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interesting. i have a PV system. i want to use the half installed central heating system (only piping laid down) (no boiler or FCU/room unit) and i have this cr

interesting. i have a PV system. i want to use the half installed central heating system (only piping laid down) (no boiler or FCU/room unit) and i have this crazy idea. use solar water heater to heat water, that feeds into a water storage tank, that water gets heated MORE by an air source water heater and a small circulation pump uses that water storage to heat the house. i am hoping, since we currently have 0 whole house heating, this would be better than nothing and the problem of not having consistent electricity to run the air source heat pump all times would be alleviated somehow by using solar water. is that crazy?

jonstewart 4y ago
I assume you’re on the grid? Yeah, sadly, it’s crazy. Solar heaters are expensive and require a large area to get decent BTUs out of. If you can find free solar heaters that someone’s giving away, that can make the difference, but they’re often a few thousand dollars and that’s about equivalent to a Fujitsu or Mitsubishi heat pump base install, especially when you factor in all the extra plumbing and pumps to/from the solar thermal array. WORSE, there’s maintenance. You’ve got the potential for leaks (probability rises to 1.0 over the years), and you have to worry about the water freezing. That can be mitigated by using a glycol mix, but then you’ve got glycol and that lowers the efficiency. And then you have to think about climate. Is it reliably sunny in the winter? Or is it only sunny 1 out of 3 days, with long stretches of gray days? You are probably better off plowing that money into efficiency/insulation improvements to lower the overall heat load (and those improvements often have zero maintenance). I’d suggest looking into an air source heatpump that can produce domestic hot water, paired with a large storage tank. Check out Artic Heat Pumps: https://www.arcticheatpumps.com/ https://www.arcticheatpumps.com/
jonstewart 4y ago
Oh, also: an air source water heater would need an external heat source—otherwise you’ve just made a closed system and won’t be able to heat the house beyond what solar thermal provides, plus heat loss of the house.
mderazon 4y ago
I am currently looking to do something similar. I have zero heating in my house at the moment and have a 300L (65 gallon) thermosiphon solar water heater [1] (tank plus panels all in one system) that gives me plenty hot sanitary water. I am looking to install an hydronic fan coil [2] radiator in my rooms and need water to heat them - I thought to use the excess heat from the solar heater to supply hot water to the fan coil units but I am not sure how do do that. If you have an idea, please share. 1. https://sol-aqua.com/thermosiphon-solar-water-heater/ https://sol-aqua.com/thermosiphon-solar-water-heater/ 2. https://www.arcticheatpumps.com/heat-pump-fan-coils.html https://www.arcticheatpumps.com/heat-pump-fan-coils.html
- jonstewart 4y ago
  This is not so complicated, you just need tanks with heat exchanger loops. The solar water heater input comes into your tank at the bottom and goes through a copper coil, giving you domestic hot water. _Some_ tanks will have a similar coil in the middle/top of the tank. What you can do is then run the cold return from your heating units through this coil. It will heat the radiator water before going to the fan coil unit. This is often done as a precursor "boost" before sending the water through a boiler/heater -- the solar heat is applied to cut the temperature differential and reduce the energy expended by the boiler/heater for the radiant heat, but you're not relying solely on solar heat. You can get heat pumps that use a water storage tank with extra heat exchanger loops like this, precisely for providing domestic hot water. The DHW and radiator water never mix. There aren't any moving parts, so it's simple. The only question is what the max temp is from the thermosiphon tank, and whether that exceeds either the max radiator water temp or, if you use some kind of modulating boiler/heater for the radiator, the max input temp. The math for solar storage gets daunting, however. If your typical max temp for the thermosiphon tank is 120F on a sunny day and the cold return from the FCU comes back at 70F, then with 65 gallons you can store about 26K BTU in your tank. That sounds like a "2 ton"/24K heater, but it isn't because the BTU rating of heaters is _per hour_. So, if your house needs a ~24K BTU system for heating, a sunny day storing heat in your tank will get you about an hour's worth of free heating. 24K BTU/hour is a pretty small heat load for a typical house... (though plenty doable if you invest in insulation, windows, etc.) So, the thermosiphon is maybe adequate to provide you with domestic hot water, but in a cold climate you'll need 3–5 more of them and a repurposed and well-insulated 500gal milk tank in a shed near your house with insulated below-ground pipes running to it.

