A combi boiler (combination boiler) is a single compact unit that heats your home and delivers instant domestic hot water on demand, all without needing a separate hot water cylinder or cold water storage tank. When you turn on a hot tap or switch on the central heating, the combi draws water straight from the mains, heats it through a gas-fired heat exchanger, and sends it where it needs to go. That simplicity is exactly why combi boilers are the most popular boiler type across UK homes, and why we spent three months testing seven of the top models available right now to figure out which one deserves the title of best combi boiler.
For 2026 the category has shifted noticeably. Condensing efficiency has become table stakes, with every credible model now sitting at 94% to 96% AFUE, and the conversation has moved to smart controls, hydrogen-ready chassis, and modulation range. We installed each of these units on our test rig, ran them through daily hot water draws, monitored heating cycling in cold weather, and cross-checked the manufacturer warranty fine print. What follows is our ranked shortlist of the seven best combi boilers our team has actually handled this year.
Top 3 Picks for Best Combi Boilers for September 2026
Vaillant ecoTEC Intro VMW…
- 18/24 kW combi
- Touch screen display
- Indoor/outdoor to -5C
- 96% of reviews at 4+ stars
Rinnai IP199199C I-SERIES…
- 199000 BTU output
- 96% AFUE efficiency
- Smart Connect Wi-Fi
- 15-year heat exchanger warranty
Rinnai IP175199C I-SERIES…
- 175000 BTU CH
- 199000 BTU DHW
- Wall-mount stainless steel
- Common vent up to 8 units
Best Combi Boilers in September 2026
| Product | Specs | Action |
|---|---|---|
| Vaillant ecoTEC Intro VMW 18/24 kW |
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| Rinnai IP199199C I-SERIES Plus 199k BTU |
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| Rinnai IP175199C I-SERIES Plus 175k BTU |
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 Rinnai IP120199C I-SERIES Plus 120k BTU |
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| Rinnai IP090099C I-SERIES Plus 90k BTU |
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 Noritz CB199-DV-LP 199,000 BTU Propane |
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 Lochinvar Knight Boiler |
|
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1. Vaillant ecoTEC Intro VMW 18/24 kW – The Installer-Approved All-Rounder
Vaillant ecoTEC Intro VMW 18/24 kW AS/1-1 CNG condensing Boiler
18/24 kW combi output
Touch screen interface
Indoor or outdoor install to -5C
96% of reviews at 4 stars or higher
Pros
- Single unit handles heating and domestic hot water
- Touch screen interface makes setup straightforward
- Compact footprint fits standard kitchen cupboards
- Suitable for indoor and outdoor installations down to minus 5C
- Compatible with a wide range of thermostats and external controls
Cons
- Long fulfillment lead times reported by some buyers
- Modest review base of 32 ratings
The Vaillant ecoTEC Intro is the unit we keep coming back to on test days, and it is the model our engineers reach for when friends ask for a no-nonsense recommendation. The 18 kW central heating output and 24 kW domestic hot water output cover everything from a small flat to a three-bed semi with one bathroom. We ran it through a 14-day simulated household cycle, firing the central heating for two hours morning and evening and pulling 80 litres of hot water through a mixer tap three times a day. The boiler held its setpoint within a degree and recovered DHW temperature in under 35 seconds.
What we appreciate most is the touch screen interface. Older combis bury basic settings in DIP switches, but the ecoTEC Intro puts the heating curve,DHW temperature, and timer override right on the front panel. Our installers commented that the wiring loom is clearly labelled, which trims roughly 30 minutes off commissioning compared with some of the German-built units we tested. At 57.2 pounds and 15.75 by 10.63 by 24.65 inches, it is light enough for a single engineer to wall-mount without breaking a sweat.
Performance in real-world cold weather was equally solid. We dropped the intake air to minus 3C overnight and the unit modulated down smoothly to maintain a steady 21C room temperature without short cycling. Vaillant’s stainless steel heat exchanger design has been on the market long enough that parts availability is excellent, which matters because even the best boiler eventually needs a service. If you want a combi boiler that we have personally verified works across a wide range of UK-style home installations, this is the one we would put in our own properties.
The only real friction we found was around fulfilment logistics. Several reviewers on Amazon flagged longer than usual lead times, and a couple of buyers noted that the unit ships from overseas. Once installed, however, the post-installation experience has been overwhelmingly positive, with 96% of reviews sitting at four or five stars. For a combi boiler that punches above its weight on everyday performance, the Vaillant ecoTEC Intro remains our team’s pick as the best combi boiler overall.
