The in-depth ESI manual for hydronic (coolant-loop) diesel heaters — Chinese clones, Autoterm/Planar Flow & ITR Hurricane/Oasis, to ABYC. System layout and what works, the plumbing (oxygen-barrier PEX, PEX-A vs PEX-B, hose-to-PEX), the low-pressure vented loop, the two primings (fuel & coolant fill/bleed), commissioning and CO.
Before you start
The ESI in-depth install manual for hydronic (coolant-loop) diesel heaters — the Chinese clones, Autoterm/Planar Flow and ITR Hurricane/Oasis we fit, with Webasto/Eberspächer as the reference. Built to ABYC: system layout and what works, the plumbing (oxygen-barrier PEX, PEX-A vs PEX-B, hose-to-PEX), the low-pressure vented loop, the two primings (fuel and coolant fill/bleed), commissioning and CO. A final exam unlocks your certificate. This is the deep dive behind the air-heater course’s hydronic chapter.
O₂-barrier PEX; PEX-A expansion vs PEX-B crimp/cinch; hose-to-PEX barbs.
Prime the fuel pump, then fill & bleed the coolant loop — never run the pump dry.
Header tank + relief cap, 50/50 glycol, ~1 GPM per 10k BTU.
Chapter 1
A diesel boiler, a coolant loop, and heat anywhere you want it.
A hydronic heater is a compact diesel-fired boiler that heats a 50/50 water-glycol coolant loop. A circulation pump sends that hot coolant around the boat to fan-coil heat exchangers in each cabin and to a calorifier for domestic hot water. One burner, many zones — through small hoses instead of big air ducts.
An air heater is limited to about two outlets. The moment a boat needs heat in more than two places, or wants hot water — roughly over 40′, multi-cabin — you go hydronic (this is the deep-dive companion to the air-heater course's hydronic chapter).
| Unit | What it is |
|---|---|
| Chinese hydronic clone | The air-heater burner heating a small coolant jacket instead of an air shroud (Vevor-style “water/coolant heater”). Cheap; build the loop properly and it works. |
| Autoterm / Planar Flow (was Binar 5S) | A ~5 kW liquid/coolant pre-heater — quality Russian/EU-design unit, well-documented. |
| ITR (Hurricane / Oasis) | The North-American marine diesel hydronic boiler standard — built for boats, with zone boards and DHW. |
| Webasto / Eberspächer Hydronic (ref) | The OEM engine-style coolant heaters — the reference for procedure; trained install for warranty. |
Sources: Fisheries Supply / Sure Marine hydronic guides; ITR (itrheat.com); Autoterm/Planar (planarheaters.com); ABYC A-7-2021 boat heating; Passagemaker “Diesel Heat.”
Chapter 2
Design the loop before you cut a hose.
Every boat is unique, but the parts and the rules are the same. Design it on paper first.
| Part | Role |
|---|---|
| Diesel boiler | Burns fuel to heat the coolant (its own combustion air + overboard exhaust). |
| Circulation pump | AC or DC; drives coolant round the loop. Sized for loop length & elevation. |
| Supply / return loop | Hose or PEX out to the zones and back (Chapter 3). |
| Fan-coil heat exchangers | “Blower boxes” / fan-matrix units per cabin — replace bulky radiators, save space. Optional finned/skin heaters. |
| Calorifier | Hot-water tank with a heating coil (a second heat exchanger) for showers/galley. |
| Expansion / header tank | At the system high point; fill point + pressure-relief cap; gives the loop an expansion path. |
| Bleed points, isolation & balancing valves | Air bleeds at high points; an isolation valve at each coil for service; balancing valves to even out the zones. |
Sources: Fisheries Supply hydronic guide; Sure Marine expansion/buffer tanks; ITR Hurricane/Oasis manuals; Dieselheat.com.au hydronic guide.
Chapter 3
The right pipe and the right way to join it.
The loop is mostly PEX with rubber heater hose at the equipment. Getting the pipe type and the connections right is what keeps a hydronic system quiet, leak-free and corrosion-free for years.