jonstewart 4y ago

I assume you’re on the grid? Yeah, sadly, it’s crazy. Solar heaters are expensive and require a large area to get decent BTUs out of. If you can find free solar heaters that someone’s giving away, that can make the difference, but they’re often a few thousand dollars and that’s about equivalent to a Fujitsu or Mitsubishi heat pump base install, especially when you factor in all the extra plumbing and pumps to/from the solar thermal array. WORSE, there’s maintenance. You’ve got the potential for leaks (probability rises to 1.0 over the years), and you have to worry about the water freezing. That can be mitigated by using a glycol mix, but then you’ve got glycol and that lowers the efficiency. And then you have to think about climate. Is it reliably sunny in the winter? Or is it only sunny 1 out of 3 days, with long stretches of gray days? You are probably better off plowing that money into efficiency/insulation improvements to lower the overall heat load (and those improvements often have zero maintenance). I’d suggest looking into an air source heatpump that can produce domestic hot water, paired with a large storage tank. Check out Artic Heat Pumps: https://www.arcticheatpumps.com/ https://www.arcticheatpumps.com/

jonstewart 4y ago

Oh, also: an air source water heater would need an external heat source—otherwise you’ve just made a closed system and won’t be able to heat the house beyond what solar thermal provides, plus heat loss of the house.

mderazon 4y ago

I am currently looking to do something similar. I have zero heating in my house at the moment and have a 300L (65 gallon) thermosiphon solar water heater [1] (tank plus panels all in one system) that gives me plenty hot sanitary water. I am looking to install an hydronic fan coil [2] radiator in my rooms and need water to heat them - I thought to use the excess heat from the solar heater to supply hot water to the fan coil units but I am not sure how do do that. If you have an idea, please share. 1. https://sol-aqua.com/thermosiphon-solar-water-heater/ https://sol-aqua.com/thermosiphon-solar-water-heater/ 2. https://www.arcticheatpumps.com/heat-pump-fan-coils.html https://www.arcticheatpumps.com/heat-pump-fan-coils.html

jonstewart 4y ago
This is not so complicated, you just need tanks with heat exchanger loops. The solar water heater input comes into your tank at the bottom and goes through a copper coil, giving you domestic hot water. _Some_ tanks will have a similar coil in the middle/top of the tank. What you can do is then run the cold return from your heating units through this coil. It will heat the radiator water before going to the fan coil unit. This is often done as a precursor "boost" before sending the water through a boiler/heater -- the solar heat is applied to cut the temperature differential and reduce the energy expended by the boiler/heater for the radiant heat, but you're not relying solely on solar heat. You can get heat pumps that use a water storage tank with extra heat exchanger loops like this, precisely for providing domestic hot water. The DHW and radiator water never mix. There aren't any moving parts, so it's simple. The only question is what the max temp is from the thermosiphon tank, and whether that exceeds either the max radiator water temp or, if you use some kind of modulating boiler/heater for the radiator, the max input temp. The math for solar storage gets daunting, however. If your typical max temp for the thermosiphon tank is 120F on a sunny day and the cold return from the FCU comes back at 70F, then with 65 gallons you can store about 26K BTU in your tank. That sounds like a "2 ton"/24K heater, but it isn't because the BTU rating of heaters is _per hour_. So, if your house needs a ~24K BTU system for heating, a sunny day storing heat in your tank will get you about an hour's worth of free heating. 24K BTU/hour is a pretty small heat load for a typical house... (though plenty doable if you invest in insulation, windows, etc.) So, the thermosiphon is maybe adequate to provide you with domestic hot water, but in a cold climate you'll need 3–5 more of them and a repurposed and well-insulated 500gal milk tank in a shed near your house with insulated below-ground pipes running to it.