New HVAC system

[Dokat]

Folks

I am in the NYC area and have a pretty poorly installed low-efficiency HVAC system that's quite leaky and blows some dust on the 1st floor. Our home is 100 years old and insulation only exists in the attic and it has a new roof (no insulation in external stucco walls which are made of lots of stucco, wood, mesh). I tried to put band aids and got an Aprilaire system installed but it hasn't helped much due to leaky duct work. Let's just say the Aprilaire installer didn't give me the right advice in the first place. Also, the indoor heating/cooling furnace is about 15 years old and in average to bad shape - it's motor wasn't held correctly and became dislodged and had to be fixed recently. The ductwork is poorly done and has leaks in numerous places and that's just what we can see and it could be much worse inside the walls. After we purchased the house, we've realized the shoddy contractors that the last owner hired including one that used a large plastic bag to protect against a leaky roof slate tile (there are 10 other examples I can provide just like this one)

My primary priority by far for the future is air quality as I am an extreme pollen allergy sufferer for 2 months of the year. Instead of putting band aids in, I've decided to take the right steps to better air quality and if possible, better efficiency during both cold and warm months given we plan to live here at least 10+ years. We have traditionally set the heating at 67 and even with that, our monthly gas bill in Feb or March is $500. We only set the AC about 10 days and in a prior test, using the AC for 30 days straight (both basement and 2nd floor units) was an increase of $400 for the month. If that can be reduced to $200 with a newer system, we'll certainly be in a position to use the AC more often during the summer


Questions
- One of the benefits that supporters of ductless mini-split state you can turn on heating or cooling in individual rooms and thus save energy. But is this really true given warm air moves into cold spaces. In my open-layout home, if I turn on heating in the office, won't the warm air simply move into the cold areas such as living room & kitchen causing the unit to stay on constantly. Hence, wouldn't energy consumption turning on a single unit be approximately the same as turning on all 3 units on the 1st floor. Secondly, even with all 3 units on, won't heat simply keep moving up and warm up the entire 2nd floor. I'm not sure if I understand why this factor is being touted as higher efficiency compared to central air and if I'm reasoning correctly

- For ductless, one HVAC company said if we go ductless, we'll need 2 external units and 6 indoor units (3 on each floor). If we place a unit in a 21x12x8 MBR which also has a door into the 8x8x8 bathroom which also has a small window, would it heat the bathroom or would we need an air handler in the bathroom too?

- Am I crazy considering a mini ductless given I already have an entire ducted system in place and should really consider migrating to a high-efficiency central air system. Of course, I'll have to replace all the duct work given it's been an extremely shoddy job 20 years ago and I've been told a high-efficiency system needs new duct work anyways

- If somebody knows a neutral party who can come to our home in NYC and do an assessment and all calculations, a referral would be appreciated


Thank you!

 

[Dokat]

I know that's quite wordy but any assistance here would be appreciated. Thanks!

 

[Sponsor]

 

[Fitter30]

Energy audit from your electric company that includes a blower door test ( tells how air tight the house is) and there might be rebates on new equipment. More attic insulation, better windows and doors even with no insulation in walls will tighten up the house. Mini splits run 24/7 the blowers and the compressors vary their speed according to how far from set point the temperature varies. Advantages: energy efficient & quite. Disadvantages: thin filters not bad to clean if wall unit is in a accessible, parts might be a few days away, cleaning the inside unit is a job (look at you tube)
and you want a contractor that reps a certain brand has installed a few systems. Bosch and others make a variable refrigerant flow heat pumps also their is a company Space Pac that uses high pressure duct system that might be able to use your duct as a chase. Two story house needs two systems one per floor.

 

[Dana]

My primary priority by far for the future is air quality as I am an extreme pollen allergy sufferer for 2 months of the year. Instead of putting band aids in, I've decided to take the right steps to better air quality and if possible, better efficiency during both cold and warm months given we plan to live here at least 10+ years. We have traditionally set the heating at 67 and even with that, our monthly gas bill in Feb or March is $500. We only set the AC about 10 days and in a prior test, using the AC for 30 days straight (both basement and 2nd floor units) was an increase of $400 for the month. If that can be reduced to $200 with a newer system, we'll certainly be in a position to use the AC more often during the summer

Gas energy is sold in ccf, therms or decatherms, not dollars. NYC utility rates are well above the national average, so it's hard to take a WAG at your actual heating load based on the dollars.

