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.
Thanks for your input. So I understand that heating the house requires much more hot water than what my solar heater can probably provide. In that case, connecting it to the radiator system is not so beneficial, and probably better to rely on a heat pump for the radiator water (the solar heater will not provide much help to justify connecting to it)
Basically. There are all sorts of neat DIY examples of people using large solarthermal arrays and big storage tanks to provide adequate heat, but they’re major projects and custom.
For your system, what I’d potentially recommend is looking at a heat pump that can provide both domestic hot water -and- hot water for your radiators. They exist! Then, explain to your vendor/installer that you want a second heat exchanger in the tank for input from the thermosiphon. Then you would be providing solarthermal to both house heating and DHW— it won’t be enough, but it is theoretically better to apply that input across both loads, in a shared manner.
However, the question is cost. You’ve got a DHW system right now that works. To install a new system with a tank that can accommodate the thermosiphon input and provide DHW and radiant heat may be too expensive to be worth it.
A second thing for you to consider is the heating area of the radiators. Old radiators are designed for very hot water from boilers, ~180F. Heat pumps for radiant heating operate between about 95F-120F (lower is more efficient, but less responsive). Because of the lower temperature, they need a larger heating surface to deliver the same heat to the house. So, typically there’s a mix of larger European-style radiator panels and in-floor heat. In-floor heat is pretty great! A room at 65F feels like 72F when the floor is warm.
Thanks a lot for the detailed reply. Old radiators operate in high temperature but from what I understand, the fan coil heaters operate in lower temperature more equivalent to radiant floor heating.
For cost reasons, installing undefloor heating is not an option, that's why I opted for low temp radiators on top of floor. However, it sounds like the heat pump system will not be cheap either. According to BTU calculator I found online, seems like I need 28K BTUs to heat the home. I thought about pellet burners as well, but here in Europe, the cost of pellets has sky rocketed this year. I don't want to be dependent on gas either, also electricity is not cheap... So I am not sure exactly what i'll go with. I'll do the calculation, but your input on the effectiveness of the theromosiphon system is valuable, thank you.
A "variable speed compressor" is what you want in a heat pump, as these modulate their speed (and electricity usage) to produce a constant amount of heat. You might be okay with a 24K BTU system, especially when coupled with the thermosiphon and then some effort spent on insulation & air-sealing?
Here in the US the base system would be about $3K USD, but with installation costs on top of that.
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.
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.
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
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/. 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.
_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.