HVAC replacement in ICF home–Hydronic A/C and radiant floor heat
Hello! We have an ICF home in USA zone 6a, which is from -10°F to -5°F, or -20.6°C to -23.3°C in the winter. Our normal winters tend to be more in the 10s°F to 20s°F and normal summers tend to be high 80s°F to low 90s°F.
The home was built in the mid to late 2000s and is around 2,300 sq. ft. on a single level. It has a full basement that essentially maintains relatively the same temperature even though it isn’t technically heated or cooled. This is due to things like the ICF, a very open stairway system, radiant upstairs floor heat in the winter, and an overall cooler temperature by nature in the summer.
All of the HVAC was installed by the previous owner. In hindsight, we probably should have negotiated something in relation to at least the air conditioning getting replaced upon purchasing.
Our current heating system is radiant and heated by our whole-house on-demand water heater. We recently replaced the water heater due to the previous one failing. It was about 10 years old and is not designed to be used in this way, but doesn’t create any problems, outside of shortening the lifespan of the water heater. It has a heat exchanger and the radiant floor is a closed system. The manufacturer of the radiant floor heat system closed before we purchased the house.
There are seven zones controlled by smart WiFi thermostats, for both the heat and air.
We also have solar panels that power the entire house, except the 240v. We attempted to put the A/C on a 240v Growatt, but the initial power up kick is too great.
We are debating our options. The company makes a compatible solar chiller with a low power up kick. But it is pricey.
We received a quote to centralize all the ducts and put in normal forced air A/C and a boiler for the floor. But it was $15-20k, depending on how “smart” we went or zoned we stayed. And that was a year or two ago money. It’s much cheaper to replace the tankless water heater ourselves every 10 years than some of the other options we’ve explored…
We would like to put one or both systems on the solar, if at all possible. We need better main inverters anyway because of flickering lights when the clothes washer is running. So we’re open to that.
I’ve learned from these forums that radiant floors in an ICF house don’t actually make a lot of sense since it doesn’t really get cold enough on average to keep the system on. Or something to that effect.
We are leaning towards a heat pump and leaving the radiant floor system on the water heater as a supplement if the heat pump can’t keep up in colder winters, but it also seems such a waste to not really use that system. And connecting the existing ducts together for the A/C. But I’d really prefer to leave them zoned. I don’t know if that’s possible or feasible on true forced air?
We are comfortable installing almost anything ourselves, as long as we can retail purchase as consumers. We are not against hiring where necessary, though.
Summer is nearing and I’d like to at least have a plan on place if we can’t get one more summer out of the current system. I’m guessing fans in the floor units will continue to die, and the compressor will be the next big thing to go.
What do you think is the best approach considering cost and energy efficiency?
Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Before you do anything else, follow the process in this article to estimate your heating load:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Look at the table here to get your heating design temperature:
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
The output of heat pumps drops as the temperature drops. The question is going to be whether a heat pump can even put out the amount of heat you need at your design temperature.
As to ductwork, it doesn't sound like you have real ducts right now. It sounds like you have a fan coil unit in each room, and they're buried in the wall. Duct systems are engineered, they have to be sized to the load and type of system. It often can be difficult to add them to existing houses because they take up a lot of space. You may be able to use the space that the existing fan coil units take up but you're still probably looking at a lot of paint and patching. And possibly some new soffits in your ceilings.
I wouldn't give up on the air handlers yet, they're probably better than any other solution. I would try harder to identify them. Can you post some pictures?
Another alternative to consider is a high-velocity air system. They're meant for situations like this where running new ductwork would be really disruptive to the house.