Antminer S21+ Undervolting Guide — Efficiency vs Hashrate Trade-off
Step-by-step undervolting and tuning guide for the Antminer S21+ on SoloFury. Custom firmware comparison (Braiins OS+, LuxOS, Vnish), voltage profiles, monitoring, and cost-per-PH/s/day impact.
The stock Antminer S21+ runs at 16.5 J/TH and 235 TH/s. With proper undervolting on custom firmware, you can hit 14–15 J/TH at the same hashrate, or maintain 16.5 J/TH at higher hashrate. Over a year of 24/7 mining, this translates to $200–400 in electricity savings per S21+ at $0.085/kWh, plus longer chip lifespan from cooler operation.
This guide covers why undervolting works, which custom firmware to use, safe voltage profiles, and how to verify the improvement on your SoloFury dashboard. Undervolting requires custom firmware — stock Bitmain firmware has very limited tuning options.
1. Why Undervolting Works
ASIC chips are designed with a safety margin in voltage. The stock voltage is chosen so that every chip from every silicon lottery can run reliably at the rated hashrate. Most chips are actually better than the worst-case silicon and can produce the same hashrate at lower voltage.
Lowering the voltage reduces power consumption quadratically (P ∝ V²), but reduces hashrate linearly at most. The result: lower power, similar hashrate, much better efficiency (J/TH).
The trade-offs:
- Some chips can’t handle aggressive undervolting and produce hardware errors
- Going too low triggers instability and the machine becomes unusable
- Custom firmware is required because stock firmware doesn’t expose voltage controls
The S21+ silicon is generally good for 5–15% undervolt depending on chip lottery and cooling.
2. Custom Firmware Options Compared
Three serious custom firmware options exist for the S21+ in 2026:
| Firmware | License | Pool fee | Best for |
|---|---|---|---|
| Braiins OS+ | Premium (paid) | 2% pool fee (mandatory) | Professional operators, best autotuning |
| LuxOS | Free / Premium tiers | Free tier available | Mid-tier users wanting good tuning without subscription |
| Vnish | Premium (paid one-time) | No pool fee | Best feature/price ratio for solo operators |
Braiins OS+
The industry standard for high-end ASIC tuning. Has the best autotuning algorithm, per-chip voltage profiling, and dashboard. However, it requires you to pay a 2% fee to Braiins on top of any pool fee. For solo mining on SoloFury, this means you’d effectively pay 1% (SoloFury) + 2% (Braiins) = 3% total fees on any block found.
For a typical S21+ owner solo mining BCH, this 2% Braiins fee on a found block costs you $30–60 per block found. Whether this is worth it depends on your block-finding frequency.
LuxOS
Free tier offers manual undervolting and basic tuning. Premium tier ($X/month per machine) adds autotuning and advanced features. No pool fees. Used by many small-to-mid operators.
Vnish
One-time license purchase per machine (typically $20-30). Full undervolting, autotuning, AsicBoost configuration, and per-chain voltage control. No pool fees, no subscription, no per-block tax. For solo miners on SoloFury, this is usually the best economic choice.
3. Pre-Flight Checks Before Flashing
Before flashing any custom firmware:
- Backup factory firmware: download the original Bitmain firmware from a trusted source so you can restore if needed
- Note serial number: write down the ASIC’s serial in case warranty needs to be invoked later
- Verify cooling capacity: ensure ambient temperature is under 30°C and exhaust is well-routed. Undervolting reduces heat output, but you want headroom in case you push hashrate up
- Identify chip count: S21+ has 3 hashboards × 110 chips = 330 chips total. All should report online before flashing
- Run baseline benchmark: record stock hashrate, stock power consumption (kill-a-watt meter), and stock J/TH. You’ll compare against this after tuning
- Reliable network connection: do not flash over Wi-Fi. Wired ethernet only. A flash interrupted by network drop can brick the machine
4. Flashing Vnish (Recommended Path)
The high-level process is the same for all three firmware options. Vnish is the most economical for SoloFury solo miners.
- Download Vnish firmware for S21+ from the official Vnish site. Verify checksum against published value.
