Air source heat pump (mini split) for narrowboat

[Lori2025]

Anyone got any experience with so-called "mini split" a/c / heat pump units on a narrowboat? I'm keen to get one for all the usual reasons (high efficiency heating from electricity, plus the benefit of cooling in the summer, no messing around with diesel, gas or solid fuel etc.) but it seems my options are limited to a R290 (basically propane) unit which can be self-installed but runs the risk of leaking explosive gas if anything goes wrong. The R290 heat pumps are also noticeably less efficient, which would stretch my electrical system. 

 

There are more efficient (and cheaper) units based on R32 gas which are available as an "easy install" version, but they're illegal to self-install in the UK. The issue here is finding anyone who will install it. None of my local (and not so local) suppliers who do units for houses and offices will install anything they haven't also supplied (and guess what, they ones they sell are twice the price) and even then, they're extremely cautious about going near a boat. 

 

Really curious to hear if anyone else has any experiences in this area. Also, does anyone know of an automotive A/C tech who might be up for commissioning an R32 unit on a boat? Or better yet someone who already works on boats? I realise this is a long shot.

 

(p.s. I'm well aware of the roof mounted units popular on motorhomes and caravans - I feel this is a risky proposition on a narrowboat and every tunnel and low bridge would be terrifying!)

[mrsmelly]

 

[Tony Brooks]

@Lori2025 Have you thought about how you are going to supply the electricity? Not an issue if you are on shore power, but if not things will at the least get difficult or even impossible. If you intend to run a generator to supply the amount of electricity needed then have you worked out the efficiency of the whole thing, generator plus unit, it is likely to be worse than using direct electric heating.

 

If you are thinking of relying on solar then even with a wide beam AND lithium batteries I think you will have problems in winter, and even more so  on a narrowboat.

 

Please give the electricity supply and battery charging part far more thought.

[IanD]

28 minutes ago, Tony Brooks said:

@Lori2025 Have you thought about how you are going to supply the electricity? Not an issue if you are on shore power, but if not things will at the least get difficult or even impossible. If you intend to run a generator to supply the amount of electricity needed then have you worked out the efficiency of the whole thing, generator plus unit, it is likely to be worse than using direct electric heating.

 

If you are thinking of relying on solar then even with a wide beam AND lithium batteries I think you will have problems in winter, and even more so  on a narrowboat.

 

Please give the electricity supply and battery charging part far more thought.

Running a genny to generate electricity to drive a heat pump is about 4x more efficient (assuming COP=4) than using that generated electricity directly for heating -- however it'll use a similar amount of fuel to burning the diesel in a conventional CH boiler, so no fuel saving but at least 10x the total installation cost... 😞 

 

On shoreline a heat pump makes more sense, but still won't save much on running costs because bollard electricity is a lot more expensive than diesel per kWh of heat, which cancels out the COP gain.

 

There's zero chance that solar in winter will provide enough energy to run a heat pump for boat heating.

Edited March 12 by IanD

[Lori2025]

Perhaps I should have mentioned in my OP that I have an abundant supply of reasonably priced electricity, otherwise this project would obviously be doomed from the outset. I'm not expecting this to be the only heat source for my boat - my multifuel stove will always be important. Also, the ability to cool efficiently is also really important for me personally as I live and work on board. In any case, the electricity is not an issue for me. Finding someone to install a heat pump on a boat really is though. . .

[DandV]

On 13/03/2026 at 04:49, Lori2025 said:

Anyone got any experience with so-called "mini split" a/c / heat pump units on a narrowboat? I'm keen to get one for all the usual reasons (high efficiency heating from electricity, plus the benefit of cooling in the summer, no messing around with diesel, gas or solid fuel etc.) but it seems my options are limited to a R290 (basically propane) unit which can be self-installed but runs the risk of leaking explosive gas if anything goes wrong. The R290 heat pumps are also noticeably less efficient, which would stretch my electrical system. 

 

There are more efficient (and cheaper) units based on R32 gas which are available as an "easy install" version, but they're illegal to self-install in the UK. The issue here is finding anyone who will install it. None of my local (and not so local) suppliers who do units for houses and offices will install anything they haven't also supplied (and guess what, they ones they sell are twice the price) and even then, they're extremely cautious about going near a boat. 

