5 ms·

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] (t

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.
- 2Gkashmiri 4y ago
  this is interesting. i have been told about using 319 steel insulated tank to store hot water, i was also told that this is a "pretty big deal in terms of cost" but i am willing to make that investment. here is my idea. 300Litre solar water heater+200Litre air source heat pump + storage + FCU.
  - jonstewart 4y ago
    I don’t have intuition for how large a 300L solar water heater would be — you mean the solar array on the roof, right? If you have infloor radiant heat with concrete floors, then you might not need much water storage — the concrete provides heat mass. I would look more at deploying PV arrays for offsetting the electricity usage of the heat pump than adding more water storage and solarthermal. Solarthermal is technically more efficient in providing heat alone than equivalent electrical energy, but a heat pump uses that electricity to gather heat from outside, operating at 200-400% efficiency. Electricity is also more useful across the whole house, and of course is a great help in the summer for providing air conditioning.
    - 2Gkashmiri 4y ago
      yeah, the problem with infloor is that the floor is already laid down so i would have to redo the flooring for all rooms. my use case is winter nov-feb which occasionally has snow, some year there is 0 snow, some year there is 12 inches, usually inbetween. electricity from the grid is often down during this time period but when it does come, we could use the heat pump to heat up the "storage". 300L water heater in this case means 300L/day of water heater (yeah, rooftop solar water) the storage can be anything, 500L, 1000L, 2000L, that is cost dependent only. if we feel more storage is needed, we just add another tank heat pump is like 200L/hour so yeah
      - jonstewart 4y ago
        Ok, you're in Kashmir and don't have a reliable grid. I'm not sure it would make sense to spend the money on a heat pump if you didn't have reliable electricity to operate it (if you had sufficient PV panels, then it might be worth it). Build It Solar has a ton of DIY projects for improving efficiency and using solar energy in various ways. You might find inspiration there: https://www.builditsolar.com/ https://www.builditsolar.com/. In particular, they have plans for a solar collector that's a wood frame box with clear plastic sheeting and window screen material — extremely cheap and simple to build. It provides hot air, and it's best if you can fit a few cheap fans (like 120mm 5v PC fans) into it, powered by a small PV panel. I want to build one of these to heat my garage. Some of the nice things about air-based solar collectors is that they're cheap, low maintenance, can't leak, and can't freeze. The downside is that storage is harder. If you have enough heat mass inside the house it might not matter, though.

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.

2Gkashmiri 4y ago
this is interesting. i have been told about using 319 steel insulated tank to store hot water, i was also told that this is a "pretty big deal in terms of cost" but i am willing to make that investment. here is my idea. 300Litre solar water heater+200Litre air source heat pump + storage + FCU.
- jonstewart 4y ago
  I don’t have intuition for how large a 300L solar water heater would be — you mean the solar array on the roof, right? If you have infloor radiant heat with concrete floors, then you might not need much water storage — the concrete provides heat mass. I would look more at deploying PV arrays for offsetting the electricity usage of the heat pump than adding more water storage and solarthermal. Solarthermal is technically more efficient in providing heat alone than equivalent electrical energy, but a heat pump uses that electricity to gather heat from outside, operating at 200-400% efficiency. Electricity is also more useful across the whole house, and of course is a great help in the summer for providing air conditioning.
  - 2Gkashmiri 4y ago
    yeah, the problem with infloor is that the floor is already laid down so i would have to redo the flooring for all rooms. my use case is winter nov-feb which occasionally has snow, some year there is 0 snow, some year there is 12 inches, usually inbetween. electricity from the grid is often down during this time period but when it does come, we could use the heat pump to heat up the "storage". 300L water heater in this case means 300L/day of water heater (yeah, rooftop solar water) the storage can be anything, 500L, 1000L, 2000L, that is cost dependent only. if we feel more storage is needed, we just add another tank heat pump is like 200L/hour so yeah
    - jonstewart 4y ago
      Ok, you're in Kashmir and don't have a reliable grid. I'm not sure it would make sense to spend the money on a heat pump if you didn't have reliable electricity to operate it (if you had sufficient PV panels, then it might be worth it). Build It Solar has a ton of DIY projects for improving efficiency and using solar energy in various ways. You might find inspiration there: https://www.builditsolar.com/ https://www.builditsolar.com/. In particular, they have plans for a solar collector that's a wood frame box with clear plastic sheeting and window screen material — extremely cheap and simple to build. It provides hot air, and it's best if you can fit a few cheap fans (like 120mm 5v PC fans) into it, powered by a small PV panel. I want to build one of these to heat my garage. Some of the nice things about air-based solar collectors is that they're cheap, low maintenance, can't leak, and can't freeze. The downside is that storage is harder. If you have enough heat mass inside the house it might not matter, though.