Ariston NUOS Side-Connect 40 Gal
#15 in 40–50 GallonFull review
A non-Midea design with a leak sensor built in and a 10-year tank and parts warranty, sold through plumbing dealers. It is louder than most here.
The Ariston NUOS Side-Connect 40 Gal is the better heat pump water heater overall, rated 3.3 to 2.8. The Ariston NUOS Side-Connect 40 Gal leads on hot water, efficiency and cold rooms. The Rheem ProTerra 15 A 40 Gal wins on noise.
Rheem ProTerra 15 A 40 Gal
#19 in 40–50 GallonFull review
The ProTerra for a home that only has a 15 A, 240 V circuit, such as one left by a small electric tank: it avoids rewiring but gives up some efficiency and first-hour delivery.
Prices were last checked October 21, 2026 and are subject to change; the price and availability shown by the retailer at the time of purchase apply.
Which should you buy?
- Choose the Ariston NUOS Side-Connect 40 Gal for hot water (66 gallons in the first hour vs 46 gallons in the first hour), efficiency (3.58 UEF vs 3.45 UEF) and cold rooms (heat pump runs down to 35 °F room air vs heat pump runs down to 37 °F room air).
- Choose the Rheem ProTerra 15 A 40 Gal for noise (45 dB(A) vs 49.1 dB(A)).
Scores side by side
Each factor is scored 0-100 against the same fixed reference points for every 40–50 gallon heat pump water heater; a clear win is highlighted. How we rank explains each one.
| Factor weight | Ariston NUOS Side-Connect 40 Gal | Rheem ProTerra 15 A 40 Gal |
|---|---|---|
| Overall rating | 3.3/5 | 2.8/5 |
| Efficiency 35% | 41 3.58 UEF | 32 3.45 UEF |
| Hot water 20% | 64 66 gallons in the first hour | 0 46 gallons in the first hour |
| Warranty 20% | 70 10-year tank, 10-year parts | 75 10-year tank, 10-year parts, 1-year labor |
| Cold rooms 15% | 40 heat pump runs down to 35 °F room air | 32 heat pump runs down to 37 °F room air |
| Noise 10% | 43 49.1 dB(A) | 69 45 dB(A) |
Specifications
| Ariston NUOS Side-Connect 40 Gal | Rheem ProTerra 15 A 40 Gal | |
|---|---|---|
| Ratings (ENERGY STAR) | ||
| Size (nominal) | 40 gal | 40 gal |
| Tank volume (measured) | 37 gal | 36 gal |
| Uniform Energy Factor | 3.58 | 3.45 |
| First-hour rating | 66 gal | 46 gal |
| Draw pattern | Medium | Low |
| Yearly energy use | 907 kWh | 643 kWh |
| Heat pump cut-off (low) | 35 °F | 37 °F |
| Sound level | 49.1 dB(A) | 45 dB(A) |
| ENERGY STAR ID | 3942613 | 3554171 |
| Installation | ||
| Power | 240 V | 240 V |
| Circuit | 30 A, 208/240 V | 15 A, 208/240 V |
| Backup element | 4,500 W | 2,250 W |
| Operating air range | 36 to 120 °F | 37 to 145 °F |
| Minimum room size | 700 cu ft | 700 cu ft |
| Size | 37.6 in tall, 18.2 in across | 62.3 in tall, 20.3 in across |
| Weight | - | 166 lb |
| Refrigerant | R-134a | R-134a |
| Features | ||
| Wi-Fi | Built in | Built in |
| App | Ariston NET | EcoNet |
| Demand response port (CTA-2045) | Yes | Yes |
| Leak detection | Yes | No |
| Modes | Comfort, Green, Fast, Electric, Vacation | Energy Saver, Heat Pump, High Demand, Electric, Vacation |
| Warranty and models | ||
| Warranty | 10 yr tank, 10 yr parts | 10 yr tank, 10 yr parts, 1 yr labor |
| Also sold as | American Standard ASHPWH-40*****, Lennox WHHP040T45MD240S**** | Friedrich PROH40 T2 FD400-15, Rheem PROPH40 T2 RH375-15, Rheem PROPH40 T2 RH400-15, Richmond 10E40-HP5U15, Ruud PROUH40 T2 RU375-15, Ruud PROUH40 T2 RU400-15 |
| Released | 2024-12 | 2020-04 |