Ever needed to get a hydronic heating system up and running before the building it serves has domestic water service? Perhaps there've been times you've hesitated to fill a sparkling new system with cloudy water from the site's newly drilled well. Or maybe you've pondered the best way to add antifreeze to a system. These are all situations in which a "purge cart" can be a valuable tool.

I built my purge cart out of necessity in 1987. I needed it to fill, purge and add antifreeze to the buried tubing loop supplying a newly installed geothermal heat pump system. The earth loop consisted of two parallel circuits of 1-inch polyethylene pipe, each 500 feet long. The pressure and flow available from my well pump just wasn't up to ridding the piping loops of entrapped air.

At that time, purge carts were commercially available for filling and purging geothermal piping loops. Typical units cost about $1,600 - a bit pricey for my needs and budget at the time. So I did what many of you have probably done while looking for mechanical inspiration, I began leafing through my GraingerR catalog.

It didn't take long before I spotted several models of swimming pool filter pumps. The 1-hp model packed quite a wallop - over 60 gpm at 30 feet of head. I decided this would be the heart of a homemade purge cart. Other components consisted of a 30-gallon RubbermaidR trash barrel, a short 2 by 8 plank, a PVC foot valve and a few stray pieces of 1.5-inch schedule 40 PVC pipe and fittings. Although any apparatus with the word "cart" in its name should probably have a set of wheels, I skipped them to keep cost down. The resulting assembly is shown in Figure 1. The whole thing cost me around $230.

Water Sports

Like a kid anxious to launch the model rocket he just assembled, I couldn't wait to "test fly" the new purge cart. I filled the barrel with water, fastened one end of hose to the pump leaving the other end in the barrel, primed the intake piping and plugged in the cord.

At that moment, I witnessed "hose power" not seen since the last fireman's field days. The loose end of the discharge hose leapt from the barrel and, like the trunk of a deranged elephant, began blasting water in every direction. I moved for cover as the contraption proceeded to empty itself in about 10 seconds. As I pulled the plug to shut it down, I was pleased both in the power of my new toy, and that I had chosen to test it on the driveway, rather than inside the basement.

Purge cart operating rule No. 1: Always hold onto any loose hose end before turning on the power.

Incidentally, purge cart operating rule No. 2 is to always use a reinforced rubber hose on the discharge side of the pump. On one occasion, I hastily hooked up a piece of washing machine drain hose instead. The resulting "aneurysm" in the hose was the size of a football before I could yank the power cord to shut down the pump. This time I was especially fortunate I wasn't on the driveway!

Taming The Beast

After entertaining yourself with its water-blasting power, there are a number of practical uses for the purge cart. First and foremost is filling and purging your hydronic systems. On most systems, you'll need to install one or two extra valves in order to connect the purge cart as shown in Figure 2. I suggest using 1-inch ball valves fitted with barbed hose connectors for the inlet and outlet purging connections, so as not to impose a high-pressure loss that limits purging flow.

Prepare to fill and purge the system as follows:

  • Rinse out the barrel, and set it down where all hoses will reach their connections before you fill it (unless you like dragging more than 200 pounds of sloshing water around the building).

  • Set up the pump, intake line assembly and hose connections. Be sure all hose clamps are tight.

  • Fill the barrel with clean water to within 3 or 4 inches of the rim.

  • Pour water into the strainer basket of the purge pump to prime the intake piping.

  • Hold the end of the return hose below the waterline in the barrel.

  • Turn on the pump.

The water will race into the piping circuit pushing most of the air ahead of it. You should immediately see massive air bubbles belching from the return hose. Be sure to hold the end of the hose just below the surface as the water level in the barrel drops. If you're filling a large piping circuit that has a volume of more than 20 gallons, have one or more 5-gallon pails of clean water ready to dump into the barrel when the water level gets below the midpoint of the barrel. Keeping the barrel at least half-full prevents returning air bubbles from being blasted down near the foot valve and getting sucked back into the pump.

Most of the (non-dissolved) air in a typical residential zone circuit will be pushed out after a few seconds of purging. Still, it doesn't hurt to keep purging for a minute or two. If possible, turn on the circulator in the piping loop after the return stream is running mostly free of bubbles. This helps dislodge any air trapped within the rotor can. When the return flow is running clear of any air bubbles, open the next zone circuit and close off the one just purged. Add water to the barrel as necessary, and purge the remaining zones in the same fashion.

When the final zone has been purged, close the valve on the return hose while the purge pump is still running. The system pressure will quickly increase to the maximum pressure of the pump (probably about 20 to 25 psi) as extra water is pushed into the system's expansion tank. Close the inlet valve and shut off the purge pump. If the system pressure is higher than you desire, bleed some water through the outlet valve.

