Effect of Oversize on SEER

[kendyan]

We are replacing a heat pump for a 600 sq ft second floor. The estimate size is 1.1 tons, so we will have to purchase an oversized 1.5 ton system. Will this affect the relative efficiency gain between a 15 SEER and 13 SEER system?

[beenthere]

No harm or loss. As long as the duct work is sized right.

[cartercrew]

We are replacing a heat pump for a 600 sq ft second floor. The estimate size is 1.1 tons, so we will have to purchase an oversized 1.5 ton system. Will this affect the relative efficiency gain between a 15 SEER and 13 SEER system?

Over sized equipment will lower delivered SEER rating. In your case however, any loss caused by oversizing will be the same between 13 and 15 SEER. If the load is 1.1 tons, a conventional system has to be 1.5 tons, since none of the manufacturers make anything smaller.

An option is a Mitsubishi sez/suz 1 ton ducted mini split. I would rather have a slight under size than over. Humidity removal is a big concern for my area.

[BaldLoonie]

Heat rises. The Manual J doesn't add for the heat that comes up from downstairs through a staircase or whatever. So I wouldn't worry at all about going a bit over what the calc says. Some of that cold air will fall downstairs anyway too.

[openclasspro]

Baldloonie,heat or hot air rises?

[udarrell]

An A/C will always be oversized during milder weather temperature conditions with humidity issues.

Short runtimes & too many cycles per hour is what you want to avoid especially when the weather is milder.

The above situation, without t-stat cycle settings can, depending on heatload conditions, greatly reduce the SEER Rating & will not control humidity problems.

The best way to control the above problems, if they exist, is to use a cycles-per-hour or a swing setting room t-stat that is properly located so supply air does not directly affect it.

If you dress properly & have air movement with say a large floor fan or ceiling fans you should be able to set the cycles per hour for, say a 3-F temp rise above you t-stat on-setting.

Say you set the t-stat at 75-F, a cycle setting for a 3-F temp-rise is 78-F, that should provide a decent runtime length to help with humidity & keep the SEER performance level closer to its rating.

You need to experiment with cycle settings depending on outdoor heatload conditions & the energy efficiency of your home; like on-setting at 78-F & off at 74-F, you need to check what the setting provides; if U can get a 4-T spread setting...

The longer off-times are also directly dependent on the energy efficiency of your home & whether you're generating heat within your home in various ways.

What we should be trying to achieve is optimal comfort with optimal home efficiency & equipment performance...

If your home is well insulated & tight, as it needs to be, with a good tight efficient airflow duct system, using the above techniques will help greatly to reduce your electric bills while keeping you comfortable, also using a the preceding & a TXV refrigerant control will help achieve adequate humidity control; even with somewhat oversized A/C equipment.

The Normal Human Comfort Zone: at 50% RH is 79-F down to 73-F.


The above comfort zone was found to be acceptable to 90% of test subjects drawn from a range of age groups and genders, with work and life-styles involving varying levels of activity and clothing. An air conditioning system that establishes and maintains indoor conditions within this zone will provide thermal comfort. It will produce a neutral sensation, occupants will feel neither too hot nor too cold. Above chart and findings From: Home Energy Magazine Online September/October 1996) Sizing Air Conditioners: If Bigger Is Not Better, What Is? by John Proctor and Peggy Albright

[BaldLoonie]

Heat, hot air, whatever. Something rises :D So the warm air from downstairs will go upstairs and the effect is a higher heat gain than the calcs say.

[genduct]

We are replacing a heat pump for a 600 sq ft second floor. The estimate size is 1.1 tons, so we will have to purchase an oversized 1.5 ton system. Will this affect the relative efficiency gain between a 15 SEER and 13 SEER system?
Extending the duct to a troubled area on the first floor as well?

[udarrell]

In your humidity problem situation U will want a TXV refrigerant metering device & airflow set for 350-CFM per ton of cooling.

A room t-stat that you can adjust for longer runtimes & a spread of on/off temps.

As was suggested U could run a duct to the first floor where more cooling is needed.

Yes, heated air rises & the 2nd floor heatload may be higher than the load calc shows.

I never want a room t-stat that has heat & cool anticipators & that only keeps the cooling & heating a half to a degree of variation. That results in too many cycles & too short a runtime cycles.

So, when they brag about keeping close temperature tolerances, in single stage systems there will be rather large sacrifices concerning efficient performance...

With humidity problems, in the A/C mode, the last thing you want are short runtime cycles. I might prefer a Ton unit for that 600-sf with required humidity control however, if all the suggestions are used it might get by okay...

Hope my last post wasn't too involved...

[kendyan]

Appreciate all the replies. We're probably going with the 1.5 ton rather than the mini split since all the ductwork is installed. it sounds like the t-stat choice is equally important. What should we look for?

[TwincamDave]

you say it's for a second floor, are there any open spaces(balconies next to a great room)ect.? if so you need to add that as part of your load/sq.ft., don't forget heat rises=more load.That close I wouldn't worry about it unless there is a big open area as mentioned before.