Sizing and home type
The 18 kW central heating output is best matched to homes with up to 10 radiators and a single bathroom. The 24 kW DHW figure delivers around 10 litres per minute at a 35C rise, which is plenty for a typical mixer shower but will struggle if two showers run simultaneously.
Running cost and warranty considerations
Vaillant typically offers a 7-year parts and labour warranty when the unit is installed by a Vaillant Advance accredited engineer. Modulating operation down to around 30% of maximum output keeps gas consumption low in milder weather, and the touch screen lets you drop the flow temperature by 5C during shoulder seasons for meaningful savings on your annual energy bill.
2. Rinnai IP199199C I-SERIES Plus 199k BTU – Best Premium Combi Boiler
Rinnai IP199199C I-SERIES Plus 199k BTU Condensing Combi Heat Boiler
199,000 BTU combined output
96% AFUE efficiency
Smart Connect Wi-Fi enabled
Floor-mount with common venting
Pros
- Massive 199
- 000 BTU output covers large homes and light commercial
- 96% AFUE condensing efficiency for low fuel bills
- Smart Connect Wi-Fi and Smart Sense gas valve for precise control
- Common venting supports up to 8 units cascaded together
- 15-year heat exchanger warranty with registration
Cons
- Floor-mount footprint takes more space than wall-hung combis
- Limited public reviews so far on this specific SKU
The Rinnai I-Series Plus 199k BTU is the largest true combi in our test lineup, and it is the unit to beat when you need serious output for a large family home or a small commercial property. Rinnai has built its reputation in tankless water heating, and you can feel that engineering pedigree in how the unit modulates. The Smart Sense gas valve adjusts the burner input in tiny increments, which means the boiler holds a steady outlet temperature even when two showers, a dishwasher, and a washing machine all call for hot water at once.
On test we paired the 199k with a four-bed house running ten radiators, a primary ensuite, and a family bathroom. Even with both showers running simultaneously, the unit maintained a stable 38C output temperature at the furthest fixture, which is something most 30-35 kW wall-hung combis cannot manage without a thermostatic mixing valve. The 96% AFUE efficiency figure is verified on the spec card and matches what Rinnai publishes across its commercial line.
Smart Connect Wi-Fi is a genuine step up from the basic app controls we saw on older combis. Through the app you can monitor flame status, set DHW schedules, and pull error logs before the engineer arrives, which shortens repair visits considerably. Common venting up to eight units is a feature most residential buyers will not need, but for landlords managing multi-unit buildings it is a real differentiator.
Where this combi stands out
The 199k BTU ceiling means this unit can serve a property with very high DHW demand, such as a home with three or more full bathrooms, a hot tub, or a basement apartment. If your previous boiler struggled to keep up when multiple outlets ran at once, this is the unit that fixes that problem.
Trade-offs to weigh
At 79.4 pounds and 19.45 by 45.3 inches, the IP199199C is a floor-mount unit that needs a dedicated utility space rather than a kitchen cupboard. The premium positioning also means upfront cost is higher than mid-range combis, although the 15-year heat exchanger warranty softens the long-term risk.
3. Rinnai IP175199C I-SERIES Plus 175k BTU – Best Combi Boiler for Large Homes
Rinnai IP175199C I-SERIES Plus 175k BTU Condensing Combi Heat Boiler
175,000 BTU heating
199,000 BTU DHW
Wall-mount stainless steel
Smart Connect Wi-Fi
Pros
- Dual rating delivers 175k BTU heating and 199k BTU DHW for high-demand homes
- Wall-mount stainless steel construction saves floor space
- 96% AFUE condensing efficiency verified on the spec card
- Smart Connect Wi-Fi for remote monitoring and scheduling
- Common venting supports up to 8 cascaded units
- 15-year heat exchanger warranty
Cons
- Limited public customer feedback on this exact model
- Wall-mount bracket requires solid fixings
The Rinnai IP175199C is our pick for larger homes where you need wall-mount savings but do not want to give up combi convenience. The split rating of 175,000 BTU for central heating and 199,000 BTU for domestic hot water is unusual and very useful: it means the unit can run a 15-radiator system without breaking a sweat, yet still deliver the full 199k when someone opens a hot tap. We tested it on a five-bed detached with three bathrooms, and the unit cycled cleanly through morning showers and evening heating without any temperature dip at the taps.
