How to Plumb a Coolant Heater: 3 Layouts That Work
Sep 27, 2026
Where you connect a coolant heater decides what gets warm. Connect it one way and your engine block warms up. Connect it another way and most of the heat goes to the heater core while the block stays cold.
This guide covers the three layouts owners actually use, what decides where the heat goes, and how to check the result. The numbers come from our Apollo C1 diesel and Apollo GC5 gasoline coolant heaters. The principles apply to any 5KW class coolant heater.
Pick the Layout by What You Want Heated
Choose the layout by your main goal: engine starts, cabin heat, or both. Every layout uses the same heater and the same pump.
| Your main goal | Layout | What you get |
|---|---|---|
| Easy starts, plus some cabin heat | 1. Engine first | Warm block first, cabin heat after |
| Fast cabin heat and defrost | 2. Cabin first | Warm air sooner, engine warms more slowly |
| Engine starts in extreme cold | 3. Engine-only short loop | All the heat stays in the engine, shortest run for the pump |
Why the Thermostat Decides Where the Heat Goes
A closed engine thermostat keeps the radiator out of the loop. On a cold engine it stays shut. The heater can then only move coolant through the block, the heads, the bypass and the heater core circuit.
The Apollo C1 and GC5 regulate their outlet at 140–176°F (60–80°C). Engine thermostats are commonly rated between 160°F and 195°F (71–90°C), and the rating is usually stamped on the part. If yours opens above 176°F (80°C), the radiator stays out of the loop for the whole preheat.
A lower-rated thermostat may crack open near the end of a long preheat, and some heat then reaches the radiator. You then lose a little heat through the front of the vehicle.
The same fact catches people out. A hose that only carries coolant when the thermostat is open, such as the radiator return hose, is the wrong place to tap in. Vanagon owners on TheSamba forum describe heating a short section of hose this way and little else.
Layout 1: Engine First, Heater Core Second
Here the heater is spliced into the heater core circuit, with its outlet pointed toward the engine. Owners on truck and overlanding forums often describe this layout for engine preheat.
Layout 1, engine first: heated coolant goes through the engine before it reaches the heater core.
Cut the hose that returns coolant from the heater core to the engine. Connect the heater inlet to the heater core side and the heater outlet to the engine side. The hot coolant passes through the block before the heater core picks up what is left.
You get a warm engine first and cabin heat once the block has soaked up the heat. It suits trucks and vans where the heater core sits close to the engine.
Layout 2: Cabin First, the Manual Layout
Our installation manual shows the heater just ahead of the heater core inlet. Hot coolant reaches the heater core first, so your blower delivers warm air and clears the windshield quickly.
Layout 2, cabin first: the layout in our installation manual, with the heater just ahead of the heater core inlet.
The engine still warms, but more slowly, because the heater core takes its share of the heat first. Pick this layout if you mostly want a warm cabin while parked and engine preheat is a bonus.
Layout 3: Engine-Only Short Loop for Extreme Cold
In the coldest climates, a short loop at the engine puts all the heat where you need it. It also gives the pump the shortest run to push through.
Draw cold coolant from a low point on the engine, using a port at least as large as the heater hose. Many block drains are too small-bore for this. Return the heated coolant at a port far from the pickup. That way it flows through the block and heads before coming back to the heater.
Layout 3, engine-only short loop: the heater circulates through the engine alone. A heater core branch with one manual valve can be added in parallel.
Keep both hoses as short as the layout allows.
If you want cabin heat as well, leave a path to the heater core with a single manual valve. Close it for engine-only preheat and open it when you want the cabin warm. Exact tap points differ on every engine, so confirm them in your vehicle's service manual or with whoever built your conversion.
Can the Pump Handle Your Coolant Loop?
For a short engine loop, usually yes. The limit to check is flow through the heater, which must stay at or above 1.1 gal/min (250 L/h).
| Pump max head (12V) | 8.2 ft (2.5 m), at zero flow |
| Pump max head (24V, Apollo C1 only) | 9.8 ft (3 m), at zero flow |
| Pump max flow | 6.1 gal/min (23 L/min), at zero head |
| Minimum flow through the heater | 1.1 gal/min (250 L/h) |
| Minimum coolant in the loop | 1.1 gal (4.0 L) |
| Heater coolant connections | 3/4 inch (20mm) hose |
| System operating pressure | 6–36 psi (0.4–2.5 bar) |
Max head and max flow are the two ends of the pump curve. In a real loop the pump settles somewhere between them, depending on how much the loop resists flow.
Most of the resistance comes from tight bends, reducers and small-bore fittings, more than from straight hose. Some vehicles use 5/8 inch (16mm) heater hose, so keep any step-down from 3/4 inch to as few fittings as possible.
For the 1.1 gal (4.0 L) minimum, count the coolant in the engine plus the hoses in the loop. Your service data lists the engine's coolant capacity. A 3/4 inch (20mm) hose holds about 1 quart per 10 feet (0.3 L per meter).
A long rear-to-front run, like a rear-engine van feeding a front heater core, is where flow can fall short. Shorten the loop if you can. Some installers add a separate inline 12V booster pump; we haven't tested one with our heaters, so we can't recommend a specific model.
Valves and Check Valves: Keep Them Out of the Heater's Path
Never let a valve shut off flow through the heater itself. If it closes while the heater runs, the heater overheats and shuts down.
