My Very Nerdy Experiment Heating with Solar via a Buffer Tank

When it's sunny, even during the dark days of November-January, each of our buildings can recharge its batteries to 100% by about 10 AM if we haven't used up most of their energy over several days with little sun, which doesn’t happen very often. We have not needed to use a generator at all since we go our PV panels fully up and running, and once the batteries are full, which happens most days, we run out of ways to use all the power we can produce. All that solar energy falling from the sky for free is just wasted! I wanted to find some way to use it, and heating our buildings seemed like the best idea.

We have two main buildings on the property: Our “main house”, which looks like a 36’x44’ barn, has a large garage, entry foyer, full bathroom, and mechanical room on the first floor, with our living space above that. The other “main” building is my artist wife’s 34’x24’ studio, which also serves as our guest house. We use heat pumps in these buildings, and for our second floor living space they’re perfect all year, for both heating and cooling. But for the first floor of the “barn”, and in the studio, we also have in-slab radiant hydronic heat, powered by a propane boiler in each building. We like this heat because it’s silent, clean, and feels great, with no drafts. It also dries up the garage floor quickly, which is all but essential in a Maine winter.

 The large mass of concrete that is heated by these systems causes the temperature in these buildings to change slowly. It can take a day or two to get things warm when you first turn on the heat each year, and once warm, the slab will continue to give off heat for many hours – sometimes a day or more after the boiler is no longer needed, without overheating the space. The concrete acts like a large “heat battery” so you normally set the thermostat for this type of heat to a comfortable temperature and don’t touch it again, all winter. This slow reaction time also means that you can pump more heat than is required into the slab, yet the room doesn’t tend to overheat easily. Instead, it can retain much of that additional heat and just release it as needed to keep the room at a temperature close to the thermostat set point. This battery-like property of the concrete slab allows heat to be pumped into the floor on a sunny day and then released over many hours as it’s needed. I just needed a way to convert our excess electricity into heat that could be pumped into that concrete slab heat battery. Enter the buffer tank…

Buffer tanks are nothing more than a tank of water that sits between the boiler and the PEX tubing in the floor. Instead of heating the water in the tubes directly through the boiler (which is how most hydronic heating systems work), you use the boiler to heat the water in the tank, and then you pump that water from the tank, through the tubes in the floor. Buffer tanks are normally used in situations where you have one or more small heating zones that you want to control separately from larger heating zones. They allow the boiler, which has been sized to heat the entire building, to heat up the water in the tank, and then the heating system pulls water from the buffer tank instead of directly from the boiler. This keeps the smaller zones from getting too much heat all at once (which is bad for the concrete), while also preventing “short cycling” the boiler, which would happen if the large boiler came on for just a very few minutes whenever the small zone needed only a certain amount of heat. Short cycling is both inefficient and bad for the longevity of the boiler, so buffer tanks serve as great ways to balance everything out in a hydronic heating system. But buffer tanks can also be equipped with electric heating elements inside them similar to those found in a traditional domestic hot water tank. These are often used in systems that might struggle to keep up with the demand for large volumes of hot water, such as those used in air-to-water heat pumps operating on a very cold day. I decided I could use those electric heating elements to convert our excess solar energy into heat that could in turn be transferred into the large thermal mass of our buildings. So that’s what I decided to do!

I installed identical 26-gallon buffer tanks in each building’s hydronic heating system. Each tank has 4 ports on it:

·      Port 1, near top of the tank: Receives hot water from the boiler

·      Port 2, near bottom of the tank, same side as port 1: Cooler water from the tank exits and goes back to the boiler.

·      Port 3, near top of the tank, opposite side of port 1: Hot water from the tank goes to the heating system manifold where all the PEX tubes come together

·      Port 4, near bottom of tank, same side as port 3: Cooled water from the manifold returns to the tank.