When you're done purging and it's time to pack up the purge cart, start by emptying the barrel. If you're purging with water, connect a garden hose to the boiler drain teed into the pump discharge. Run the hose to a drain or outside. Leave the discharge hose connected to the closed inlet valve for the time being. Turn on the purge pump and let it empty the barrel through the garden hose until the foot valve starts drawing air. At this point the barrel probably contains only 4 or 5 gallons of water, and can be carried out of the mechanical room.

Before dumping the barrel, put it on a level surface and use a permanent marker to put a line on the outside of the barrel 1 inch above the level where the foot valve started to draw air (see Figure 3). This establishes the minimum operating volume of the purge cart. Pour water into the barrel to bring the level up to this line. Now, carefully empty the barrel into a container of known volume, and record how many gallons the barrel contains when filled to this line. Write this number on the barrel for future reference. You'll need it when using the cart to add antifreeze to a system. More on this later.

Another use for the purge cart is filtering turbid water before it's pumped into the system. Hook up a cartridge filter assembly to the pump discharge hose, and route the filtered water back into the barrel through another hose. The water can be recirculated through the filter as many times as necessary. Be sure to have extra filter cartridges on hand if what you fill the barrel with looks more like diluted cocoa than clean water.

Bring On The Brine

A purge cart is also great for adding antifreeze to a system. I use the following procedure, which assumes the system is already filled with water:

Step 1. Calculate the volume of the system based on pipe size(s) and length, as well as the water content of the boiler.

Step 2. Calculate the volume of antifreeze needed using Formula 1 below:

Formula 1

Vantifreeze = p x (Vsystem + VBmin)


Vantifreeze = volume of antifreeze required (in gallons) p = percentage of antifreeze (by volume) needed for the desired freeze protection (decimal percent) Vsystem = volume of system piping from Step 1 (in gallons) VBmin = minimum operating volume of barrel discussed in previous section (in gallons)

Step 3. Shut off any make-up water system.

Step 4. Drain an amount of water from the system equal to the required volume of antifreeze calculated in Step 2. The water can be removed from a low point in the system such as the boiler drain. If necessary, use a small air compressor to coax the required water volume from the system.

Step 5. Set up the purge cart and connect the hoses as usual.

Step 6. Fill the barrel to its minimum operating level (e.g. the line on the barrel) with water, then pour the required volume of antifreeze into the barrel.

Step 7. Turn on the purge pump. The fluid level in the barrel will drop to and hold at the minimum operating level as the partially emptied piping is refilled. Do not add water to the barrel. Hold the end of the hose just under the fluid surface and away from the foot valve. Continue circulating fluid through the purge cart for at least 15 minutes to ensure a thorough mixing. After the fluid has circulated free of returning air bubbles for several minutes, close the outlet valve to pressurize the system. Then close the inlet valve and turn off the pump.

Assuming you've kept track of the fluid volumes going into and out of the system in the above procedure - what should be left in the barrel when purging is completed? Answer: The minimum operating volume of a well-mixed water/antifreeze solution the same as in the system.

One option is to pour this left over fluid into a clean polyethylene container, mark it with the name and percentage concentration of antifreeze it contains, date it and leave it in the mechanical room where it may be needed to replace fluid lost during a future service call. Another possibility is to use it on another system that requires the same antifreeze mixture.

In this case, use the left over antifreeze solution (rather than water) to fill the barrel to its minimum operating level as described in Step 6. This reduces the antifreeze needed to meet the requirements of the system. Use Formula 2 (rather than Formula 1) to calculate the amount of antifreeze needed by the system.

Formula 2

Vantifreeze = p x Vsystem


Vantifreeze = volume of antifreeze required (in gallons) p = percentage of antifreeze (by volume) needed for the desired freeze protection (decimal percent) Vsystem = volume of system piping from Step 1 (in gallons)

Finally, when you're done charging the system with antifreeze, make sure you thoroughly rinse down the barrel, hoses and pump, anything that has glycol on it. In my experience, dried up antifreeze solution runs a close second to fly paper when it comes to stickiness.

Have It Your Way

The purge cart I built back in 1987 is still ticking. It's an easy-to-build, basic design that's been used on dozens of systems, and paid for itself many times over.

After reading this perhaps you'll think of some hardware that could improve the basic design. Maybe you'll decide to build a fully-loaded version with accessories like wheels, hose racks, a built-in filtering loop - even an umbrella in case you forget to hold onto the end of the return hose. Good luck, and pleasant purging.