 

Really curious to hear if anyone else has any experiences in this area. Also, does anyone know of an automotive A/C tech who might be up for commissioning an R32 unit on a boat? Or better yet someone who already works on boats? I realise this is a long shot.

 

(p.s. I'm well aware of the roof mounted units popular on motorhomes and caravans - I feel this is a risky proposition on a narrowboat and every tunnel and low bridge would be terrifying!)

As you are on a boat, a split system air to water would provide more efficient heating at any temperatures when the water is warmer then the outside air. 

[GUMPY]

On 12/03/2026 at 15:49, Lori2025 said:

I'm well aware of the roof mounted units popular on motorhomes and caravans - I feel this is a risky proposition on a narrowboat and every tunnel and low bridge would be terrifying!)

It was never a problem on Parglena, slightly bigger than a narrow boat so closer to all the low bridges and tunnel roofs. 

 

[David Mack]

On 12/03/2026 at 15:49, Lori2025 said:

p.s. I'm well aware of the roof mounted units popular on motorhomes and caravans - I feel this is a risky proposition on a narrowboat and every tunnel and low bridge would be terrifying!

Most narrow boats have plenty of space between the cabin top and the underside of bridges and tunnels - as demonstrated by the number of dog boxes, storage boxes, solar panels, sacks of coal, piles of firewood and other stuff which you see on the roofs of many boats that move regularly (as well as on boats which rarely move).

Edited March 19 by David Mack

[Cheese]

The outdoor compressor unit would usually (i.e. on a house) be bolted down to something solid, typically concrete, in order to reduce both noise and vibration. That wouldn't work quite so well on a narrow boat.

[Lori2025]

3 hours ago, GUMPY said:

It was never a problem on Parglena, slightly bigger than a narrow boat so closer to all the low bridges and tunnel roofs. 

 

Thank you for sharing that. I may consider roof mount if it proves impossible to get a mini split installed

[Technical Tony]

As Cheese has mentioned, finding a suitable location for the outdoor unit might be an issue unless you have a decent size cruiser stern.

The other issue will depend on how much you actually move your boat. Mini split systems are designed to stand still and not move so will not tolerate a lot of vibration. Decent AV mounts will help and will reduce noise transfer.

You say that the units installers will supply are a lot dearer but are you comparing apples with apples? Before I retired (HVAC Engineer), quite often I would get a call from someone who had bought a cheap unit online and wanted me to install. I always refused, and it's not about the money. The cheap units are cheap for a reason, and when (not if) they breakdown, parts are not available. There is also the issue with warranty, I could warrant my install, but you can bet that if the unit fails, the customer will expect me to repair free of charge!

Mitsubishi, Daikin, Hitachi and Toshiba have been in the business for years and their products have the technical support required. Many, like Glowworm, Worcester Bosch and Vaillant did not make heat pumps and rebadged other manufacturers. At the time I retired, those companies could supply and install a unit but had no technical expertise to diagnose any breakdowns if it failed under warranty, which was a good earner for me!

The pipe installation requires specialist tools, (don't ever consider premade pipe runs, the copper used is inferior and excess pipework ends up being coiled somewhere) pressure testing at quite high pressure (42 Bar), and then the indoor unit and pipework requires a deep Vacuum to ensure no moisture remains in the installation. New legislation probably prohibits the outdoor unit being shipped with any refrigerant and the required charge would need calculating and weighing in. ( Units use to be sold with a charge sufficient for up to 15 Metres pipe run).

The other issue which no one seems to point out to gullible house owners when they are being brainwashed by the efficiencies, is the monumental cost of parts if the unit fails outside of warranty. If the compressor fails, that used to require new control PCB's be fitted as well or the new compressor would have no warranty.

Heat pumps DEFINITELY have their uses but a lot needs to be considered before spending large sums of money.

[Up-Side-Down]

I would suggest (taking account of IanD's comments) the tried and tested way to go is with a Kensa Shoebox water source heat pump – 

kensa.co.uk.

 

Several examples already up and running on the inland waterways and the new FCCT wide-beam at Edinburgh is just about to have one fitted.

[IanD]

2 hours ago, Up-Side-Down said:

I would suggest (taking account of IanD's comments) the tried and tested way to go is with a Kensa Shoebox water source heat pump – 

kensa.co.uk.