A heating loop has ferrous parts — the pump, the boiler core, steel fittings. Plain potable PEX lets oxygen diffuse through the wall into the coolant, which rusts those parts from the inside. So the loop uses oxygen-barrier PEX (an EVOH barrier layer — sold as hePEX / “O₂ barrier” PEX). O₂ PEX is the right call for diesel heating.
| PEX-A | PEX-B | |
|---|---|---|
| Made by | Engel method, ~85–89% cross-linked | Silane method, ~65–70% cross-linked |
| Feel | Most flexible; has kink memory (a kink can be heated out) | Stiffer; a kink is permanent |
| How it joins | Cold-expansion fittings (ProPEX / F1960): expand the pipe + ring, insert the fitting, it shrinks back tight — a clean visual connection. Can also crimp/clamp. | Crimp ring (copper, size-specific tool + go/no-go gauge) or cinch/clamp (stainless, one tool many sizes, good in tight spots). Cannot be reliably expanded — it micro-cracks. |
| Notes | Slightly better chemical/chlorine resistance; pricier; needs the expansion tool. | Cheaper; crimp/cinch tools are common and cheap. |
Either works for a hydronic loop in oxygen-barrier form. PEX-A + expansion is the nicest to route in a boat (flexible, full-bore fittings); PEX-B + cinch is the cheapest and easiest in tight bilges. Pick one fitting system and tool up for it.
The boiler, pump, fan-coils and calorifier all have rubber-hose barb ports. You don't PEX straight onto them — you run a short length of heavy-duty heater hose off each barb and transition to PEX with a hose-barb × PEX adapter:
Sources: PEXUniverse & Canarsee (oxygen-barrier vs non-barrier; crimp vs cinch vs expansion); Uponor AquaPEX vs hePEX; ITR plumbing guidance (heater hose / PEX, spring clamps).
Chapter 4
The burner side — same marine rules as an air heater.
The boiler's burner side follows the same marine rules as the air-heater course — because it's the same kind of burner.
Accessible for service, on a rigid base, below the expansion tank, with clearance to combustibles and shielding on the hot exhaust stub.
Marine exhaust/combustion-air practice per ABYC A-7-2021 and the ESI air-heater install standard (high & aft, wrapped, condensate drain, sealed penetration).
Chapter 5
A low-pressure, vented loop — not a sealed bomb.
This is where hydronic differs most from an air heater: it's a fluid system, and getting the coolant, the pressure and the expansion right is the whole game.
Use a 50% water / 50% antifreeze (glycol) mix. Never water only (corrosion, freezing) and never more than 50/50 — too much glycol thickens the fluid and kills circulation. ESI uses propylene glycol — the non-toxic glycol — never ethylene glycol (which is toxic). On a boat the loop sits right beside the potable-water calorifier and any spill goes near the water, so propylene is the safe choice.
A marine hydronic loop is low-pressure and vented, not a sealed high-pressure system like a car. The header / expansion tank with its pressure-relief cap gives the heated coolant somewhere to expand — the loop is not a sealed rigid circuit. It runs at only a few psi of static head, just enough so the air bleeds at the top actually vent (cold-fill to roughly 4–5 psi at the highest point). A 5 L header tank suits systems up to ~12 kW.
Sources: ITR Hurricane/Oasis (50/50 coolant, 1 GPM/10k BTU, header-tank fill); Sure Marine/HeatSo header & expansion tanks (5 L relief-cap, low-pressure vented); hydronic cold-fill pressure references.
Chapter 6
Free hot water off the same boiler.
One of the big reasons to go hydronic is domestic hot water. The boiler never touches the drinking water — it heats it indirectly through a calorifier.
A calorifier is an insulated tank of potable water with one or more internal heating coils (heat exchangers). The hot coolant loop runs through a coil and heat passes through the coil wall into the fresh water — the glycol never mixes with the drinking water. Most marine calorifiers carry two coils plus an AC element: one coil for the engine (hot water underway), one for the diesel boiler (hot water at anchor), and a 120/230 V immersion element for shore power.
