Jason Murphy
Selecting a heat source for your radiant heating system is easy for large projects since efficient boilers and hydronic geothermal heat pumps offer exceptional value and performance.
Problems arise when you try to select a heat source for small radiant systems of less than 1,000 square feet.
This is a problem for our customers since many radiant heating systems are indeed small. Many customers are adding radiant heating systems to new rooms, barns, and garages.
Radiant Heat Source Options
You have the following options for radiant heat sources:
- Boiler (Electric/Gas/LP)
- Tankless Hot Water Heater
- Traditional Water Heater (Electric/Gas/LP)
On-demand hot water heaters, or instant hot water heaters, are typically the first choice since they are small and most users consider these devices to be cost effective. Water heaters are often considered as an option to reduce costs. Each option has important considerations before you decide on a heat source for a radiant system.
Instant Hot Water for Radiant Systems
If you are planning to use an instant hot water heater, you need to make sure the heat loss of the radiant zone exceeds the minimum output of the water heater. For example, if your water heater modulates down to 15,000 BTUH but the 300 SF radiant zone only needs 5,000 BTUH on the coldest day, your water heater will short cycle and overheat. The solution for a small radiant zone would be to use a buffer tank or to use a traditional tank style water heater.
Water Heaters for Radiant Systems
Tank style water heaters for radiant heating systems present a much different problem. You need to make sure the water heater recovery time is sufficient to satisfy the radiant zone’s heat loss on the coldest day. Water heaters are designed to make cold water hot, and radiant systems operate with a 20 degree temperature differential, which sends warm water back to the tank. When incoming water is warm, a water heater cannot transfer heat in the needed amount of time or at the required flow rate.
Water heater recovery is measured in gallons per hour. Flow rates for a radiant system can be calculated at 1 gallon per minute for each 10,000 BTUH of heat loss. Due to recovery time, water heaters will work only with very small radiant systems. Larger radiant systems can use water heaters but it is essential to correctly calculate heat loss and select equipment based on recovery time for a 20 degree temperature differential.
Boilers for Radiant Systems
Boilers are an efficient way to make warm water hot. However, when used with a small radiant systems, the heat produced by a boiler may greatly exceeded the radiant zone requirements. Boilers such as the Argo Electric Boiler work great for small radiant systems as you can configure the 2nd heating element with a delay based on the heat loss, thus reducing risks of short cycling. Another solution to this problem is to use a deactivated water heater as a buffer tank.
There are many reasons to use a boiler for small radiant systems. Many boilers can simultaneously heat water for both radiant systems and domestic hot water. Additionally, a boiler provides the opportunity for future expansion of your radiant system.
Important Considerations
When selecting a heat source for a small radiant system, it’s important to consider the heat loss of the radiant zone, the flow rate required to satisfy the heat loss, and the correct equipment needed to make sure your system meets both current and future requirements.
If you require any assistance please contact Eagle Mountain at 1-800-572-7831 or contact us.
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Tags: boiler, instant hot water, radiant heating, tankless
I looked at your article about small radiant systems. I think you need to add a little to explain exactly how it is possible for this to be true: “It takes much longer to make warm water hot than to make cold water hot.” Especially considering that at some point cold water becomes warm.
Lacking a time warp of some kind physics does not support the statement. Perhpas you need to discuss something of differentials on two elements of an electric heater. With a gas or oil fired unit that puts out a constant BTU of heat warm water simply requires fewer BTU to become hot than cold water (and with less potential heat loss in the system.)
It’s not that it takes longer, I’ll update the blog post, It’s that it can’t transfer the heat in the needed amount of time or flow rate. Remember, the water is flowing continuously through the tank, not sitting there waiting to be used again like domestic hot water. A hot water tank is not efficient at raising the water only 20 degrees because it has only a small amount of surface area for the volume of water. Once a dhw tank’s temperature has been “dumped”, it can take a long time to have hot water again, which is what the recovery rate is. It can’t heat the water while you are using it due to the low surface area per quantity of water.