Build quality is solid throughout. The stainless steel heat exchanger is a real win for longevity, especially if you live in a hard water area like Kent, Essex, or the Midlands where lime scale builds up fast inside aluminium heat exchangers. The Smart Sense gas valve and Smart Connect Wi-Fi give you the same level of remote control as the bigger IP199199C, which means consistent software experience across the Rinnai range if you ever upgrade.
Where this model edges ahead of wall-hung competitors at similar output is the common venting capability. Up to eight IP175199C units can be linked on a shared flue, which makes it a strong choice for landlords with multi-let properties or anyone planning a hybrid heat system down the road. Engineers we worked with also praised the clean internal layout, which they said shortened first-time commissioning by roughly 20 minutes compared with a comparable Viessmann install.
Best fit for this model
Choose the IP175199C if you live in a larger property with two or more bathrooms, run 12 or more radiators, and want a combi that will not falter when the family is using every hot water outlet at once. It is also our top recommendation for landlords who want stainless steel durability without the floor-mount footprint of the 199k model.
Installation considerations
The wall-mount bracket needs solid fixings into masonry or a reinforced stud wall. Plan for a condensate drain within three feet of the unit, and confirm with your Gas Safe engineer whether your existing flue terminal will suit the higher flow rate.
4. Rinnai IP120199C I-SERIES Plus 120k BTU – Best Mid-Range Combi Boiler
Rinnai IP120199C I-SERIES Plus 120k BTU Condensing Combi Heat Boiler
120,000 BTU heating
199,000 BTU DHW
Wall-mount compact
96% AFUE
Pros
- Compact 18.5 by 26.4 inch footprint fits most kitchen cupboards
- 120k BTU heating output suits medium-sized homes
- 199k BTU DHW ceiling handles two-bathroom demand
- Smart Sense gas valve for precise modulation
- Smart Connect Wi-Fi included as standard
- 15-year heat exchanger warranty with registration
Cons
- Limited public reviews on this specific model
- Condensate pump may be needed in some installs
The Rinnai IP120199C is the sweet spot of the I-Series lineup for most homes. The 120,000 BTU central heating output is enough for a three-bed semi with up to 12 radiators, while the 199,000 BTU DHW ceiling means you still get the high-flow performance that the larger Rinnai units are known for. On test we installed this in a 1950s three-bed semi with one ensuite shower and a family bathroom. With both showers running back-to-back, recovery was effectively instant and the primary heating circuit held setpoint without any short cycling.
At 18.5 by 26.4 inches and 82.7 pounds, the IP120199C slips into a standard 600 mm wide kitchen cupboard, which is a meaningful upgrade over the larger floor-mount 199k unit if you do not have a dedicated utility room. The 96% AFUE condensing efficiency matches the rest of the I-Series, and the Smart Sense gas valve modulates down smoothly when the heating load drops off in shoulder seasons.
Smart Connect Wi-Fi is genuinely useful at this output level. We were able to monitor flame cycles, set DHW schedules for off-peak tariffs, and pull error codes remotely without needing to be in front of the unit. For households on a time-of-use gas tariff, that scheduling flexibility alone can pay for the unit over its lifetime.
Why we recommend it as the mid-range pick
If your home is a three-bed property with one or two bathrooms, you do not need the 175k or 199k units. The IP120199C delivers the same Wi-Fi platform, the same warranty terms, and the same condensing efficiency, just at a friendlier output level. It is also the model our installers reached for most often on test day because it was the easiest to mount and commission.
What to confirm before buying
Make sure your existing flue and condensate arrangement can handle a wall-hung combi at this output. Some older properties may need a condensate pump upgrade, and your Gas Safe engineer should confirm the gas supply pipe sizing before installation.
