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Controlling Water Temperatures in PEX Radiant Heat Systems
There are many ways to control the water temperature in a radiant heat application.
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- The simplest and cheapest way is with a water heater
Also - the least efficent way to heat your house - by far!
Domestic water heater output temperatures
work very well with radiant heat. Some water heaters even come with a set of heating taps
on the side as well as the top taps for domestic hot water. In some areas using the
water heater for your domestic hot water and heating is not allowed (check with your local
authority). You can always use a separate water heater for your heat source. When using a water
heater you only need a bronze circulator, a flow check, automatic air remover, a manifold
(a manifold is used to connect PEX to piping and is also called a header), a relay, and a
thermostat. PEX Tubing with no oxygen barrier can also be used to keep the cost down.
This simple system uses your house water pressure and your houses water expansion tank
(water when heated expands and needs a place to go). The PEX
Tube must be approved for potable water.
- A better way to use a water heater
The best way to use a water heater for radiant floor heating is treat it as a boiler and
set it up as one. First you isolate it from your domestic water with a back flow
protector.
This is a one way valve that keeps the water in your heating system from mixing with your
domestic water. Back flow protectors are a must if you install anti-freeze
in your heating
system. We recommend installing anti-freeze as frozen water in PEX Tubing can
shatter a concrete floor.
Next you install a pressure regulator to maintain a lower pressure within
your heating system. This makes it easier on all the valves, gaskets, connections, etcetera, in
the system. Now that you have isolated your heating system from your domestic water supply
you need to provide for water expansion by installing an expansion tank. You need some type
of air removal system. Some installers put in a micro bubble air remover but I have installed
this type of system with a simple high vent automatic air remover above the hot water tank
and have encountered no problems. We recommend installing PEX with an oxygen barrier because
even though water heaters are made to withstand corrosion I have replaced many over the years
used for domestic hot water from rusting out. PEX Tubing without an oxygen barrier lets fresh
oxygen permeate into the water and this oxygen corrodes any ferrous metals in a heating system.
Using PEX with an oxygen barrier allows for any type of heat source to be used in the future.
OK, back to water heaters. Any type of tank type water heater can be used - gas, oil and even electric.
What must be kept in mind though is matching the BTU output to your heating needs. You
must know the heat loss of the area being heated and have a water heater having a output
that is equal to
or greater than your needs. A simple rule of thumb is that a radiant floor can comfortably
deliver 25 BTU per square foot of floor space. If you try to get more than this the floor
gets too hot. If the heating load is more than the radiant floor can handle
another heating device must be installed to make up the difference. This can be done simply with
some baseboard heat installed on the water heater also.
- A better way to use a mixing valve
Mixing valves can be used for controlling the water temperature in your radiant floor
when the heat source temperature is greater than the radiant floor needs.
This is accomplished by mixing some of the radiant floor return water with the radiant heat floor
supply water. With mixing valves you push the water through the PEX radiant heat tubing.
By attaching the suction side of the circulator to the mixing valve the mixing valve
adjusts the water temperature by either drawing off high temperature
water (from a boiler or
other heat source) or the return water (from the radiant floor) and mixing them to
provide a constant temperature for the PEX tubing in your radiant floor.
You can make your own mixing valves using ball valves.
This is accomplished by mixing some of the return water with the supply water before
the circulator pushes the heated water through the radiant PEX floor tubing.
There are however commercially available mixing valves designed for radiant heat
that can be purchased through HouseNeeds.com.
These valves supply the radiant floor with a more constant heat and are much
easier to adjust.
Using an indoor/outdoor resetting mixing valve to control the water temperature
that is supplied to your radiant PEX tubing the resetting mixing valve will
constantly adjust the radiant
floor water temperature to the outside temperature and avoid over heating of your house
during large outdoor temperature swings. This happens when the thermal storage mass
(your floor) is heated to provide enough heat for its coldest design
temperature on a cool night and then can not stop releasing the stored heat during a warm day.
This means if you have a radiant tile floor
in your kitchen, a wood radiant floor in the living room, a carpeted radiant floor in
the den and baseboard heat in your bedrooms you need an indoor/outdoor resetting mixing valve to
provide the different water temperatures each radiant heat floor system needs.
We liked blend tanks the best. Blend tanks blend the water being supplied to and returned
from the radiant floor PEX tubing and the water being supplied from and returned
to your boiler. Modern-day boilers contain very little water and require
cooled return water supplied to them to avoid short cycling. Short cycling is when
the boiler heats up to its high limit very quickly and is constantly
turning itself on and off. Mixing valves usually draw the water from the boiler slowly.
This can make a low mass boiler short cycle. Most modern-day gas boilers are low mass
and most modern-day oil boilers are high mass.
Blend tanks have many advantages. They can except heated water from any heat source
or multiple heat sources and still provide the radiant floor with constant
temperatures.
Using an indoor/outdoor resetting aquastat to control the water temperature
that is supplied to your radiant PEX tubing the blend tank will constantly adjust the radiant
floor water temperature to the outside temperature and avoid over heating of your house
during large out door temperature swings. This happens when the thermal storage mass
(your floor) is heated to provide enough heat for its coldest design
temperature on a cool night and then can not stop releasing the stored heat during a warm day.
Another advantage of using a blend tank is it can provide multiple water temperatures
necessary for different radiant heat floor types. This means if you have a radiant tile floor
in your kitchen, a wood radiant heat floor in the living room, a carpeted radiant floor in
the den and baseboard heat in your bedrooms it can provide the different water
temperatures each radiant floor system needs. This accomplished by mixing some of
the return water with the supply water before the circulator pushes the heated water
through the radiant PEX floor tubing. Using ball valves to accomplish this and a
outdoor/indoor reset, each radiant floor system will get the temperature it needs
in relationship to the outdoor temperature.
Injection mixing is accomplished by constantly circulating the water
through the radiant floor and injecting heated water into the loops.
This is done with a variable speed injection pump.
It is quite complicated and needs a large knowledge of hydronics heating.
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