Heat pump & back up options
We are expanding our house by 50% and planning to re-install an 8 yr old 13 SEER 3 ton AC with 92% efficient 80 kBtu/h furnace. The house envelope will be upgraded, so the system should be sufficient, at least one contractor said it would be oversized.
We live near Los Angeles where it doesn't get very cold. It has only frozen a couple times in the 15 yrs I've lived here. So far this year (Nov 1 ~ Dec 11)we've accumulated ~180 hrs with the temp below 45F, which is the most chill hours for this period in at least the last 15 yrs.
The remodel will also bring an oversized PV system (so electricity is free) and solar DHW with 90% efficient gas backup.
So I have 3 questions
1) Is it worth it to re-install an 8 yr old system. It is not a high end system and not from a large brand, but it worked fine previously and all ducts will be new. How much longer should the system work?
2) If we go with a new system, what are the pros and cons of the following options
a) AC with hydronic heat
b) heat pump with electric back up
c) heat pump with hydronic back up
3) We will have to run some ducts between 1st and 2nd floor. If we re-install the old system and have to replace it with a similarly sized or smaller system, would those ducts have to be replaced?
I did something similar here in Tucson. First I did a load calculation on the newly expanded home. After that I was able to see what air flows were required and what capacity I needed. As is typical on most remodels the old equipment was over sized and the existing duct system was under sized.Since the equipment at that time was a 6 year old high end Trane unit I re-installed it on an additional 600 square feet of expanded home with a new duct system.See Smith remodel 1/2 way down on this page.
Originally Posted by airbare
Work like this requires a contractor that can evaluate your load and design a duct system that will "fit" your home now and in the future.
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Keep the fire inside the fireplace.
I like the heat pump with hydronic backup option. If you are putting in a large PV system why not utilize the benefit fully with electric heating. If your doing solar DHw and a 90% gas backup why not utilize that system instead of an 80% furnace? The Trane TAM air handlers match up great wit Tranes heat pumps and allow for a backup hot water coil instead of electric.
An 8 year old system may work ok but you really went utilizing you upgrades with it. Why put an 8 year old engine in a new car?
Well, the excess PV capacity is intended to cover all gas used for commuting in an electric car. replacing gasoline provides a better return than replacing natural gas.
The existing furnace is already 92%, so the back up option won't increase efficiency, but would change heating capacity from 80 kBTU/h to 40 kBTU/h.
Would making a change like that in the future require all new ducts?
Can you explain why re-installing the previous system required larger ducts? Will those larger ducts need to be replaced when you eventually replace the older system with one that is properly sized?
The previous mentioned system required more duct because it supplied more area.The previous duct system was removed and a new system was installed in space below the thermal and air barrier (semi conditioned space).
Originally Posted by airbare
My point is that your space requires a certain amount of air to make you comfortable. Your duct system will remain and give you good service after your new unit is dead and gone replaced by the latest and greatest furnace or air handler.Your new duct system if designed and installed right will have a long life span giving you and your family many years of comfort no matter what type of unit you put on it, as long as it is sized right.
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