Layouts 1 and 2 put the heater in line with the heater core. Some vehicles have a heater control valve in that hose. Set the dash to full heat before running the heater, or the valve may be closed.
Use one manual valve on the branch you want to switch, usually the heater core branch. Owners on Expedition Portal warn that layouts with several valves can shut off the engine's own cooling if one valve fails.
Some installers fit a check valve so the engine's water pump doesn't push its full flow through the heater while driving. That's a judgment call for your layout. It isn't needed for the heater to work.
Bleed the Air: the Top Cause of Overheat Shutdowns
Trapped air is a common problem after installation. Boat and truck owners describe the same pattern: the pump keeps running, the burner shuts off after 10–15 minutes, and the heater reports an overheat.
- Mount the heater as low as practical, with the hoses rising away from it.
- Don't point the coolant connections downward.
- Remove dips and loops in the hose where an air pocket can sit.
- On a cold engine, fill with the cap off and run the engine with the heater valve open. Squeeze the hoses until no bubbles come up.
- Start the heater for the first time with the coolant below 86°F (30°C), then recheck the level once everything cools.
On the Apollo C1 and GC5, an overheat shows as code 11110. If you see it on a new install, check for air first, then for a closed valve or a pinched hose.
Fuel: Fit a Standpipe, Don't Tap the Engine Line
Give the heater its own fuel pickup in the tank. A Vanagon owner on TheSamba took gasoline from the engine's return line, and the engine stumbled after the heater ran for 45 minutes.
A fuel standpipe drops into the tank and feeds only the heater. It comes in the box with both the Apollo C1 diesel coolant heater and the Apollo GC5 gasoline coolant heater. Keep the tank-to-pump line under 4 ft (1.2 m) and the pump-to-heater line under 19 ft (5.8 m).
Match the heater to the fuel in your tank. The C1 burns diesel and comes in 12V or 24V. The GC5 burns gasoline and comes in 12V only.
Check the Flow With a Temperature Test
Once installed, compare the coolant temperature going into the heater with the temperature coming out. Start the test on a cold engine, shortly after start-up, while the heater runs at full output. It throttles back as the coolant nears 176°F (80°C).
At full output and exactly 1.1 gal/min (250 L/h), coolant leaves the heater roughly 32–38°F (18–21°C) hotter than it went in. That's our own calculation for water and for a 50/50 antifreeze mix.
A difference well below that means you have more than the minimum flow. A difference clearly above it means the loop is too restrictive.
An infrared thermometer on the two hoses right at the heater is enough for this check. Readings through rubber hose can be a few degrees off, so look at the size of the gap, not the exact number.
Before You Start
- Route the exhaust so it exits away from doors and windows. One Vanagon owner had exhaust drift in through the sliding door.
- Read our five installation precautions for mounting, fuel and exhaust basics.
- If you only need engine preheat and have shore power, a plug-in heater may be simpler. See how to install an engine block heater.
Frequently Asked Questions
Will a coolant heater warm my engine if I splice it into the heater hoses?
Yes, if the heated coolant flows into the engine before it reaches the heater core. Connect the heater outlet toward the engine side of the hose. If you connect it the other way, most of the heat goes to the heater core and the block warms slowly.
Does a coolant heater send heat to my radiator?
Not much while the engine thermostat is closed. The heater regulates its outlet at 140–176°F (60–80°C), below the opening point of many thermostats. A thermostat rated lower than 176°F (80°C) may crack open near the end of a long preheat.
How long does it take to preheat an engine with a coolant heater?
It depends on engine size, how cold it is and how the loop is plumbed. Owners on overlanding forums commonly report 30–45 minutes with the whole engine in the loop. Treat that as a field report, not a guaranteed figure, and start earlier on the coldest mornings.
Is the built-in pump strong enough for a long rear-to-front coolant run?
The pump is rated at 8.2 ft (2.5 m) max head on 12V and 9.8 ft (3 m) on the 24V Apollo C1. Max flow is 6.1 gal/min (23 L/min). The heater needs at least 1.1 gal/min (250 L/h). A short engine loop is easy for it, but check a long run with the temperature test after installation.
Why does my coolant heater shut down on overheat after 10–15 minutes?
Trapped air is a common cause. The pump keeps running, the burner shuts off, and the heater reports an overheat code (11110 on the Apollo C1 and GC5). Bleed at every high point, check that no valve is closed across the heater, and look for restrictions such as kinked hoses or undersized fittings.
Can I take fuel from my engine's fuel line instead of fitting a standpipe?
We don't recommend it. Tapping the engine feed or return line can let air into the heater's fuel line and starve the engine while both run. A dedicated standpipe gives the heater its own pickup, and one comes with the Apollo C1 and GC5.
Do I need a diesel or a gasoline coolant heater?
Match the heater to the fuel in your tank. The Apollo C1 burns diesel and comes in 12V or 24V. The Apollo GC5 burns gasoline, comes in 12V only, and shares the C1's 5.2KW output and circulation pump.
Get the Right Coolant Heater for Your Fuel
The Apollo C1 (diesel, 12V or 24V) and the Apollo GC5 (gasoline, 12V) are 5.2KW coolant heaters. The pump, fuel standpipe, hoses and exhaust come in the box. Both carry CE and RoHS certification.
Not sure which layout suits your vehicle? Email info@rvheater.com with your engine, fuel type and a photo of the coolant hoses, and we'll comment on the heater side of the plan.
