Keeping the Heat In
More and more these days, Solar Panels or other renewable energies such as Heat Pumps are being combined with solid-fuel and back-boiler systems. Either as a new build, or as part of a retrofit. It wouldn’t be unusual to have a system combining solar, an oil boiler and a solid fuel stove into the one tank – one like in the diagram below:
Both the solar and the boiler circuit are pumped circuits – water is pushed around them by a circulating pump. The stove circuit is different; there’s no pump. For safety reasons it uses a gravity flow to heat the tank. The temperature difference between a hot stove and a cooler tank creates a natural circulation of water between the two. This is sometimes called a Thermosiphon.
This, however, can cause some unusual problems.
The Cold Stove Problem.
On good days the solar panels will heat the bottom of the tank, warming the water inside. On other occasions, this water may be heated by the boiler. Especially if it is warm out, the stove will likely not be lit – so will stay cold. In the diagram above, its outlet temperature is a cold 20°C.
The temperature difference between the heated water in the tank and the unlit and still cold stove will be high enough that circulation between the two begins. This is the same Thermosiphon effect, but operating in the reverse direction. The cold stove will begin to leach heat out of the tank, cooling it down. This heat is lost either up the flue, or into the air in the room where the stove is located
If this heat came from your solar panels, then you will have to make up for the loss with an immersion heater or by calling in the oil boiler. If it came from your boiler alone, then you will have to run your boiler for longer.
Either way this is costing you money. And nobody likes the surprise that comes from turning on what was expected to be a nice-hot shower, only to be met by a rush of lukewarm at-best water.
A Solution?
It seems like the quick and easy solution is to simply swap the solar and stove heating connections at the tank. This will mean that the stove is now heating the entire tank from the bottom. The solar will now only heat the top half of the tank. The modification would look like this:
The bottom of the tank will remain cool unless the stove is lit, meaning there will be no temperature difference to start the Thermosiphon. This solves the Thermosiphon problem, but introduced a new one.
This approach effectively halves the amount of hot water available from the solar panels. Whatever would’ve been lost to the Thermosiphon, is never received in the first place. Instead of losing hot water, now you never have it. Also, if this is a modification to an existing system, you would have to factor in the cost of hiring a plumber to do the work, or the time to do it yourself. Three to Five hours would be a good minimum for a skilled plumber.
Clearly, not an ideal solution. It solved one problem, but in a way creates another one in the process.
The Greentherm Way.
At Greentherm, we’re an engineering company first and foremost. We’re all engineers here. What this means is, we’re always looking for the best solution to a problem. Both in terms of cost-effectiveness, and long-term efficiency.
A customer came to us with a Thermosiphon problem on their system. This was the solution we offered:
A Solid Fuel Loading Valve
A Solid Fuel Loading Valve is a device which is designed to automatically regulate the flow of water coming out of a solid-fuel boiler or stove. The purpose of the loading valve is to allow the stove to heat up quicker and burn hotter, reducing coking of the flue and minimising internal corrosion. It does this by diverting the flow of water coming from the stove back towards the stove in a short loop, bypassing the rest of the heating system.
This diagram shows how the valve operates. When cold, the valve switches to divert, recirculating water back to the stove rather than through the tank:
When it warms up, the valve switches open, allowing water to flow to the tank, as seen below.
But what relevance is this to our Thermosiphon problem?
When the solid fuel stove in the above system is cold, the Loading Valve will be closed. It will effectively isolate the stove in its own short loop until the stove heats up. This means that cold water in the stove cannot flow towards the tank. This also means that warm water from the tank cannot flow back to a cold stove.
Therefore, the modification we ultimately made to the original system above, looked like the following:
If the stove is cold, the valve is switched to divert. When the stove is lit the valve will open once the water has reached a high enough temperature, allowing heat to move up from the stove to the tank. If the stove cools down, the valve closes, preventing circulation through the tank.
It’s that simple.
In a nutshell
Fitting a solid fuel loading valve to the system eliminated heat losses from a Thermosiphon. The efficiency of the system was improved. The Loading Valve also improved the combustion efficiency and lifespan of the stove by fulfilling its original function.
Physically adding the valve and divert loop took a competent plumber two hours. Including the cost of materials, this solution not only worked out cheaper for the customer to implement, but left her with a much more efficient system to boot.
Most importantly, this solution is safe*. The Solid Fuel Loading Valve is fully automatic. It isn’t reliant on electrical power or any external control unit- it’s operated solely by the heat in the water. This valve won’t be left stuck in position by a power failure, or quietly forgotten when the homeowner goes to sleep.
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