System sizing for heat and manual J

I am replacing broken and aging equipment. The first floor system was shy of heating balance point below 30 degrees, calling for electric heat backup.

I think I might reduce excess heating cost by increasing the first floor heat pump by 1/2 ton, so that I stay off of the electric heat strips most of January and February season. The electric meter spins like crazy when the heat strips cut on. The old urban tail that heat pumps don't bring heat in just isn't true. They bring in more than you spend on electricity, with. They are sized for cooling season, so they are not large enough to handle January and February heating.

I built a spreadsheet that shows through interative and hourly weather data calculations against known heat loss, COP and run hours on the system. It shows that another 6000 btu/hr heat pump capacity on the first floor would drop my elecricity use by 15% per year. It does make a difference to keep the resistance heat turned off. Electricity costs will double or tripple in the next three to five years.

The impossible notion to calculate is the lost efficancy for the moments the heat pump or A/C is running but not producing conditioning.

In four years, I will run more electricity through this unit that the unit costs me. Selecting the correct size is fairly important. To me, operating cost considerations are more than occasional higher humidity.

Do I really want to size this first floor unit for best humidity demand and latent heat removal when that demand is mostly attended to by the second floor system and the home has fairly low latent heat gain and low infilatration by design? Most of the heat gain is insulated glass.

Manual J shows this home has a sensible load of 70,560 and a latent load of 2295. This is shared between upstairs and downstairs systems. I proposed to make them 3-1/2 tons each. The open home architecture causes some of upstairs cooling to attend to downstairs needs and there for reduces downstairs run time. One guess is a ton upstairs treats downstairs. That would make even a three tons downstairs excessive.

If I replace the existing 3 ton downstairs system with 3-1/2 ton, and replace the existing upstairs 3-1/2 ton system with an identical unit (that system has a four ton compressor hacked in), what are the pitfalls?

I was planning to use a variable speed air handler and humidistat on the first floor to facilitate humidity control for the oversizing I propose on floor one.

I am correcting the inadequate air flow through the second floor system at the same time, probably to the detriment of that systems latent/sensible ratio.

I actually thought I would fix floor one with the 3-1/2 ton larger system now, and see how things work through the summer and then replace that system in autumn. By then, the ducts will be sorted out up upstairs and properly working.

Has anyone balanced this difference between winter and summer operating cost in a relatively dry home and can advise.. Thanks! :)

My advice, which have cost of electricity in mind, would be to consider buying two duct type split-system inverters of some Japan manufacturer like Daikin, Toshiba, Fujitsu and you could heat your home without much need of additional heat from electric heaters all the way to -10 to -15°C outdoor temperature.
Example:
http://www.fujitsu-general.com/globa...uct/index.html

Install a geothermal heat pump is the answer. maybe


Best regards from
yangchenchen

If you oversize it for heating have a dehumidifier on hand. Get an April Air whole house model.

You guys are right. But the payback on best choices can be fairly long, way longer than I will stay here. My motivation to replace the old gear instead of patching old stuff is because I expect energy will cost much more in the future.

I looked at ground sourced systems. I have the land and I have no objection to stripping the dirt to set the pipe coils. The choices of contractors are pretty limited so far and they are very expensive. For instance, three systems here can be replaced for around $23K-$25K with good air sourced gear. The wild guess from a ground sourced contractor was $60,000+ nearing $70,000. I may be misinformed, but excavation, laying pipes etc doesn't seem like it should be that exensive, even five or six feet deep. (wait till he discovered the rock though, lol) There's a lack of competition in this field that makes this choice very costly. It seems like the equipment itself is very reasonable. If I would live here ten or fifteen years more, I would study that option more closely, because I expect electricity here to rise from 9-1/2 cents to 25 cents per kwh. My use is 32,000 kwh/year with old gear that spec'd at hspf 8.75.

The inverter technology looks attractive. It can adjust to meet peak loads while providing dehumidification during light loads. The efficiancy is somewhat better too, perhaps by eliminating the cycling. This inverter technology has limited distribution here in the US. In the larger sizes, it's really expensive as well.

I made the hard decision to up-size to meet heating needs on floor one by adding 1/2 ton. This house is well sealed, so I hope my gamble works out for the latent heat side. If the first floor becomes humid, I'll add the system dehumidifier Abby mentions. The lion share of my expense is for heat, so keeping the heat strips turned off is key.

Hi I Need Some Easy Way To Size Ducting In A House ,thnks