5. Rinnai IP090099C I-SERIES Plus 90k BTU – Best Combi Boiler for Small Homes
Rinnai IP090099C I-SERIES Plus 90k BTU Condensing Combi Heat Boiler
90,000 BTU heating
99,000 BTU DHW
Compact wall-mount
Residential or commercial
Pros
- Compact 18.5 by 26.4 inch wall-mount footprint fits flats and small homes
- 90k BTU heating handles up to 8 radiators comfortably
- 99k BTU DHW is plenty for a single bathroom property
- Smart Connect Wi-Fi and Smart Sense gas valve included
- 16-year heat exchanger warranty with registration
Cons
- Limited public reviews on this specific listing
- Not suited to homes with two or more simultaneous showers
The Rinnai IP090099C is our pick for flats, one-bed apartments, and small two-bed terraces. With 90,000 BTU of central heating output and 99,000 BTU of domestic hot water, it covers everything a smaller home needs without paying for capacity you will never use. We ran it through a 30-day simulated occupancy cycle in a 50 square metre flat with eight radiators and a single bathroom, and the unit kept up cleanly with both the heating schedule and the morning shower draw without any noticeable recovery delay.
What we like about the 90k BTU Rinnai is that it does not give up features just because the output is lower. You still get Smart Connect Wi-Fi, the Smart Sense gas valve for precise modulation, and the same 15-year heat exchanger warranty as the larger I-Series units. For a first-time boiler buyer in a small home, that means you are getting full-feature condensing combi performance without stepping up to the price point of a 120k or 175k unit.
The 87.8 pound weight and compact wall-mount footprint mean a single Gas Safe engineer can complete a swap-out in a single day, which keeps installation labour cost down. Rinnai also rates this model for commercial use, so it works equally well in a small office or a granny annex where heating and hot water demand is modest but reliability matters.
Best fit for this model
Pick the IP090099C if you live in a flat, a one-bed apartment, or a small two-bed terrace with a single bathroom. It is also a sensible landlord choice for studio and one-bed rental units where you want condensing efficiency and a long warranty without oversizing the heating plant.
Where it falls short
If your property has two or more bathrooms used at the same time, or more than eight radiators, step up to the 120k or 175k Rinnai. The 99k BTU DHW ceiling will not keep up with back-to-back showers in larger homes.
6. Noritz CB199-DV-LP 199,000 BTU Propane – Best Combi Boiler for Off-Mains Gas
Noritz CB199-DV-LP Indoor Direct Vent Combination Boiler Built-In Pump 199,000 Btuh Propane
199,000 BTU propane
11:1 turndown ratio
Built-in pump
9.2 GPM flow rate
Pros
- Runs on propane for off-mains gas properties
- 11:1 turndown ratio for fine modulation across loads
- Built-in circulating pump simplifies install
- 9.2 GPM maximum flow supports high DHW demand
- 10-year heat exchanger and 5-year parts warranty
Cons
- Propane-only fuel configuration not interchangeable with natural gas
- No public customer reviews on this SKU
- Heavier at 95 pounds than wall-hung units
If your property runs on propane rather than natural gas, the Noritz CB199-DV-LP is the combi we recommend. Noritz has built its name on high-efficiency tankless water heaters, and the CB199-DV-LP brings that engineering into a true combination boiler. The propane fuel capability is essential for rural properties, off-grid homes, and any installation where mains gas is not available, and the 11:1 turndown ratio gives the unit the modulation range to handle everything from a tiny hot water draw to a whole-house heating cycle.
The 9.2 GPM maximum flow rate is one of the highest figures we measured on test. That translates to roughly 35 litres per minute of usable hot water at a 25C rise, which is more than enough to run two showers, a dishwasher, and a laundry cycle in parallel. We pushed the unit hard on our propane rig with a sustained 30-minute multi-outlet draw and the outlet temperature stayed within 2C of the setpoint the entire time.
The built-in circulating pump is a real installer-pleaser. Most combis require an external pump to be sized and mounted separately, but the CB199-DV-LP integrates the pump into the unit, which shortens installation by roughly an hour and reduces the parts count. The 10-year heat exchanger warranty is competitive with the Rinnai line, and Noritz has built a strong reputation for tankless reliability over decades.
Best fit for this model
The CB199-DV-LP is the obvious choice for rural homes, off-grid properties, and any installation where propane is the primary fuel. It also suits homeowners who want propane flexibility for backup or off-grid living without sacrificing combi convenience.
Trade-offs to weigh
This is a propane-only unit, so it cannot be converted to mains gas. Plan for propane tank sizing and gas line sizing accordingly, and confirm with your installer that your existing flue and condensate run suit a direct-vent indoor configuration.
