The best systems do this by reducing airflow and output from the condenser (reducing refrigerant mass-flow) so as not to reach temperature satisfaction while "wringing out the sponge".

That is the analogy I use when teaching: consider air a sponge. It can absorb water from the air when relatively dry and when it has taken on some moisture (return air) it is run through a wringer (the AC coil) to go back to work because it has more room to absorb moisture..

OK I am rambling now...

Brad