HVAC replacement and 2 stage units

[target2019]

Answering my own dumb question after some research, 1 ton=12,000 btuh, so my 4-ton system looks good for the main floor calculation ~45,000. I'm surprise my upstairs heat calculation was 45,000 also but A/C was only 15,000. Don't know what that means for my 2-ton system.
There's no urgency to replace my system, since it's working, but I will have to try to figure out the payback period of a new system to see if it makes sense to upgrade even though the current system is still operational.

The 2 ton A/C rating is not used for heat. The furnace will have another rating. For example, my A/C is 2.5 ton, but the furnace is 80,000 BTU.

 

[Masquernom]

When we bought our current house, the home inspector said he thought the AC was too small. I called an HVAC company to do an inspection. They said our AC was too small. I contacted the owner with the concerns. He was a nuclear engineer and sized the system himself. I reviewed his numbers. We bought the house.

Every few months the local utility sends us a letter informing us the utility cost for our home is lower than any other house around. The previous owner was right.

If you read the HVAC forums, there is a big difference between what the HVAC engineers say to do, and what the local dealers actually do. The reasoning is that if you oversize the AC system, you never get a call back from the customer with a complaint. But the systems work better if they run for a longer time. They dehumidify the house better and are smaller and more efficient.

 

[ExitLeft]

But the systems work better if they run for a longer time. They dehumidify the house better and are smaller and more efficient.

This isn't true for single-stage systems, though, right? Or if it is true, should I leave the fan in the "on" position, rather than auto? I don't see how that would save energy though, unless it results in the A/C rarely running as a result.

I went so far as putting in my own manual baffles in the ducts for our upstairs systems, so that I could turn off the kids' rooms when they were away at college, etc. (keeping their doors closed as well). I wonder if that saves energy or just screws up the "balance" of the system?

 

[samclem]

I'm surprise my upstairs heat calculation was 45,000 also but A/C was only 15,000. Don't know what that means for my 2-ton system.

You may want to check to be sure about your design temp for the summer at your location. I couldn't find Thomas Point listed in the tables I found, but there was >nowhere< in Maryland with a 1% design cooling temperature as low as the 85 deg F you used. Baltimore is 91 deg F, and the "coolest" summertime town I found in MD had a design temp of 88 deg F. Here's the EPA table.

 

[travelover]

This isn't true for single-stage systems, though, right? Or if it is true, should I leave the fan in the "on" position, rather than auto? I don't see how that would save energy though, unless it results in the A/C rarely running as a result..........

The fan control only runs the fan. What dehumidifies is running the interior air over the cold coil in the air handler / furnace. If the compressor is not running, the coil doesn't stay cold for long. So, a smaller compressor allows more run time in the cold coil condition.

 

[Texas Proud]

When we bought our current house, the home inspector said he thought the AC was too small. I called an HVAC company to do an inspection. They said our AC was too small. I contacted the owner with the concerns. He was a nuclear engineer and sized the system himself. I reviewed his numbers. We bought the house.

Every few months the local utility sends us a letter informing us the utility cost for our home is lower than any other house around. The previous owner was right.

If you read the HVAC forums, there is a big difference between what the HVAC engineers say to do, and what the local dealers actually do. The reasoning is that if you oversize the AC system, you never get a call back from the customer with a complaint. But the systems work better if they run for a longer time. They dehumidify the house better and are smaller and more efficient.

Not always.... my friend is having my AC guy come by because the last person oversized his and it cycles way too much... he said he has put up with it for 10 years and finally has gotten so frustrated he is going to do something about it...

 

[ExitLeft]

You may want to check to be sure about your design temp for the summer at your location. I couldn't find Thomas Point listed in the tables I found, but there was >nowhere< in Maryland with a 1% design cooling temperature as low as the 85 deg F you used. Baltimore is 91 deg F, and the "coolest" summertime town I found in MD had a design temp of 88 deg F. Here's the EPA table.

Thanks for that info. We are in Anne Arundel County, 91 deg F. The 'coolcalc' website pulled that Thomas Point data based on their GPS info.