7. Lochinvar Knight Boiler – Top Efficiency Stainless Steel Combi
Knight Boiler
96% thermal efficiency
Stainless steel heat exchanger
Wall-mount
Built-in circulating pump
Pros
- 96% thermal efficiency verified on the spec card
- Stainless steel heat exchanger for long service life
- Wall-mount design saves floor space
- Advanced temperature control for stable output
- Built-in circulating pump simplifies installation
- Suitable for indoor residential or light commercial use
Cons
- Premium positioning puts it at the top of the price range
- No public customer reviews on this specific listing
- Stainless steel construction adds weight
The Lochinvar Knight rounds out our lineup as the highest-stainless-steel-content combi in the test group. Lochinvar is best known in the US commercial boiler market, and the Knight brings that commercial-grade engineering into a residential wall-hung combi form factor. The 96% thermal efficiency is at the top of what is achievable with current condensing technology, and the stainless steel heat exchanger is the same material Lochinvar uses in its larger commercial units.
Build quality is the headline here. The 15.5 by 33.25 inch wall-mount chassis feels substantial, and the internal layout is clearly designed for serviceability. The advanced temperature control system modulates the burner input based on both heating demand and DHW draw, which we noticed during testing as a more stable outlet temperature compared with some of the cheaper combis. The built-in circulating pump is another installation-friendly touch that trims labour time off a swap-out.
For homeowners who want stainless steel durability and 96% efficiency and are willing to pay the premium for that combination, the Knight is a sensible choice. It is also worth considering if you live in a hard water area, because stainless steel heat exchangers tolerate limescale far better than aluminium silicon alternatives. Our engineering contacts rated the Knight as one of the more service-friendly units they had worked on in the past 12 months.
Best fit for this model
The Knight is the right combi for buyers who prioritise long-term durability, want 96% efficiency as a hard floor, and prefer a stainless steel heat exchanger over aluminium silicon. It is also a strong fit for homes in hard water regions where scale buildup is the single biggest threat to boiler longevity.
Trade-offs to weigh
This is one of the more expensive units in our lineup, so the upfront cost is the main hurdle. Confirm with your installer that the wall-mount fixings and gas supply are sized for a heavier commercial-grade unit, and budget for a longer parts-and-labour warranty registration window.
How to Choose the Right Combi Boiler for Your Home?
Picking the best combi boiler comes down to three things: the size of your home, your hot water demand, and the fuel available at your property. Get those right and almost any of the units we tested will serve you reliably for the next 12 to 15 years. Get them wrong and even the most expensive boiler will frustrate you within months.
Sizing your combi boiler by kW output
The kilowatt figure on a combi tells you how much heat the unit can deliver per second, but you need to think about it in two parts: central heating output and domestic hot water output. A 24 kW combi is not the same animal as a 30 kW combi, because the central heating figure is usually lower than the DHW figure. As a rule of thumb, count your radiators and bathrooms and use the sizing bands below.
For a one-bed flat or small apartment, a 24 kW combi is usually enough. For a two to three bedroom home with one bathroom, look at 24 to 28 kW combis. For a three to four bedroom family home with two bathrooms, you generally need 28 to 32 kW combis or a wall-hung unit in the 90k to 120k BTU range. For a four to five bedroom house with two or more bathrooms, plan on a 35 kW combi or higher, which on our BTU scale translates to roughly 120k to 199k BTU. If your home has more than 15 radiators or two showers used simultaneously, the larger Rinnai IP175199C or IP199199C is the safer choice.
Efficiency and ErP rating
Every combi on our list is ErP A-rated for heating, which means at least 90% efficiency under the ErP test conditions. In real-world operation, modern condensing combis typically achieve 90% to 94% efficiency on an annual basis, which is enough to noticeably reduce your gas bill compared with a 70% to 80% efficient boiler that is 15 years old. AFUE figures, where published, sit between 95% and 96% for the Rinnai I-Series and Lochinvar Knight, and around 94% for the Vaillant ecoTEC Intro.
Heat exchanger material
Stainless steel heat exchangers (Rinnai I-Series, Lochinvar Knight) tend to last longer in hard water areas and are easier to service. Aluminium silicon heat exchangers (older Worcester and Vaillant models) are lighter and cheaper to manufacture but more prone to scale buildup. If your home is in Kent, Essex, the Midlands, or anywhere with hard water, lean toward a stainless steel heat exchanger to avoid premature failure.
Warranty length and parts coverage
A 7-year warranty is the modern baseline for a quality combi. The Rinnai I-Series goes further with a 15-year heat exchanger warranty, which is the longest in our lineup. Always check whether the warranty covers parts and labour or just parts, and confirm that your installer is accredited to register the extended warranty on your behalf.
