Hydronic Stove Video Script
The main house heating system in the Seed Eco Home is the Hydronic Stove - a wood burning stove with a water pipe heat exchanger. The hot water is circulated in PEX pipes under the floor, providing comfortable heat - no cold feet.
The main parts of the heating system are the stove itself - the heat exchanger on the top part of the stove - the chimney for exhaust gases - the PEX tubing under the floor, and the control panel. Here's how it works.
The stove generates heat. The heat exchanger - or more specifically - 16 sections of 1" black pipe - captures the heat and transfers that heat to water that is circulating through the pipes. The water then circulates the heat through the house. There are 4 coils of tubing, each 300 feet long, under the main floor. We are using 1/2" PEX.
For the heat exchanger, we are using pipe elbows to make the connections. When you do that, use thread tape and pipe dope both - the pipe dope makes it much easier to screw the fittings on to avoid leaks. We did not use pipe dope. I had many leaks, which I sealed with JB Weld high temperature epoxy by pasting over the leaking joints. The joints are outside of the stove. The reason is ease of build - and access for serviceability. Because you don't know if you will have leaks until you actually run the system at temperature. As the joints are outside the stove, they don't get hotter than the water temperature. To seal the air holes around the pipes, we welded on a collar and stuffed it with rock wool insulation. Rock wool, unlike fiberglass insulation, can take the heat of a stove. The heat exchanger has a total of 16 square feet of area, or an estimated 200,000 BTU.
We lined the inside of the stove with CEBs - and also added a heat shield panel for the sides of the stove next to the wall. The heat shield consists of a frame of 1.5" steel tube, stuffed with rock wool insulation, and the sides are made of aluminum flashing.
The stove door is hinged and closes with a latch. A circular opening on the door allows either a blower - to get more heat - or a pellet burner attachment that shoots pellet heat into the stove. As such, this stove can run either on pellets or firewood. We plan on installing an open source pellet burner before winter ends. The pellet burner is more efficient, and can last days as opposed to loading a wood stove a few times per day. Underneath the door is an ash tray, which comes right out, with a handle on both ends for easy carrying.
The pumping and control of the system is done through the open source Hydronic Control Panel. It includes the pump, water lines to the house, and other control elements. Our design has the pump on the return side of the control panel, which pumps water through the stove, then to the forward side of the control panel - then through the house - looping back to the return side.
To get the stove running for the first time, you fill the system with water from a water supply line and turn on the pump. A fill valve is very useful - it allows you to reduce the pressure - typically 12 PSI for normal operation. This is important because it helps the system stay within pressure not tripping the relief. In our case, the over-pressure relief would activate easily when we did not use the fill valve - but that may have been due to air trapped in the system.
During initial startup, you have to prime the system - meaning get all the air out so that the pump can work properly. The fill valve appears to be indispensible for this. When you lift the lever, the fill valve allows you to fill the system at the full pressure of the supply line (44 PSI in our case) - which allows you to purge the system of air much more easily. In our case, it was not possible for us to get all the air out until we added the fill valve - at which point all the little air bubble noises stopped in about 5 minutes of priming - and the pump went into silent operation. Once the system is full, we release the lever and the system pressure can drop back down to 12 PSI - but we need to let some water out. And we release the air - for which we have an air purge valve at the very top of the forward line.
Next is the Expansion Tank. Upon heating, water expands - about 1% total until it reaches operating temperatures (about 1/10000 per degree rise [1]. This means that the water needs to go somewhere. The expansion tank is used to absorb that extra volume of water. If there is a total of about 10 gallons in the PEX tubing and the heat exchanger - that means about about a pint or half liter of expansion volume.
Another important feature is a pressure-temperature gauge. You want to verify that you are at 12 PSI - and you want to read the temperature on the hot forward side and the cold return side so you can see how much heat you are feeding into your house. Here the return line temperature gauge is missing.
A critical safety feature of the system is the pressure-temperature relief valve, which opens if the system temperature gets too high. Remember that we are dealing with water and heat - and water can boil. The stories of hot water heaters taking off like a rocket appear to be real - upon the condition of a failed pressure relief and large pressure buildup. In our case here, this is physically impossible - even if the relief valve fails. In our system, the plastic PEX would burst before any significant pressure buildup could occur. PEX is rated for 80 PSI at 200F, and less when hotter. For code compliance, it is required that you can see the outlet of your pressure relief valve so that you can observe if there are leaks. Leaks would indicate a broken pressure relief, which should be replaced.
The 4 lines going to the house are here and the 4 return lines are there.
That is the heart of the heat delivery - and everything else in the system is there to support the delivery of hot water through the pipes under the floor.
In this system, there are more unused channels. First, we will run 2 more lines out of this system to heat the biodigester. This means we will have to shift the expansion tank to the extra outlets on the forward line. Second, we can add the thermoelectric generator and a hot water storage tank to the large exits on the forward line. This would allow us to generate electricity from the hot water side, where we would have to add a cooling line from the aquaponic pond to the thermoelectric generator. The same circuit could also be used for adding a hot water storage tank for household hot water.
You can say that we have over-succeeded with the heat exchanger - because we can't put too much wood into the stove - even without a blower - or otherwise, the pressure relief triggers from too much heat. This should be resolved once we add pond heating, biodigester heating, and potentially add more heat lines - we simply have way too much heat for what we are using right now.
A big lesson for us is that we should have also put sand under the floor to retain heat longer. In MicroHouse 3 - one of our former builds - we put in about 5 cubic yards of sand around the hydronic tubes. This gets us about a day of comfortable heat after the stove dies out. Righ now, in the seed eco-home - it's 66 F indoors, and it's 5F outside. [2]. Once the heat goes out, it gets cold in about 3 hours on a freezing day like today.
Future additions to the hydronic stove will include a pellet burner, as shown in the Seed Eco-Home Infographic. A pellet burner is more efficient - and it allows for automated operation - so the fire won't burn out on you in the middle of the night on a very cold day like today. If you have a big hopper full of pellets - you may not need to load the stove several times per day - but instead - you can go for days or even weeks - depending on the size of your wood pellet hopper.