 

[ExitLeft]

The fan control only runs the fan. What dehumidifies is running the interior air over the cold coil in the air handler / furnace. If the compressor is not running, the coil doesn't stay cold for long. So, a smaller compressor allows more run time in the cold coil condition.

Is it the constant dehumidifying that results in the lower electric bill? I just want to reduce my electricity consumption. According to utility, we use 56% more electric than the top 20% "most efficient" neighbors, but use 5% less than "all neighbors." Doesn't make me feel good when I have a $280 bill to pay this month...

I should note that we keep the main living area set to 77 deg and the upstairs at 85 until evening and then upstairs drops to 74 and main living gets turned off. Always have left the fan in "auto" position. In the winter, we use about 1,300 kwh/month and peak summer goes up to 2,300, I assume 90%+ driven by the A/C usage.

 

[ERD50]

Is it the constant dehumidifying that results in the lower electric bill? I just want to reduce my electricity consumption. According to utility, we use 56% more electric than the top 20% "most efficient" neighbors, but use 5% less than "all neighbors." Doesn't make me feel good when I have a $280 bill to pay this month...

I should note that we keep the main living area set to 77 deg and the upstairs at 85 until evening and then upstairs drops to 74 and main living gets turned off. Always have left the fan in "auto" position.

It really depends on your circumstances. There is no general answer.

Someone in Phoenix isn't dealing with humidity, but they are dealing with heat.

In my case, we really can't have much in the way of shade trees, and the west Sun just beats on our house, two stories with a basement (so 2 stories getting hit by the sun. But my Mom's house, in the same climate, had a lot of old oak shade, a ranch with full basement, so LOTs of humidity coming up from that basement. I think their unit was over-sized (don't know the rating), and I'd go there and it would be like 67 F, and clammy. She had to keep turning down the thermostat in an attempt to get the humidity lowered. And had a de-humidifier running in the basement.

You want long cycles for efficiency and humidity removal, and that means a smaller unit. But on a really hot summer day, with sun hitting the house, and people over, you need a large unit. That's why I went for the 2-stage and variable speed fan. But not everyone needs that.

It depends.

-ERD50

 

[samclem]

Is it the constant dehumidifying that results in the lower electric bill? I just want to reduce my electricity consumption.

If we take two identical homes and one has a "too big" AC unit and the other has a "correct size AC unit, and both homes set the thermostat to the same temperature, then the home with the "too big" AC unit will have higher indoor humidity. And that will be less comfortable, likely prompting the owner to set the temperature lower (in order to get the same "wet bulb" indoor temperature, which is what really determines the comfort to occupants). And setting the temperature lower will increase electricity costs. So, indirectly, a smaller AC unit will generally reduce operating costs.

 

[ExitLeft]

If we take two identical homes and one has a "too big" AC unit and the other has a "correct size AC unit, and both homes set the thermostat to the same temperature, then the home with the "too big" AC unit will have higher indoor humidity. And that will be less comfortable, likely prompting the owner to set the temperature lower (in order to get the same "wet bulb" indoor temperature, which is what really determines the comfort to occupants). And setting the temperature lower will increase electricity costs. So, indirectly, a smaller AC unit will generally reduce operating costs.

Ah, brilliant. We have extreme humidity in Maryland. I guess I need to do some hard math comparing how long my A/C runs per day*kw used vs assumptions of a 2-stage or variable in lowest electric mode running over a longer period set at a couple degrees higher (assuming higher temp could be comfortable with lower humidity) to figure out the potential electric cost difference.

 

[Texas Proud]

If we take two identical homes and one has a "too big" AC unit and the other has a "correct size AC unit, and both homes set the thermostat to the same temperature, then the home with the "too big" AC unit will have higher indoor humidity. And that will be less comfortable, likely prompting the owner to set the temperature lower (in order to get the same "wet bulb" indoor temperature, which is what really determines the comfort to occupants). And setting the temperature lower will increase electricity costs. So, indirectly, a smaller AC unit will generally reduce operating costs.

I have no idea if this is true, but our AC guy said that the start up of an AC system takes a lot of electricity..... so in your example even if they did not turn the temp down, the larger unit would cycle many more times and also use up more electricity...