This means we effectively have two loops operating through the tank: One that cycles water through the boiler whenever the pump for that “primary” loop is activated, and another that cycles water through the tubes that make up the heating system, or “secondary” loop, whenever the pump for that loop is activated. In hydronic heating systems that use a buffer tank, the primary loop is the one that contains the heat source (the boiler), while the secondary loop is the one that uses the heat. This 4-port configuration is the most common configuration for a buffer tank in a heating system, and it works well. It’s just one big tank of water that tends to stratify based on temperature, with hot water at the top, and cooler water at the bottom.

Each of these buffer tanks also have two 3KW electric heaters in them, one near the top, and another near the bottom, almost identical to the 4.5KW electric heaters found in traditional electric hot water tanks, except that in a traditional hot water heater these elements don’t turn on at the same time, while in a buffer tank, they’re on simultanesously unless you take steps to prevent that. We’ll talk about those heating elements in a moment, but for now just know that unless electricity is being supplied to them, they just sit there, doing nothing, and that’s just fine because unless there is an excess of solar electricity that can be used to power them, the hot water in the tank will be supplied by the propane boiler.

When the system is operating in “normal” mode, which occurs whenever we don’t have an excess of solar electricity, the propane boiler heats the water in the buffer tank up to about 125F before it shuts off. But hydronic radiant heat in a well-designed concrete slab that’s heating a well-insulated building doesn’t need to be very hot. Such a slab needs water that’s only about 90F to keep the building at around 68-70F, even when outside temperatures are hovering near 0F. To keep the water going into the floors near that 90F temperature all the time, we installed a mixing valve in each system. Regardless of the temperature of the hot water coming out of the buffer tank, the mixing valve keeps the water going into the floor at about 90F by mixing the hot water from the tank with the cooler water returning from the floor loops, with any cooler water not being used just going back into the tank. This mixing valve allows us to keep the buffer tank at almost any temperature between 90F up to the safe limit of the tank, which for our tanks is about 165F.

To make the buffer tank work with the propane boiler work I installed an aquastat on the tank. An aquastat is basically just a thermostat that has a temperature probe that you insert into a designated tube in the buffer tank. That probe measures the temperature of the water in the tank (at about the vertical midpoint of the tank in our case, which is a common configuration). When the temperature of the water in the tank reaches the upper limit you set on the aquastat, the aquastat opens or closes a dry contact circuit depending on whether you have the aquastat set to operate as normally open or normally closed. When the temperature falls to a different preset temperature, the aquastat does the opposite (open or close) to the dry contact in the circuit, and the pump controlled by that aquastat circuit turns on, firing up the boiler. This means that the boiler has no direct connection to the thermostats in the heating system. Its only job is to keep the buffer tank water hot, and it doesn’t even “know” that a heating system exists on the other side of that buffer tank.

When the thermostat in any zone calls for heat, little manifold valves for that zone open up and the thermostat turns on the secondary loop pump. These thermostats also have no direct connection to the boiler, as their only job is to tell the pump on the secondary loop to turn on, and to tell the zone vales to open. When that happens, water from the buffer tank flows through the mixing valve out to the zone or zones that need heat.

So, where does the solar-generated heat come into play?

Well, I forgot to mention that I am a home automation hobbyist. I use software called Home Assistant to automate a variety of things on our property, and one of those is a high amperage relay that controls the heating elements of the buffer tank. The tank circuit is turned on only when Home Assistant sees that our battery banks have recharged up to a predetermined level (90%), yet we still have the ability to produce enough power for the tank heating elements, the other loads required by the house at that time, and continued recharging of the batteries. When that happens, power is supplied to the tank but the heaters themselves are not yet turned on.

I should also note that the aquastat I installed is a two-stage device. So, instead of having just a single stage that turns the primary (boiler) loop pump on and off, a separate stage circuit in the aquastat can be programmed to turn something on and off within a different temperature range. I set this separate stage to turn on the electric heaters in the tank through a high amperage relay whenever the tank temperature falls to 130F, and then turn those heating elements off at 140F.