- Open ASIC web UI at
http://<asic_ip>and log in - Navigate to System → Firmware Upgrade
- Upload the Vnish .bin file and start the flash. Do not interrupt the power or network during flashing (5–10 minutes)
- Wait for reboot. The ASIC may take 2–3 reboots during firmware initialization
- Log into the new Vnish UI (default credentials in Vnish documentation)
- Activate license by entering the license key you purchased
If flashing fails, the recovery procedure is documented in Vnish’s manual — typically involves connecting via UART or pressing the reset button while powering on.
5. Safe Voltage Profiles for S21+
After flashing, the S21+ runs at stock voltage by default. Now you tune.
The S21+ uses a 3-stage voltage system: chip voltage, frequency, and per-chain voltage adjustment. Custom firmware exposes all three. Below is a conservative starting profile — always start conservatively and test, never push aggressive numbers without monitoring.
Profile A — “Efficient stock hashrate” (recommended starting point)
| Parameter | Stock | Profile A |
|---|---|---|
| Chip voltage | ~14V | 13.0–13.5V |
| Frequency | stock | -2 to -3% |
| Expected hashrate | 235 TH/s | 232–236 TH/s |
| Expected power | 3,877 W | 3,400–3,550 W |
| Expected efficiency | 16.5 J/TH | 14.5–15.0 J/TH |
| Power savings | 0% | ~10% |
This is the safest starting point. It targets the same hashrate as stock but uses less power. Run this profile for 48–72 hours before pushing further to verify long-term stability.
Profile B — “Peak hashrate, stock efficiency”
| Parameter | Stock | Profile B |
|---|---|---|
| Chip voltage | ~14V | 14.0V (stock) |
| Frequency | stock | +5 to +8% |
| Expected hashrate | 235 TH/s | 245–255 TH/s |
| Expected power | 3,877 W | 4,100–4,300 W |
| Expected efficiency | 16.5 J/TH | 16.5–17.0 J/TH |
| Power increase | 0% | ~10% |
This profile pushes hashrate up while accepting more power consumption. Not the goal for most solo miners (efficiency is what matters), but useful if you have very cheap electricity and want maximum hashrate per machine.
Profile C — “Aggressive efficiency” (only after weeks of stability on Profile A)
| Parameter | Stock | Profile C |
|---|---|---|
| Chip voltage | ~14V | 12.5–13.0V |
| Frequency | stock | -5 to -7% |
| Expected hashrate | 235 TH/s | 220–230 TH/s |
| Expected power | 3,877 W | 3,000–3,200 W |
| Expected efficiency | 16.5 J/TH | 13.5–14.5 J/TH |
| Power savings | 0% | ~17–20% |
Only attempt this profile after at least 2 weeks of stable operation on Profile A. The hardware error rate often climbs significantly on aggressive undervolting, and the savings may be offset by increased rejected shares.
6. Per-Chain Tuning (Advanced)
Custom firmware lets you set different voltage per hashboard. This matters because the 3 hashboards in an S21+ are not identical — silicon lottery means one board might handle aggressive undervolt while another can’t.
Approach:
- Start all 3 boards at Profile A
- Watch hardware error rate per board for 48 hours
- Boards with error rate <0.3% can be pushed further (-0.5V or so)
- Boards with error rate >1% should be raised slightly back toward stock
- Iterate until each board is at its individual sweet spot
Result: typically 1–2 J/TH better than uniform tuning, because you’re matching each board’s silicon quality.
7. AsicBoost — Free Efficiency on Top
All three custom firmware options (Vnish, LuxOS, Braiins OS+) support AsicBoost on the S21+. AsicBoost is a Stratum protocol extension that exploits the SHA-256 algorithm’s overt structure to compute roughly 3-13% more hashes for the same power.
SoloFury supports AsicBoost on all 5 coins (BTC, BCH, BC2, BCH2, XEC). To enable it:
- In your custom firmware’s Mining Config, locate the AsicBoost or Stratum extensions setting
- Enable it
- Restart the ASIC’s mining service
That’s it. The pool advertises support automatically. The S21+ uses AsicBoost transparently and your hashrate goes up roughly 5-10% with no other config change.
