Hydronic Heating System Design

Jay designed the hydronic heating system following the principles in the book Modern Hydronic Heating For Residential and Light Commercial Buildings. The design of this system is relatively straightforward, and similar to that of many in-floor heating systems with supplemental solar heat. It is sized smaller than would be typical for a house of this size because our house has much more insulation and better windows, and thus a much smaller heating load than a typical house. Click here to see more pictures of the system.


The following shows the schematic design of the hydronic plumbing, and a photograph of the actual system.

Heat Sources

When water enters the heat exchanger shown at the top of the diagram, it first travels through copper tubing to the bottom of the heat storage tank. From there the copper tubing spirals upward to the top of the tank where the water is warmest and then returns through the pipe on the right-hand side of the diagram. It then flows through the Ecosmart Tankless Water Heater. This is an on-demand water heater that will raise the temperature of the water to 80 degrees F. If the water is already at or above this temperature when it exits the heat storage tank, the heater does not switch on and we heat entirely with solar energy. We hope that will be the case most of the time, but the heater is necessary for cold cloudy weather when we can't collect enough solar energy to keep the tank warm. Even in those times we can keep from using electricity for heat by using the wood stoves, so the electric heater is there just as a backup and to meet building code requirements. Our heater provides only 6400 watts, which is about 22,000 BTU per hour. This is equivalent to the heat output of a very small furnace, but it is sufficient to keep the house warm even in cold, cloudy conditions because the house is so well insulated. This small heater cost only $250 versus several thousand for a high-efficiency furnace or geothermal heat pump, an example of how spending more money on insulation saved us money on the heating system.

Mixing Valve

The Mixing Valve is used to prevent extremely hot water from entering the floors when the heat storage tank is hot. We want to supply water to the floors at about 80 degrees F, but if the storage tank is at its maximum of 140 degrees then the water returning from the heat exchanger could be much too hot. This valve automatically adjusts the mix of hot water and cool return water that enters the pumps, to provide a relatively constant supply temperature that can be adjusted by turning the knob atop the valve. We can adjust the valve to the desired temperature using the temperature gage installed between the valve and the pumps.


The two Recirculator Pumps run on 120 volts A.C. and consume 60 to 90 watts each depending on their speed setting (each pump has a 3-speed selector switch). Each pump is controlled by a thermostat, with one thermostat in the cottage and another in the main house, but typical home thermostats are designed to switch only 24 volts A.C. at low current so they can't control the pumps directly. The green box is a Switching Relay that enables the thermostats to control the pumps.  Each pump has a built-in check valve that prevents water from flowing backwards through it when it is off. This is essential so that when only one zone calls for heat we don't get cool water flowing back through the other pump, because it would allow some water to circulate uselessly without passing through the rest of the system.


The supply and return Manifolds are designed to distribute the water flow through the six loops of in-floor tubing. Warm water enters the supply manifolds on the top, and cooler water returns through the manifolds on the bottom. There are two loops for the cottage and three for the main house, plus a small loop for the entryway that is fed from the main house manifold. We'll cut down the flow rate on that loop in order to keep the entryway somewhat cooler than the main house.

Air Eliminator

The Air Eliminator is a microbubble resorber, which is designed to extract even microscopic bubbles from the water as it flows through. The air collects at the top of the air eliminator where it activates a float valve that automatically releases the air from the system. The Expansion Tank below it has a diaphragm inside, and is pre-pressurized to about 12 PSI. This is similar to the pressure tank found on most domestic water wells, but much smaller with a capacity of 2.2 gallons in this case. If we didn't have an expansion tank, even a small change in the temperature of the water could cause a large change in the pressure of the system. The expansion tank keeps the system pressure relatively constant.


It is necessary to keep the water under pressure, because otherwise the pumps will experience cavitation. This can happen as the pump's impeller moves through the water and causes a pressure drop on the trailing edge of the impeller, much like the wing of an airplane. The boiling temperature of water gets lower as the pressure drops, and if the pressure drops enough the water can literally boil and create tiny bubbles of water vapor along the trailing edge of the impeller. As they move out of the low-pressure area the bubbles change back to a liquid, and they do this so suddenly that a tiny shock wave is created. These shock waves, although tiny, are intense enough to erode the surface of the impeller over time. That's why cavitation is very bad, and the simple solution is to keep the water under enough pressure to prevent the pumps from cavitating. How much pressure depends on the temperature, and at the low 80-degree operating temperature of our system it only requires about 2 PSI according to the pump manufacturer. We pressurized our system to 15 PSI just to be on the safe side. The Pressure Relief Valve next to the heater is set at 30 PSI so we have plenty of margin in case the pressure increases a bit as the system warms up.

In-Floor Tubing

The heat tubing in our radiant floors is all 5/8" PEX (cross-linked polyethylene) Pipe. Most systems use 1/2" diameter tubing, but going to 5/8" enabled us to use smaller (lower wattage) pumps to get the necessary flow rate. If the flow rate is too low, or if a given loop is too long, the water gets cold before it reaches the end of the loop resulting in uneven heating. The larger diameter tubing also has 25% more area over which it exchanges heat, which enabled us to use less tubing than if we had used 1/2". Most of the tubing is spaced 16" apart in the floors, which is much farther apart than in typical systems because the house is designed to have a much lower heating load per square foot than a typical house. This is another example where spending more money on insulation saves us money on the heating system. In all we used about 2000' of tubing for all the floors, including some for the workshop floor which will not be heated with this system but could be used to dump surplus solar heat in the spring and fall.

The tubing we used has an oxygen barrier, which keeps oxygen from diffusing into the water through the plastic tube wall. This prevents cast iron components such as the pump housings from corroding, which they would do if there were dissolved oxygen in the water.

This page was updated on Tuesday March 18, 2014