I checked my unit for yesterday.... it ran 77% of the day... most of the time I looked it was in the 30% range for the compressor with some times going as high as 70%...

Have not had it long enough to know if less electricity... but it does seem a bit more comfortable... not as good as they were making it out, but more even...

BTW, I have mine set to 45% humidity, but it struggles to get below 50%... one of the techs who came out to move a sensor said that it is almost impossible to get lower on an 'old' house (built in late 70s or early 80s)... the newer houses are tighter and easier to get moisture out...

 

[ERD50]

I have no idea if this is true, but our AC guy said that the start up of an AC system takes a lot of electricity.....

I think that gets exaggerated a bit (but not 'wrong'). Yes, start up currents are high for a motor (maybe not so much for variable speed motors though?), but that's only a few seconds. If it were much higher for much longer, your circuit breaker would pop.

so in your example even if they did not turn the temp down, the larger unit would cycle many more times and also use up more electricity...

But yes, cycling often does lower efficiency - it takes time to circulate the 'cool'.

I checked my unit for yesterday.... it ran 77% of the day... most of the time I looked it was in the 30% range for the compressor with some times going as high as 70%...

Have not had it long enough to know if less electricity... but it does seem a bit more comfortable... not as good as they were making it out, but more even...

BTW, I have mine set to 45% humidity, but it struggles to get below 50%... one of the techs who came out to move a sensor said that it is almost impossible to get lower on an 'old' house (built in late 70s or early 80s)... the newer houses are tighter and easier to get moisture out...

That sounds pretty reasonable to me. Running that long and that low indicates it is working to get the humidity down. The bumps up to 70% are probably the peaks of the summer sun, or when running that long just isn't quite keeping up.

The worst of the heat/humidity we get here in N IL is probably similar to what you experience on a 'bad' (but not excessively bad) day in Texas. I think our is set for 45% humidity, and low 50's seems to be what we can expect. Even though I hate the heat/humidity, I find that by the time the inside is low 50's in RH and 77F or lower, we are comfortable. Seems to be a big difference between that and ~ 65 RH inside (it might be 70's/80's/90's RH outside, with temps in the 90's).

Ahhh, one more thing I noticed. These smart thermostats are pretty flexible. The old ones, you got a temperature and a range (say set to 76F, it maintains 75-77, turning on/off at those points). Another way they work to keep the humidity controlled, and keep the cycles so they are not too short or too long, is they will make that range wider or tighter dynamically. I found a setting in the thermostats called CPH (cycles per hour). It was set to 5, I set it to 4. Meaning that at "4" it will try (within other limits/conditions) to be on for 7.5 minutes, off for 7.5 minutes, so 4 cycles per hour. So if it's hot out, it might tighten that range a bit. If it's cool out, it widens the range to keep the A/C running a little longer. I wish I understood all the algorithms, but it seems to work well.

-ERD50

 

[target2019]

Is it the constant dehumidifying that results in the lower electric bill? I just want to reduce my electricity consumption. According to utility, we use 56% more electric than the top 20% "most efficient" neighbors, but use 5% less than "all neighbors." Doesn't make me feel good when I have a $280 bill to pay this month...

I should note that we keep the main living area set to 77 deg and the upstairs at 85 until evening and then upstairs drops to 74 and main living gets turned off. Always have left the fan in "auto" position. In the winter, we use about 1,300 kwh/month and peak summer goes up to 2,300, I assume 90%+ driven by the A/C usage.

It's a combination of things. I think your d/s unit is oversized, and u/s is possibly undersized. Is the house well-insulated? High celings? How about the distribution system?

My 1975 house is 3000SF on 2 floors. Can't really fix the original design, but high ceilings and features like cathedral ceiling mean significantly more space to fill with heat, and dehumidify.

Replace all windows when moved in. Attic insulation made an incredible difference. Replace large sliders and entrance door. Replaced both 40-year old systems.

Finally, last year we reached comfort. And gas/electric bills are down 25% or more.

Note: if you lower upstairs system under 80 in the summer, downstairs might work less, and be more comfortable.