Take the time to run a fuel use based load calculation. Since you keep it at 67F, use base 60 heating degree days, not base 65F. The fuel use based heat load calc will tell us roughly how much total compressor it would take to do it with ductless.

Questions
- One of the benefits that supporters of ductless mini-split state you can turn on heating or cooling in individual rooms and thus save energy.

That's a common misconception promoted by the ductless vendors. Most rooms don't have nearly enough load to run an individual ductless head efficiently. Without sufficient load it'll spend most of it's time cycling on/off rather than modulating with load, which is bad for efficiency, bad for comfort due to bigger room temp swings.

But is this really true given warm air moves into cold spaces. In my open-layout home, if I turn on heating in the office, won't the warm air simply move into the cold areas such as living room & kitchen causing the unit to stay on constantly.

Ideally, yes. Having it run constantly is what you WANT with a modulating ductless system, ramping it's output up and down with load. With an open floor plan it's easier to right-size the system, running fewer heads/cassettes than the "head in every room" approach which grotesquely oversizes the system.

Hence, wouldn't energy consumption turning on a single unit be approximately the same as turning on all 3 units on the 1st floor. Secondly, even with all 3 units on, won't heat simply keep moving up and warm up the entire 2nd floor. I'm not sure if I understand why this factor is being touted as higher efficiency compared to central air and if I'm reasoning correctly

The difference between ductless and central ducted systems is the parasitic duct loads. To move air in ducts requires creating pressure differences between the supply & return registers. When ducts aren't perfectly balanced (which is always the case) it results in pressure differences between rooms, and between the indoors & outdoors, driving outdoor air infiltration (and dust) whenever the system is running. With ductless the air supply & return are in the same room less than 2 feet away from each other, so no indoor to outdoor pressure differences are generated.

The other advantage to ductless is the modulation of output. At low to mid output the efficiency can be twice as high as when running at max capacity and duty cycling on/off.

- For ductless, one HVAC company said if we go ductless, we'll need 2 external units and 6 indoor units (3 on each floor). If we place a unit in a 21x12x8 MBR which also has a door into the 8x8x8 bathroom which also has a small window, would it heat the bathroom or would we need an air handler in the bathroom too?

Lose that company's number- they're only interested in making their boat payments.

- Am I crazy considering a mini ductless given I already have an entire ducted system in place and should really consider migrating to a high-efficiency central air system. Of course, I'll have to replace all the duct work given it's been an extremely shoddy job 20 years ago and I've been told a high-efficiency system needs new duct work anyways

If any of the ductwork is outside of the conditioned space (say, in an attic above the insulation), it needs to be scrapped. If it's in a basement it might be useable, but it has to be made air tight at every seam and joint, and the supply ducts need to be insulated.

It's risky to retrofit insulate a stucco clad house in your climate, but it's not risky to make the house more air tight. A blower door and infra-red imaging directed round of air sealing is usually money well spent. The more air tight the house, the less dust & mold there will be.

If the basement walls aren't insulated, it's usually very LOW risk and moderate to high reward to insulate them to the current IRC code minimum R10 continuous insulation (in your climate zone), even if not actively heating the basement. The most important air leaks in the house are at the attic floor, and at the basement, since the difference in height defines how much stack effect drive there is. I can go into that in more detail if you're interested.

 

[Dana]

Energy audit from your electric company that includes a blower door test ( tells how air tight the house is) and there might be rebates on new equipment. More attic insulation, better windows and doors even with no insulation in walls will tighten up the house. Mini splits run 24/7 the blowers and the compressors vary their speed according to how far from set point the temperature varies.

That's only true if the mini-split has a high turn down ratio and is perfectly sized for the load. The minimum modulated output of the compressor varies (a lot!) from vendor to vendor, model to model. The "hyper heating" vapor injection type compressors tend to have bigger turn down ratios than simpler compressors, and would be appropriate but not absolutely necessary in an NYC location. Multi-zone compressors tend to have much lower turn down ratios on the order of 3:1, sometimes only 2:1. Single zone mini-splits with vapor injection compressors often have turn down ratios of 8:1 or higher, making them a bit easier to work with.

New windows really never pay off on energy savings unless they're ultra-leaky trash in need of replacement for other reasons. Tight low-E storm windows can bring a reasonably tight antique wood-sash single pane pretty close to the current code-min U0.32 at a fraction of the cost. But air sealing is by far the better bang/buck, and that will reduce the amount of dust & allergens brought into the house no matter what the HVAC system is.