Combine AsicBoost with Profile A undervolting and you can hit 14 J/TH at 245 TH/s — significantly better than the stock 16.5 J/TH at 235 TH/s.
For background on the AsicBoost technique, see the AsicBoost Explained article. For full setup details, see the AsicBoost Setup Guide.
8. Monitoring After Tuning
After applying any tuning profile, monitor for 48–72 hours before considering it stable. Watch:
| Metric | Where to check | Target |
|---|---|---|
| Hashrate (custom firmware) | ASIC dashboard | Within ±3% of target profile |
| Hashrate (SoloFury) | /miner/?addr=...&coin=... | Should match ASIC reading |
| Hardware error rate | ASIC dashboard, per-chain | <1% per chain |
| Reject rate (SoloFury) | Worker stats page | <0.5% baseline, <2% acceptable |
| Stale rate | SoloFury worker stats | <1% |
| Chip temperature | ASIC dashboard | 60-80°C, never above 85°C |
| Fan speed | ASIC dashboard | <90% — if pegged, cooling is inadequate |
If any of these are off-target, see the Reading Your Worker Stats guide for diagnosis.
9. Quantifying the Improvement
Let’s translate efficiency gains into actual dollars saved.
Setup
- Antminer S21+, $0.085/kWh electricity
- Continuous 24/7 operation, 720 hours/month
Stock baseline
- 235 TH/s @ 3,877 W
- Monthly power: 3.877 × 720 = 2,791 kWh
- Monthly electricity: 2,791 × $0.085 = $237/month
Profile A (Vnish, no AsicBoost)
- 234 TH/s @ 3,475 W (about -10%)
- Monthly power: 3.475 × 720 = 2,502 kWh
- Monthly electricity: 2,502 × $0.085 = $213/month
- Savings: $24/month, $288/year per ASIC
Profile A + AsicBoost
- 248 TH/s @ 3,475 W (+6% hashrate from AsicBoost, same power)
- Same monthly electricity: $213/month
- Effective cost/PH/s/day improvement: from $44.7 (stock) to $36.0 — a 19% reduction
Multi-ASIC fleet
If you run 4× S21+ (a real-world reference fleet operates this configuration), Profile A + AsicBoost saves:
- ~$1,150/year in electricity
- Adds ~50 TH/s effective hashrate (more lottery exposure)
- Reduces heat output by ~1.6 kW (easier cooling)
10. Rolling Back to Stock Firmware
If you decide custom firmware isn’t working for you, rolling back to Bitmain stock is straightforward:
- Download the original Bitmain firmware for S21+ from the official source
- In the custom firmware’s web UI, navigate to System → Firmware Upgrade
- Upload the Bitmain .tar.gz / .bin file
- Wait for flash and reboot
- The ASIC will be at factory defaults — reconfigure your SoloFury pool URLs
You will lose any tuning profiles and settings from the custom firmware. Re-apply them if you decide to come back later.
11. Combining Undervolting With SoloFury Strategies
Once your S21+ is running efficiently, you can combine the savings with smart pool strategies:
- Lower cost/PH/s/day means owned hardware beats more rental options — recheck the Owned vs Rental Cost Comparison with your new effective rate
- More effective hashrate means smaller mean time-to-block on any coin — see the Coin Selection Guide for the new expected times
- AsicBoost is enabled on all SoloFury coins, so you keep the efficiency gain regardless of which coin you mine
- Use the savings to fund occasional rental bursts during difficulty windows — see Difficulty Adjustment Calendar
Next Steps
- After tuning, verify shares are arriving cleanly: see the Reading Your Worker Stats guide
- If you encounter chip-level issues, run a full diagnostic: Miner Health Check guide
- Enable AsicBoost specifically: AsicBoost Setup Guide
- Recalculate your economics: Owned vs Rental Cost Comparison
- Pick the coin that benefits most from your improved efficiency: Coin Selection Guide
- For the AsicBoost theory background, read the AsicBoost Explained article