 

[target2019]

Ah, brilliant. We have extreme humidity in Maryland. I guess I need to do some hard math comparing how long my A/C runs per day*kw used vs assumptions of a 2-stage or variable in lowest electric mode running over a longer period set at a couple degrees higher (assuming higher temp could be comfortable with lower humidity) to figure out the potential electric cost difference.

Your humidity problem is worse than ours, in South NJ. When I bought these VS systems, I recall the contractor mentioned I would not benefit from two-stage. However, if I was near the ocean, he would recommend it.

One thing I can do with these VS systems, is run the fans continuously at a very low speed. This helps early and late in the seasons, before heating or A/C is required to be on.

 

[ExitLeft]

It's a combination of things. I think your d/s unit is oversized, and u/s is possibly undersized. Is the house well-insulated? High celings? How about the distribution system?

My 1975 house is 3000SF on 2 floors. Can't really fix the original design, but high ceilings and features like cathedral ceiling mean significantly more space to fill with heat, and dehumidify.

Replace all windows when moved in. Attic insulation made an incredible difference. Replace large sliders and entrance door. Replaced both 40-year old systems.

Finally, last year we reached comfort. And gas/electric bills are down 25% or more.

Note: if you lower upstairs system under 80 in the summer, downstairs might work less, and be more comfortable.

Thanks. House was built in 1999 by Toll bros, so just the cheapest materials they could get away with! Lol. We are the original owners and knew nothing about building, so made lots of mistakes. We have lots of windows and a 2-story family room+9' ceilings, but a ceiling fan to help circulate. Our humidity indoors averages mid-60's% in the summer. Congrats on your big bill savings with your improvements, but given the age of our house, in our case I don't think we need to change things out structurally at this point.

 

[ExitLeft]

Sorry if this is too tangential for this thread, but have to ask... We've had a musky smell in our master bedroom and I got to reading about mold in the A/C/ductwork and so I bought UV-C lights, but haven't installed them yet. Anyone here have experience with them and their effectiveness? I read that hospitals use them and I'm hoping it will take care of the smell, which is only in the MBR for some reason... I couldn't find any other source of water or anything so thinking it might be a mold in the duct issue.

 

[samclem]

Sorry if this is too tangential for this thread, but have to ask... We've had a musky smell in our master bedroom and I got to reading about mold in the A/C/ductwork and so I bought UV-C lights, but haven't installed them yet. Anyone here have experience with them and their effectiveness? I read that hospitals use them and I'm hoping it will take care of the smell, which is only in the MBR for some reason... I couldn't find any other source of water or anything so thinking it might be a mold in the duct issue.

The UV lights will definitely kill mold on any surface that are directly in line with the bulb, and they will also help to kill mold/inactivate mold spores in the air that goes past it. But it seems unlikely that it will be much help for your problem.
1) Mold/dirt/dust in ducts is a problem that is frequently diagnosed. Its big business for duct cleaners. In fact, removing the dust/mold seldom improves indoor air quality.
2) Mold spores are everywhere. Killing them as they fly by in the duct air is probably not going to do anything about active mold that is growing someplace.
3) If there was active mold in your ducts to a problematic degree, I'd think you'd be smelling it everywhere.
4) If you have mold somewhere, then you have a water problem in that place. If what you are smelling is mold, and it is only in your bedroom, check there very carefully. Anyplace that doesn't get a lot of air exchange (under a mattress, clothes in a walk-in closet, etc) could be a candidate, especially if it's a place with slightly higher relative humidity than the rest of the house.
It's summer, and if the >outside< of your uninsulated/underinsulated (cool) ducts are in contact with warm air (in an attic, a crawlspace, inside walls, etc), they could have condensation on the outside which is dripping somewhere and feeding a mold colony. Or, you could have a small leak in a water pipe/fitting, etc.

The most healthy range for indoor humidity is between 40% and 60%. Most fungi cannot survive with RH below 60%. You've said your indoor RH is in the mid-60s during the summer, and there are places in most homes that exceed the average RH by quite a bit. So, in your shoes I'd do the common sense things (use exhaust fans when showering/boiling water, leave doors open if needed to encourage air circulation, etc) and also check to see that your AC system is doing its part to keep things in the healthy range.