Most people who call us about hot water assume it means a new machine. It usually does not. If you already own a decent cold water pressure washer, a diesel-fired hot box turns it into a hot water rig with one extra hose and two wires. The skid in the video above is a 48 by 48 mini we built for a customer doing trash compactor cleanouts, and it is a good example because there is nothing hidden. You can follow the water from the pump to the reel with your finger.
What a hot box actually is
A hot box is a diesel burner wrapped around a coil of pipe. Cold high-pressure water goes in one side, runs through the coil while the burner fires on it, and comes out the other side hot. It has no pump of its own. Your pressure washer does all the pushing; the box just heats what passes through.
The ones we build with are 12V diesel-fired units like the BE stainless steel diesel-fired hot box and the BE vertical hot box, both sized for 8 to 10 GPM machines. Twelve volt matters because it means the box runs off the same marine battery that already powers your soft wash pump and electric start. No generator, no shore power.
The plumbing: one extra line
On a cold water build, the pressure washer outlet goes straight to the hose reel. With a hot box in the loop, it goes like this instead:
- Pressure washer outlet runs to the hot box inlet. On the skid in the video that line tucks under the box and comes up into the side, where it passes a small pressure switch on the way in.
- Water runs up through the coil inside the box.
- Hot box outlet, on top, runs to the hose reel.
So you need one more whip line than a cold setup: the short run from the machine to the box. You can hard-plumb it, or have any hydraulic shop crimp one if you give them the length. If you are buying the box from us we will size it with you.
We put every one of those connections on quick connects, and this is the detail worth copying. With quick connects on both sides of the box, you can pull the line off the box outlet, plug the pressure washer straight back into the reel, and run cold in about ten seconds. Nothing to unscrew, nothing to drain. It also means every hose on the skid can come off for service in a driveway instead of at a shop. If you have never seen how we run whip lines, the 6.5 ft skid walkthrough shows the same approach on a full build.
The wiring: two wires
A 12V hot box has a positive lead and a negative lead. That is the whole electrical job. They land on the battery, and they need a real marine-grade 12V battery behind them, not a lawn tractor battery. We run Interstate Group 24 marine batteries on our builds because the burner, the electric start, and a 12V soft wash pump all pull from the same battery and it needs to hold up to that. If your rig has an upgraded charging system off the engine, even better; it keeps that battery topped up through the day.
When we deliver a build with a hot box, the leads are already run and tucked. If you are retrofitting one yourself, size the wire for the run length and use proper marine lugs. A hot box that browns out mid-job is almost always a wiring or battery problem, not a burner problem.
How it fires
Flip the switch on, turn the thermostat dial to the temperature you want, and go wash. Nothing happens until you pull the trigger. When water starts moving through the box, the pressure switch on the inlet closes and tells the burner to light. Let off the trigger, flow stops, the switch opens, the burner shuts down. You are not managing the burner while you work; it follows the gun.
Give it a minute the first time you fire it. The coil has to come up to temperature before what reaches the reel is actually hot.
The part people skip: everything downstream has to be hot-rated
This is the one thing in the video we would underline twice. Once the box is in the loop, the hose, the reel swivel, the gun, and the fittings between the box and the nozzle are all seeing hot water. Standard cold water hose will soften, bulge, and eventually let go. The FierceJet hose we spool on our builds is rated for hot water, and we spec reels with stainless internals for the same reason. Before you light a hot box on an existing rig, look at the rating stamped on your hose jacket and check your gun. If you are not sure, call us with what you have.
When a hot box is the right call, and when it is not
A hot box is the right move when you already own a good cold water machine and the jobs coming in need heat: grease on restaurant pads, trash enclosures and compactors, fleet washing, oil on concrete. It is a few thousand dollars added to a rig you already trust, and if that side of the business dries up you can unplug it and go back to cold.
If you are starting from scratch and hot water is going to be your main business, a dedicated hot water machine with the burner built in is cleaner and takes less room on the skid. We go through the money side of that decision in Hot Water vs. Cold Water Pressure Washing: When It's Worth the Money.
Parts in this setup
- BE Stainless Steel Diesel-Fired Hot Box, 8–10 GPM
- BE Vertical Hot Box, 8–10 GPM
- Honda GX690 8 GPM gear drive with block-mounted Pulsar unloader — the machine we pair with these boxes
- Hot water units — hot boxes and complete hot water machines
- Pressure washing skids and truck setups — hot water can be spec'd on any of them
Further reading
- Hot Water vs. Cold Water Pressure Washing: When It's Worth the Money
- How Much Does a Pressure Washing Trailer Cost in 2026?
- Pressure Washing Rig Maintenance Guide: Skids, Trailers, and Vans
Want hot water on a build, or help retrofitting a box onto the rig you have? Call the shop at 503-403-6185 (landline, no texts), Monday through Thursday, 8am to 4pm, or come see us at 13896 S Fir St Suite B in Oregon City, Oregon.
