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As part of my next solar panel addition discussed in another thread, I'm installing about 4kw of panels I picked up very cheaply onto my metal shop roof. This will be backfed (AC coupled) into my Outback Skybox via another separate grid tie inverter (probably a growatt hybrid or an SMA, but that is another discussion for later)
The main purpose in the winter months will be to heat my floor with my existing radiant floor heat setup. The floor heat works well with my Takagi tankless propane heater, but that is a very expensive option for heat. So we didn't use it this year as it added up to about $300/month for propane to keep the floors and house comfortable last year. That was even burning some wood in the stove to keep things toasty when it was very cold.
Anyway, I am now looking for ideas to utilize the 4kw of solar panels to heat the floor. I can definitely exceed the 4kw if needed as I will have most of the total capacity of the skybox available to use ( roughly 10kw total panels). But I'd like to keep the average to about 25kwh per day or less as my new panels will cover that. My existing panels cover about 30 kwh/day which is about what we use on average including heat and cooling. So actually I'll have a bit more than 25kwh excess if the floor is doing the heating, maybe up to 40 kwh. I should also mention it is a net metered grid tie system so I can pull from the grid bank on cloudy days.. And in turn on sunny days I won't be inputting as much heat of course.
One problem is figuring out what kind of heater to use with my system & calculating heat capacity of the equipment. I was going to try to re plumb and use an electric tank water heater, but I don't think even with a 4-5kw heating element that the floor will stay very warm even if I run it all day. So I saw in some posts here that people recommend using a heat pump water heater with solar because they are much more efficient. I looked up some specs on the Rheem line. They have a 4200btu/h heat pump which is almost nothing compared to what a 2500 sqft house would require but I guess it also has a backup 4500 watt element. So that could provide me with about 4200btu/hour of the heat pump plus the 4500 watts (1 watt=3.4 btu/h so that is 15,000 btu/h). For a total of about 19,000 btu/h. That seems like a decent amount of heat and may be enough. Running that for 8 hours/day might go over my 40kwh limit of my excess production. But on average it may work out. If anyone is so inclined to check my calcs, let me know if anything stands out wrong as I'm sure I could have made some mistakes. I haven't thought through all these numbers yet.
The next question is are these heat pump water heaters designed to work 8+ hours/day? I guess I can read more on that in their specs and warranty information. But figured I'd pop in here and see what folks had to say about this. As I said earlier the tankless worked fine for the season I used it. One advantage is you aren't constantly adding minerals to the system in a closed loop so you don't have as much potential to produce scale that tends to shorten water heater life..
Any thoughts appreciated.
The main purpose in the winter months will be to heat my floor with my existing radiant floor heat setup. The floor heat works well with my Takagi tankless propane heater, but that is a very expensive option for heat. So we didn't use it this year as it added up to about $300/month for propane to keep the floors and house comfortable last year. That was even burning some wood in the stove to keep things toasty when it was very cold.
Anyway, I am now looking for ideas to utilize the 4kw of solar panels to heat the floor. I can definitely exceed the 4kw if needed as I will have most of the total capacity of the skybox available to use ( roughly 10kw total panels). But I'd like to keep the average to about 25kwh per day or less as my new panels will cover that. My existing panels cover about 30 kwh/day which is about what we use on average including heat and cooling. So actually I'll have a bit more than 25kwh excess if the floor is doing the heating, maybe up to 40 kwh. I should also mention it is a net metered grid tie system so I can pull from the grid bank on cloudy days.. And in turn on sunny days I won't be inputting as much heat of course.
One problem is figuring out what kind of heater to use with my system & calculating heat capacity of the equipment. I was going to try to re plumb and use an electric tank water heater, but I don't think even with a 4-5kw heating element that the floor will stay very warm even if I run it all day. So I saw in some posts here that people recommend using a heat pump water heater with solar because they are much more efficient. I looked up some specs on the Rheem line. They have a 4200btu/h heat pump which is almost nothing compared to what a 2500 sqft house would require but I guess it also has a backup 4500 watt element. So that could provide me with about 4200btu/hour of the heat pump plus the 4500 watts (1 watt=3.4 btu/h so that is 15,000 btu/h). For a total of about 19,000 btu/h. That seems like a decent amount of heat and may be enough. Running that for 8 hours/day might go over my 40kwh limit of my excess production. But on average it may work out. If anyone is so inclined to check my calcs, let me know if anything stands out wrong as I'm sure I could have made some mistakes. I haven't thought through all these numbers yet.
The next question is are these heat pump water heaters designed to work 8+ hours/day? I guess I can read more on that in their specs and warranty information. But figured I'd pop in here and see what folks had to say about this. As I said earlier the tankless worked fine for the season I used it. One advantage is you aren't constantly adding minerals to the system in a closed loop so you don't have as much potential to produce scale that tends to shorten water heater life..
Any thoughts appreciated.
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