It doesn’t matter whether you’ve finished that last joint in a new installation or it's 6:39 p.m. on a Friday and your customer is ready to put the heat on. You need to get the system full of water and move on to the next job or go home. Even better, you want to fill it up and not encounter a single leak! In either case, there are several ways to complete the task of filling hydronic systems.

Let’s touch on ways you’re familiar with and some tips that may get you out of a bind when the way “you’ve always done it” isn’t going to cut it.

The most typical method for filling residential and light commercial hydronic systems is with a pressure-reducing valve (fill valve) connected to a pressurized potable water feed (see Figure 1). The valve will be set to maintain your desired static pressure. If pressure is lost in the system, the fill valve will automatically allow fluid to pass and bring the pressure back up to the desired setting.

When filling a system with this setup, you’ll typically have some type of shut-off valve and drain combination in your piping to allow the fill valve to push water through your system and out the drain. The careful placement and use of the shut-off valve/drain ensure that the water is going where you want it. During this procedure, you’ll also purge any air and flush debris out of the system. 

Same old song and dance with a twist or two

If you do plan on using a typical automatic fill valve, my first tip is to pipe the system in a fashion so you can use a supplemental pump to fill and purge your systems. Why would you want to do this? It’s only two drains with a shut-off valve in between, but it has the potential to make your system so much friendlier to fill and service (see Figure 2). With the shut-off valve closed, you can push in one side and out the other using your supplemental pump.

Debris inside your hydronic systems can require in excess of 5 feet/second velocities to get moving. There is a good chance your fill valve has enough flow to produce the velocity needed to push air bubbles around and out of your system, but the chances of it pushing around debris are a lot slimmer. If this can’t be accomplished with your fill valve, it’s unlikely your system circulators will accomplish the task as they’re sized for heat transfer and not pipe scouring. This is where that supplemental pump will come in handy!

A good fill-and-purge setup allows you the option to forgo the fill valve entirely. This may sound like heresy to some as it’s usually on every job, but fill valves are not needed all the time. There are also scenarios where I would strongly discourage the use of a fill valve and potable water in hydronic systems. 

Potable water can be consumed by your customers, but it doesn’t mean it’s fit for your boiler. Be sure to check your manuals for water-quality requirements. Treating site water or bringing your own may be the best option to safeguard your work and your customer’s investment. 

Another scenario is the use of glycol. Having an automatic fill valve connected to potable water on a glycol system is plain reckless. In the case of a leak, the automatic fill valve will surely dilute your mixture and compromise your freeze protection.

You can use a utility pump or flush cart to push your preferred fluid into the system with only a fill-and-purge configuration. You also have the potential to really push air and dirt around and out of your systems. Trust me, a flush cart is a great tool to keep in your arsenal. For systems slightly larger than your typical residential job, a flush cart designed for geothermal systems can be used. These can often be rented from wholesalers.

Filling with a flush cart is all well and good, but without a typical automatic fill valve, how do I maintain static fill pressure? Some say you don’t need a fill valve on a system that doesn’t leak. It is a true statement, and I agree with it to some extent, but there are always those times when you intentionally create a leak. 

This could be during the blowdown of a strainer or when your air separator is doing its job and letting air out. Both of these cases will cause a loss of volume and pressure in the system that must be made up.

System feeders are typically used to maintain static pressure in those systems without a fill valve and potable connection. These devices store a limited amount of fluid and incorporate a pump and pressure-reducing valve. You’ll see them (thankfully!) in many glycol systems. They also can be used in systems without glycol. 

In jurisdictions where connecting your water supply to your boiler requires a testable backflow preventer, a system feeder is a good way to isolate your boiler from your potable. It also means you can pass on the testable backflow preventer and the costly annual inspections.

Cutting the potable cord, defying convention

Some years ago, I was enlightened by my colleague, Bob “Hotrod” Rohr, about a way to make a system feeder using common hydronic components that I thought was spectacular. With some fittings, two gauges, a fill valve and an expansion tank, you could accomplish a system feeder without a pump. Instead, you use the air side of the expansion tank to “pump” in the fluid if makeup water is needed. 

On these DIY system feeders (Figures 3 and 4), the outlet of the fill valve connects to the system like normal; you need a drain to refill the wet side of your expansion tank. If you go this route, you’ll want to make sure that the air side of the tank is set higher than your desired static pressure on the automatic fill valve. Also, make sure you don’t exceed the max acceptance volume of the tank with whatever fluid you’re putting in there. 

Pretty awesome for a low-tech system feeder, right?

These feeders not only allow you to isolate your system from the potable water supply, but they also can save a lot of grief if you spring a leak. With an endless supply of water behind it, an automatic fill valve can wreak a lot of havoc with a system leak. I’ve seen it; it’s definitely something you want to avoid. 

System feeders will maintain pressure, but once you’ve used up the stored fluid, the waterworks are over. This may not bode well for those in the water damage restoration industry, but they really save the day in conjunction with a low-water cut-off.

So, when you’re trying to finish up at that last circus sideshow on a Friday night, remember there are many ways to get the job done! Fill ‘er up!