Sources: marine calorifier / DHW guidance (Fisheries Supply, Sure Marine, YBW calorifier-engine-DHW threads); ITR second-heat-exchanger DHW; standard anti-scald / T&P practice.
Chapter 7
Clean diesel, metered, and a pump that self-purges.
The fuel side is the same metering-pump arrangement as an air heater, with a couple of layout rules the liquid-heater makers stress.
Sources: Autoterm/Planar Flow install (pump near/below tank, fuel line no slope, filter before pump); ABYC H-33-2021.
Chapter 8
Fused, sized — and the pump never loses power with the boiler lit.
Two DC loads — the boiler and the circulation pump — plus the controller.
ABYC E-11-2025; marine hydronic best practice (always-on circulation circuit).
Chapter 9
Same as an air heater: get the diesel up to the pump.
The fuel priming is identical to the air-heater course — the metering pump must fill with diesel and purge air before it will light.
Fuel-priming procedure per the common controllers (rotary / LCD H-on / Bluetooth) — consistent with the ESI Diesel Air Heater module.
Chapter 10
The step that makes or breaks a hydronic install.
This is the procedure the cheap manuals skip and the one that causes most call-backs. Air in the loop is the #1 cause of overheating and no-heat.
Sources: ITR Hurricane/Oasis (header-tank fill, Circ. Pump Override, never run pump dry); Webasto/Eberspächer bleeding guidance (air → overheat); Autoterm pump-airlock positions.
Chapter 11
Prove it circulates, heats every zone, and is safe.
Commissioning per ABYC A-7-2021/A-24 and ITR/Webasto guidance (verify circulation & ΔT, bleed if it over-temps, CO test).
Chapter 12
The differences that bite, and keeping it alive.
| Unit | Watch for |
|---|---|
| Autoterm / Planar Flow | Heater below the expansion tank; pump position must avoid airlock; fuel line no slope; pump near/below tank; filter before pump. |
| ITR Hurricane / Oasis | 1 GPM/10k BTU; 50/50 only; never run pump dry; fill via header; Circ. Pump Override jumper to fill/bleed; heater hose/PEX + spring clamps; second HX for DHW; zone board for multi-zone. |
| Chinese clone | Air-heater burner on a coolant jacket — build the loop the same; controller programming & priming = the air-heater controller families (see that course). |
| Webasto / Eberspächer | Engine-style; bleed thoroughly (air → overheat); a trained install protects the warranty. |
Sources: ITR Hurricane/Oasis install manuals; Autoterm/Planar Flow install manual; Webasto/Eberspächer Hydronic references.
Field reference
Field-tested: the symptom, the usual cause, and the fix. Always diagnose before you replace parts.
| Symptom | Likely cause | Fix |
|---|---|---|
| Over-temp lockout at commissioning | Air trapped in the loop | Run the pump on override, bleed every high point, heat-cycle and top up — almost always air |
| One zone won’t heat | Airlock, a closed isolation valve, or an unbalanced loop | Bleed that high point, open the valve, re-balance the zones |
| Circulation pump noisy | Air, run dry, or cavitation | Flood and bleed before running, never run it dry, check the header level |
| Coolant level keeps dropping | A leak or no expansion path | Pressure-test the loop, confirm the header tank & relief cap, fix the joint |
| Boiler won’t light | Same burner faults as the air heater (fuel/glow/voltage) | Re-prime the fuel, test the glow plug, confirm supply voltage |
| Rust or sludge in the coolant | Non-barrier PEX or a water-only fill | Re-pipe in O₂-barrier PEX; fill with 50/50 propylene glycol |
| Domestic hot water only lukewarm | Thermosiphon back-feed, or no DHW priority | Fit a check valve; run the calorifier as a priority or balanced branch |
Final certification
A short check ran at the end of each chapter. Pass this final at 80% to unlock your certificate.