Can You Turn a Diesel Heater Into a Condensing Boiler?

After testing dozens of hydronic diesel heaters, the 20kW HLN unit I reviewed last week delivered the highest stock efficiency I’ve ever recorded: 19.4 kW output at 71% efficiency under a 100-gallon thermal load.

During that long test, one thing stood out:

The exhaust was extremely hot — hot enough to suggest that a significant amount of waste heat was still escaping the system.

That raised an obvious engineering question:

Can you reclaim exhaust heat from a diesel hydronic heater to boost efficiency?

Today, I set out to answer that question using a 3-inch air-to-water intercooler — a component normally used to cool turbocharged intake air, not diesel exhaust.

The results were far more dramatic than I expected.

Why Exhaust Heat Recovery Matters

Diesel heaters typically lose 25–40% of their total energy out the exhaust stack. A good hydronic heater recovers most of the heat through its internal exchanger, but the exhaust gas leaving the unit is still hundreds of degrees.

If you can safely extract some of that heat before the exhaust leaves the system:

• You increase total energy capture

• If cooled far enough, you hit the dew point and create a condensing boiler effect

• Condensed vapor releases additional latent heat

This is exactly how modern high-efficiency natural gas boilers hit 90–96% efficiency.

Could a diesel heater do the same?

The Intercooler Exhaust Heat Recovery Setup

To test this safely, I built a closed-loop hydronic cycle:

1. HLN 20kW diesel hydronic heater mounted on a test bench

2. Exhaust routed through a 3” air-to-water intercooler core

3. Cold water drawn from a reservoir

4. Water flows: Reservoir → Intercooler → Heater → Reservoir

5. Thermal load:

   • 10 gallons (initial test)

   • 100 gallons (long-duration test)

6. Precision fuel measurement using a graduated flask

7. Water temperatures measured before and after each test

This is not a recommended real-world installation — purely an engineering experiment.

Test #1: 10-Gallon Intercooler Test

Data

• Initial water temperature: 8.2°C

• Final water temperature: 75.5°C

• Runtime: 8 minutes 49 seconds

• Diesel used: 284.8 g

Results

• Output: 17.2 kW

• Efficiency: 74%

A slight gain compared to the stock 5-gallon test, but the load was too small to meaningfully utilize the intercooler’s full potential. The intercooler was just getting warm when the test ended.

So I moved on to the real test.

Test #2: 100-Gallon Intercooler Heat-Recovery Test

Data

• Initial water temperature: 17.8°C

• Final water temperature: 61.3°C

• Runtime: 45 minutes 43 seconds

• Diesel consumed: 2 liters

Calculations

Energy absorbed by the water

Water mass: 100 gallons × 3.785 = 378.5 kg

ΔT = 61.3 – 17.8 = 43.5°C

Energy (Q) = mass × specific heat × ΔT

Q = 378.5 × 4.186 × 43.5

Q ≈ 68,800 kJ

= 19.11 kWh stored in the water

Output

Power = 25.1 kW

BTU/hr = 85,700 BTU/hr

Efficiency

Diesel energy (2 liters × 0.84 kg/L × 43,000 kJ/kg) = 72,240 kJ = 20.07 kWh

Efficiency = 19.11 ÷ 20.07

≈ 95%

And here’s the key:

We crossed the dew point of the exhaust and entered true condensing-boiler operation.

I witnessed visible condensation dripping from the intercooler exit, proof that the exhaust gas temperature dropped below the water vapor condensation threshold.

This recovery of latent heat is the only way to achieve efficiencies above 90%.

Why Efficiency Jumped from 71% → 95%

The intercooler captured:

1. Sensible heat (temperature drop of exhaust gases)

2. Latent heat (condensation of exhaust moisture)

Latent heat is huge. Every gram of water vapor condensed releases ~2,257 joules of energy.

In this test, it condensed enough vapor to form puddles on the floor — not great for the intercooler’s longevity, but great for efficiency.

This means the system behaved almost exactly like a condensing natural gas boiler, despite being diesel-powered.

Potential Problems With This Design (You MUST Consider These)

1. Condensate Is Acidic

Exhaust condensate contains carbonic acid. Over time, this can:

• Pit aluminum

• Corrode welds

• Shorten intercooler lifespan

2. Soot + Moisture = Sludge

If the diesel heater ever runs slightly rich, soot can mix with condensate and create acidic mud that clogs the intercooler.

3. Freezing Risk

Condensed water trapped inside the intercooler could freeze during winter cycles.

4. Maintenance Required

A condensing diesel system would likely require:

• Periodic flushing

• Internal descaling

• Inspection for corrosion

5. Manufacturer Warning

HLN does NOT recommend restricting or cooling exhaust.

This test violates their installation guidelines.

Would I Use This on My Own Bus or Cabin?

As a primary heating system:

Probably not — too many long-term risks.

As a secondary system feeding:

• Radiant floors

• Potable water heating

• A small buffer tank

Maybe.

If the heater failed, you wouldn’t freeze, and the maintenance load is manageable.

The efficiency gains are absolutely real — but so are the tradeoffs.

Final Verdict

Using an intercooler to scavenge exhaust heat from a diesel hydronic heater does work — better than expected.

Stock heater performance:

• 19.4 kW

• 71% efficiency

With exhaust heat recovery:

• 25.1 kW

• 85,700 BTU/hr

• Up to 95% efficiency (condensing operation)

This pushed the diesel heater into a territory normally reserved for high-end condensing boilers.

Is it safe long-term?

That depends on maintenance, climate, and installation.

Is it impressive?

Absolutely.

Would I recommend it for everyone?

No — but for experimenters and off-grid engineers, it’s a fascinating avenue for further development.

Interested in a 20kW Diesel Hydronic Heater?

IT IS NOW AVAILABLE ON THE WEBSITE!

https://www.hlnind-shop.com/products/hln-aquano-12k-coolant-heater-24v

🎁 HLN Heaters are expensive, Ive asked them for some help on the price. here is a Discount Code: Use %Jasonhurst for 20% off. — and support the channel at the same time.

The air to water intercooler - https://amzn.to/3XlSDpJ

These are affiliate links and support the channel if you use them.If you’re seriously considering something like this for a cabin or project:

👉 Contact Kevin at HLN: kevin.hln.int@gmail.com

Tell him you saw Jason’s 20kW test and what you’re trying to heat. He can help spec a system for your application.

Use code %JasonHurst at checkout to save 20% on any HLN product at:

https://www.hlnind-shop.com/collections/diesel-heater-water-heater

Have Questions? Want More Experiments?

Leave a comment on YouTube or send me an email.

I’m always willing to test:

• Radiant floor loops

• Heat exchangers

• Exhaust scavenging systems

• Alternate fuels

• Engine preheat integration

• Full hydronic bus heating systems

Thanks for reading —

Jason