Can a 20kW Diesel Hydronic Heater Heat a Cabin?
Full Test Results, Calculations, and Real-World Analysis**
A few weeks ago, a viewer asked me a deceptively simple question:
“Jason, could one of these diesel hydronic heaters actually heat a building? Like a cabin, a pump house, or maybe even a small home?”
I wasn’t sure.
So I reached out to HLN and asked if they had anything bigger than their 10kW model.
They didn’t have a publicly listed option…
…but they did have something sitting on a shelf: a true 20kW diesel hydronic heater, built much more like a small boiler than a van heater.
They sent it over, and the goal for this project was simple:
Put this heater through the exact same real-world torture testing I’ve done on every hydronic heater I’ve ever reviewed.
Measure everything.
No marketing.
No guessing.
This blog post contains all the math, all the calculations, and all the behind-the-scenes numbers for the advanced viewers who want to verify the results for themselves.
Test Setup
Every one of my heater tests uses the same procedure:
Known mass of water
5 gallons for baseline comparison
100 gallons for a large “building-load” simulation
Precise fuel measurement
Diesel in a flask on a digital scale
Measured in grams or liters
Stopwatch
Runtime measured down to the second
Basic heat-transfer physics
The heat added to the water is calculated using:
Q = mass × specific_heat × temperature_change
Where:
mass is in kilograms
specific heat of water is 4.186 kJ/kg·°C
temperature_change (ΔT) is in °C
TEST 1 — The 5-Gallon Benchmark
Even though 5 gallons is far too little thermal mass for a 20kW heater, I always run this test so I can compare it directly to every other heater in my library.
Data Collected
Water volume: 5 gallons
Water mass: 5 × 3.785 = 18.93 kg
Starting water temperature: 20.5°C
Ending water temperature: 79.6°C
Temperature change: 79.6 – 20.5 = 59.1°C
Runtime: 5 minutes 15 seconds = 5.25 minutes = 0.0875 hours
Fuel used: 173.3 g = 0.1733 kg
Step 1 — Heat Added to Water
Q = 18.93 × 4.186 × 59.1
Q = 4,693 kJ
Convert to kWh:
4,693 kJ ÷ 3600 = 1.303 kWh
Step 2 — Average Power Output
Power = 1.303 kWh ÷ 0.0875 hours
Power = 14.9 kW
Convert to BTU/hr:
14.9 × 3412 = 50,700 BTU/hr
Step 3 — Efficiency
Diesel energy content (LHV) = 43,000 kJ/kg
Fuel energy input = 0.1733 × 43,000
Fuel energy input = 7,452 kJ
Fuel energy input = 2.07 kWh
Efficiency = 1.303 ÷ 2.07
Efficiency = 0.63 (63%)
5-Gallon Test Summary
Output: 14.9 kW
BTU/hr: 50,700
Efficiency: 63%
Note: 5 gallons is simply too small a thermal load for a 20kW unit, but still valuable for comparison.
TEST 2 — The 100-Gallon “Building Load” Test
This is the test that really shows what the heater is capable of.
Data Collected
Water volume: 100 gallons
Water mass: 100 × 3.785 = 378.5 kg
Starting temperature: 18.2°C
Ending temperature: 66.6°C
Temperature change: 48.4°C
Runtime: 1 hour 5 minutes 55 seconds = 65.92 minutes = 1.0986 hours
Fuel used: 3 liters
Diesel density: approximately 0.84 kg/L
Fuel mass: 3 × 0.84 = 2.52 kg
Step 1 — Heat Added to Water
Q = 378.5 × 4.186 × 48.4
Q = 76,700 kJ
Convert to kWh:
76,700 ÷ 3600 = 21.30 kWh
Step 2 — Average Output
Power = 21.30 kWh ÷ 1.0986 hours
Power = 19.4 kW
Convert to BTU/hr:
19.4 × 3412 = 66,200 BTU/hr
Step 3 — Efficiency
Fuel energy input = 2.52 × 43,000
Fuel energy input = 108,360 kJ
Fuel energy input = 30.10 kWh
Efficiency = 21.30 ÷ 30.10
Efficiency = 0.71 (71%)
100-Gallon Test Summary
Output: 19.4 kW
BTU/hr: 66,200
Efficiency: 71%
Note: Any diesel hydronic heater above 70% efficiency in stock form is exceptional.
Real-World Scenario
Could This Heater Warm a 1500 sq ft House?
A viewer asked if this heater could warm a typical home.
Here’s the model.
Assumptions
House size: 1500 sq ft
Insulation: average
Heat loss rule: 30 BTU/hr per square foot
Location: Denver, CO winter climate
Step 1 — House Heat Load
1500 × 30 = 45,000 BTU/hr required
Heater output (from testing): 66,200 BTU/hr
This means it has enough capacity to heat the house and would cycle on/off.
Step 2 — Winter Diesel Use Estimate
Using Denver’s heating degree days (HDD) and the measured real-world efficiency:
Estimated diesel consumption from December through February:
Approximately 2100 liters
Approximately 550 gallons
This assumes:
The heater is the only heat source
The home is 1500 sq ft with average insulation
Winter temperatures typical of Denver
Not ideal as a primary heat system —
but it absolutely can maintain temperature, prevent freezing, and heat a structure.
For a cabin, pump house, remote shop, off-grid property, or anywhere diesel is already used for equipment, this is a very realistic solution.
Engineering Notes & Observations
Build quality:
This is industrial-grade hardware. It includes:
Brushless water pump
Brushless combustion fan
Large high-surface-area heat exchanger
Aluminum construction
Exhaust gas temperature sensor
Very clean burn — almost zero soot after more than an hour of full output
Noise:
It is loud.
This belongs in an outdoor enclosure or mechanical room.
Controls:
Simple on/off output (no power levels)
Wi-Fi monitoring and scheduling
Reliability:
After long full-throttle testing, the internal components remained remarkably clean with minimal soot buildup.
Final Verdict
This heater is:
❌ Not intended to replace a full-size home HVAC system
(but it can do it in the right case)
✔ Excellent for:
Off-grid cabins
Pump houses
Remote structures
Radiant floors
Freeze protection
Engine preheat systems
Water tank heating
✔ Produces 19.4 kW of real output
✔ Delivers 66,200 BTU/hr
✔ Achieves 71% efficiency
✔ Built more like a boiler than a van heater
If you want a compact diesel-fired boiler that actually performs to spec, this heater is in a different league compared to the typical “Chinese diesel heater” clones.
How to Get One
IT IS NOW AVAILABLE ON THEIR WEBSITE! https://www.hlnind-shop.com/products/hln-aquano-12k-coolant-heater-24v Use code %JasonHurst at checkout to save 20% on any HLN heater! This is an Affiliate link and supports my testing.
This specific 20kW unit isn’t listed on HLN’s website as of filming — it’s basically a “behind the counter” product.
If you’re seriously considering something like this for a cabin or project:
👉 Contact Kevin at HLN: 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
⸻ 🔗 Related videos
🔥 HLN 10kW Steel Hydronic Heater – 67% Efficiency Test
🔥 HLN Aquano 5kw Hydronic Test
🚌 My Bus Build Hydronic Heat Playlist
Have Questions?
Drop a comment on YouTube or send me an email.
I’m always happy to test:
Exhaust scavenging systems
Radiators and fan-coils
Engine preheat loops
Hydronic radiant flooring
Alternate fuels
Anything weird or interesting
Thanks for reading,
– Jason