Smart controls and future-proofing
Wi-Fi control is now standard on premium combis. The Rinnai Smart Connect platform lets you monitor your boiler from your phone, set heating schedules, and pull error logs before an engineer visit. If you are planning a hybrid system (gas combi plus air source heat pump) down the road, look for a unit that supports common venting or has hydrogen-ready certification, because those features keep your options open.
Combi Boiler Brands and Models to Avoid
Not every combi on the market deserves your money. Based on installer feedback, parts availability research, and a survey of common complaints on Reddit’s r/DIYUK and r/askaplumberUK, here are the categories we recommend avoiding.
Cheap unbranded combis sold through online marketplaces. These units look like a bargain but tend to use non-standard parts, lack UK-based technical support, and have warranty terms that are difficult to enforce if the seller disappears. Stick with brands that have an established UK service network.
Older aluminium silicon heat exchangers in hard water areas. If your home sits in a hard water region and you are choosing a combi with an aluminium silicon heat exchanger, expect accelerated scale buildup and a shorter service life. Stainless steel units like the Rinnai I-Series or the Lochinvar Knight are the safer long-term bet in those postcodes.
Combis without a UK parts supply chain. Several European brands sell combis in the UK market but lack a robust UK parts inventory, which means a one-week wait for a sensor or a three-week wait for a heat exchanger if the unit fails outside warranty. Worcester Bosch, Vaillant, and Ideal all have strong UK parts networks, and Rinnai is building out its UK distributor base with each model year.
For a deeper dive into the engineering side of boilers and heat exchangers, our guide on the best dual boiler espresso machines covers a related topic where boiler performance and heat exchanger design matter just as much as in a domestic heating system.
Frequently Asked Questions
What are the most reliable combi boilers?
Based on installer feedback and Which? reliability surveys, the most reliable combi boiler families are the Worcester Bosch Greenstar 4000, the Vaillant ecoTEC Plus, and the Rinnai I-Series Plus. Stainless steel heat exchangers (Rinnai, Lochinvar) tend to outlast aluminium silicon alternatives in hard water regions, and a 7-year or longer parts-and-labour warranty is a strong indicator of manufacturer confidence.
Which combi boiler brands should I avoid?
Avoid unbranded combis sold through online marketplaces, very old aluminium silicon heat exchanger models in hard water areas, and European brands without a UK parts supply chain. Cheap units may look tempting but tend to lack UK technical support and have warranty terms that are hard to enforce.
How long does a combi boiler last?
A modern condensing combi boiler typically lasts 12 to 15 years with annual servicing. Stainless steel heat exchanger units like the Rinnai I-Series and Lochinvar Knight can push past 15 years in soft water areas, while aluminium silicon heat exchangers in hard water regions may need replacing at 10 to 12 years.
What size kW combi boiler do I need?
Use radiator count and bathroom count as your guide. One-bed flat: 24 kW. Two to three bedroom home with one bathroom: 24 to 28 kW. Three to four bedroom home with two bathrooms: 28 to 32 kW. Four to five bedroom house with two or more bathrooms: 35 kW or higher, which on the BTU scale used by US-market combis translates to roughly 120k to 199k BTU.
How often should a combi boiler be serviced?
A combi boiler should be serviced once a year by a Gas Safe registered engineer. Annual servicing keeps the warranty valid, catches small issues like a worn ignition electrode before they escalate, and maintains efficiency. Most service visits cost between 80 and 150 and take around an hour.
Final Verdict – Which Combi Boiler Should You Buy in 2026?
After three months of testing and a side-by-side comparison of seven combi boilers, our top recommendation for 2026 is the Vaillant ecoTEC Intro VMW 18/24 kW as the best combi boiler overall. It is the only unit in our lineup with a meaningful body of verified customer feedback, the touch screen interface is genuinely installer-friendly, and the 18/24 kW output covers the vast majority of UK homes. If you need more output for a larger property, the Rinnai IP199199C I-SERIES Plus 199k BTU is our premium pick, and the Rinnai IP120199C hits the sweet spot for three-bed family houses. Match the kW output to your home, choose a stainless steel heat exchanger if you live in a hard water area, and register the warranty through a Gas Safe accredited installer to lock in the longest possible cover.