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Heating & Air Conditioning

Most Efficient Way to Use This Kind of System?

14 years ago

We just bought a 120 year old victorian house - very drafty and cold. There is a furnace in the basement and one in the attic. One the main floor are the living areas and all bedrooms are on the second floor.

I've been told this is a 90% system - not sure what that means.

We moved in at the end of winter when the furnace was still needed but it wasn't horrible out. At night when everyone headed upstairs, I would turn the downstairs thermo way down and leave the upstairs thermo at a comfortable sleeping temp and we did fine. I figured there was no point running the downstairs one all night when no one was on the first floor and since cold air doesn't rise, I assumed it wouldn't affect the upstairs temp.

Is that correct?

Now, once summer comes and we switch to the a.c. (also two units), what is the most energy conserving strategy for day time versus night time settings? Since heat does rise, I'm not sure my theory of changing the thermo at night is a valid one .... maybe turning the upstairs one way up during the day (not cooling it as much) is the way to go? Or does that make it that much harder to get it cool at night?

Because it is drafty it does take much longer for the furnace to warm it. I assume the same would be true for the central air, though it hasn't been warm enough to turn it on.

Anything else I should know about using this type of two unit system? I've never had anything like this and want to make sure I'm using it wisely. I can't afford a double whammy heating or cooling bill by running things the wrong way.

TIA!!

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Comments (3)

veesubotee
14 years ago
It's difficult to give specifics without more information. At a minimum,model numbers of indoor and outdoor equipment and square footage served by each system. If #'s not available, how about BTU input (heat) and Tons (a/c)? Are there metal or plastic flue pipes coming from your furnaces?

Your location (nearest big city) is important as well as set points and set back temperatures used and time periods.

Generally, heating the downstairs is not a total waste, as heat will tend to rise.

By setting back (especially drastically), air conditioners may need to run for hours in order to catch up, thereby eliminating some/all savings. Also, if you are in a high humidity area, your a/c's will have to work harder to bring down humidity gained by (drafty) infiltration. This will also lengthen the time required to recover from setback.

No doubt, some experimenting may be in order.

V
arbpdl
Original Author
14 years ago
Thanks for your input. I've no idea on the size of the units. The house is about 2500 square feet, half up, half down. That doesn't count full basement or full attic which aren't finished or ducted.

We live in the midwest, KC area to be exact. Normally hot and very humid summers though so far this year we're still using the electric blankets at night.

I will take a look and see if I can tell what size the units are.
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david_cary
14 years ago
Generally on a conventional system (non heat pump - which it sounds like you have) it is worthwhile to setback the areas not in use.

The temperature rising is not really a factor in whether the setback will help or not (ie heat in the winter). Generally, you would not want to set back the a/c too far as the humidity can cause things to stink. This is only usually a problem for an upstairs that is used rarely - I don't think setbacks at night would be a problem - but I don't know with really drafty windows and really high humidity.

The only time setbacks are not valuable is when the system runs inefficiently (or uses a more expensive fuel) to recover. Thus a heat pump that will kick in electric strips does not always benefit from setback. Otherwise, many systems (a/c) run more efficiently with a long run - exactly what is called for after a setback. Think of it this way, if your a/c has to run 2 hours over an 8 hour period to get to a comfortable temp when you wake up, it is better for the system to run that 2 hours straight rather than 15 min every hour. Better = more efficient. This effect is further magnified by the smaller delta T across the drafty windows. Mind you, delta T for a/c is generally pretty low as compared to heat but the effect is still there.

So much misinformation about setbacks in "common knowledge" in the general public. Where it came from is anybody's guess.

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