So, on a sunny day, the water in the buffer tank stays in the 130-140F range, heated by the electric elements in the tank. The mixing valve can still keep the water going into the floor at about 90F because the valve just allows less of this very hot water to be used, and more of the cooler water returning from the floor tubes. The other stage of the aquastat – the one that controls the primary loop pump that makes the propane boiler come on – won’t turn on until the water in the buffer tank drops to 95F, and then the boiler turns off when the water in the tank reaches 125F. Whenever the electric heaters are activated on a sunny day, and assuming they can keep up with the demand for heat (which they can do with ease when both elements are operating), the water in the tank stays well above 95F and the boiler never needs to turn on.

It is also important to keep in mind the “battery” effect of the large thermal mass of concrete, discussed earlier. By setting the thermostats just a degree or two above the “set it and forget it’ setpoint we would normally use, I force the heating system to keep calling for heat, which keeps the secondary loop pump on and the electric heating elements in the buffer tank turned on. That small difference isn’t enough to be noticed in the actual living space so we don’t feel too warm, but it forces more and more heat into that large thermal mass of concrete. When the sun starts to go down, Home Assistant turns back the thermostats to their normal setting. That deactivates the relay that turns on the secondary loop pump and the relay that powers the buffer tank heaters. As an extra precaution, another relay turns off the power to the tank itself, making it impossible for the electric heaters to turn on. The heat stored up in the thermal mass of the slab while the sun was shining continues to be given off very slowly, so the systems won’t call for heat for several hours – sometimes several days – beyond the point where they would otherwise be calling for heat that the propane boiler would need to supply.

Does it work? I’m happy to say that it does. The PV system for the house was originally designed to produce a maximum of 13.6 KW at any given time, and for the year before I installed this buffer tank system it produced enough power to keep our home operational without a single minute of generator use. But it would not have produced enough power in the darkest months of the year to allow for that level of self-sufficiency and frequent use of solar for heating, so I installed another 6.4 KW of PV panels in late 2025, before the federal tax credits expired at the end of the year. The system can now easily produce enough power for the home while also supplying all the heat we can use, on sunny days. On not so sunny days, the propane boiler operates just as it always did. We just need it less often.

The studio has a 6.7 KW PV system. Unfortunately, that’s not quite enough to power both 3 KW heating elements in the buffer tank while still supplying enough power for the building’s loads and full battery charging, so I disabled one of those two elements. That one element can keep the water above 95F most of the time, but I notice the boiler does sometimes run once or twice per day for about 10 minutes each time. If the federal tax credits were being extended for another year, I would solve this by adding more panels, as the roof of the studio has space for more panels. But absent that, I’ll likely address this by installing another relay to control that second heating element. It will be needed only a few times each day because it will come on only when the water temperature falls to near the 95F point that would activate the propane boiler. By throttling that second heating element on and off as needed I’m confident I can have this system operate as efficiently as the system in the house.

Some folks have noted that I could also produce heat by simply having the heat pumps in these buildings come on whenever it’s sunny. That’s true, but once you’ve experienced the comfort of a heated floor on a cold day, other heating systems don’t quite measure up when you’re walking around on a concrete slab (even one covered by “finish” flooring). We also like the drying power of the heated slab in the garage. So, while this system was more expensive and complicated than our mini splits, it allows us to enjoy our heated floors, for free. We’ll use the mini splits for heat in shoulder season months, and for cooling when the weather turns warm.

Others have noted that I should just install an air-to-water heat pump, as I would then be able to power the radiant system with electricity almost all the time, even when there is little or no sun. This is largely true, but in the US those systems are not yet common, very expensive, and there are few technicians familiar with them in our rural area. I hope to use such a system one day, but for now, the expense is just too great to justify the cost. For now, this little experiment is helping to reduce our need for propane or any other fossil fuel, heating our main living areas with free energy that falls from the sky. It’s pretty darn hard to beat that!