 

Several examples already up and running on the inland waterways and the new FCCT wide-beam at Edinburgh is just about to have one fitted.

A couple of things to bear in mind...

 

The Kensa Shoebox 3kW is probably too small for a narrowboat, the 6kW is probably a bit too big. CoP is around 3.5 or a bit less, but also this is with an output temperature of 35C which is far lower than a conventional boiler, and will mean much bigger radiators than normal are needed -- with 55C output (still lower than normal so needs bigger radiators) CoP drops to about 3.0. This puts heating cost from shoreline somewhere over 10p/kWh, depending on the price charged by the marina.

 

Still cheaper than diesel though, 1l of diesel has around 10kWh, a typical diesel boiler is maybe 80% efficient so that's 8kWh really, which at £1.20/l is 15p/kWh.

 

Also note that if you're still relying on the heat-pump for heating when away from shoreline (cruising) and the power has to come from a generator the running cost is higher, 1l of diesel makes about 2.5kWh which allowing for losses ends up as at most 7kWh of heat, so less efficient than burningthe diesel in a boiler.

 

The other problem with the Shoebox is it's heating only, no aircon in hot weather which is the other reason for installing a heat pump.

[wakey_wake]

I considered heat pumps for a while (steel nb, sprayfoam, on a shoreline) and may still go there some day.

Perhaps a summary of where I got to will help your thoughts?

 

I didn't want an ASHP, split unit or otherwise. Water sourced heat is usualy better, thermodynamically, because you can almost always source heat at 4°C (winter) or sink it at 25°C (summer). Ground sourced heat is better still because it should be steadier at 13°C all year round, provided it's big enough for the job.

 

However, WSHPs seem to be prohibitively expensive. Presumably because they aren't used en masse with houses, as WSHP from running water because it's often not available, or as GSHP because the ground interface is naturally expensive? Less volume, more specialist product, higher prices, then add the word "boat" to boost the price some more.

 

I saw an eco-promo article about some boat which had a compact WSHP and was used as a ?museum or classroom. I think the capital cost was £8k or something! Maybe it was economical in their case, but 8k buys a lot of smokeless fuel.

I looked again and can't find it. This another boat, but maybe the supplier was the same? https://kensa.co.uk/homes/case-study/marine-heat-pump-tulak-barge

 

What I thought I wanted was a beer chiller or some other vapour compression heat pump of a few hundred watts electrical input, coupled to glycol loops on both sides. Something where the f-gas certified engineer has already done the legally protected work and I can safely mess with the plumbing side without the fancy tools or legal risk. They do exist but I didn't find anything in my price range before I gave up.

 

 

On 20/03/2026 at 22:36, Technical Tony said:

finding a suitable location for the outdoor unit might be an issue unless you have a decent size cruiser stern.

(Mine's trad stern - no space there.)

 

Putting it on the roof looks like a recipe for trouble. Periodically a boat may need the roof clearing, whether for painting or low bridges. Anything left on the roof may be subject to the brutality of trees and bridges if you allow someone less careful than yourself 😉 to take the helm. Solid fuel stoves usually have the flue on the port to avoid the trees, but anyway you can remove the sticky-uppy bit any time the fire is cold and if it does get written off by a tree the damage is not huge.

 

Once a split unit is installed (I believe) you don't ever expect to move it. The pipes may bend a little but you're going to work-harden the copper and then fracture them if there is a lot of movement. It's not something you can unplug and then re-install.

 

When I was talking to aircon folks about how to get a WSHP, and they would suggest that ASHP is more available, these are the factors which caused me to ignore that branch of the decision tree.

 

On 20/03/2026 at 22:36, Technical Tony said:

New legislation probably prohibits the outdoor unit being shipped with any refrigerant and

This is the f-gas licence issue. For most folks, having the tools is not enough unless you want to take the exams or break the law.

 

On 20/03/2026 at 22:36, Technical Tony said:

the indoor unit and pipework requires a deep Vacuum to ensure no moisture remains in the installation

Bizarrely (I think) R290 is covered by these regulations despite being f'less.

In addition to excluding moisture, you have to ensure there is no oxygen. 💥

 

 

Nearby and recently we considered Peltier heat pumps, and note how appallingly inefficient they are.

 

I did consider a Peltier WSHP system, because at least I could make that myself. For heating the theory is sound provided you have plentiful and cheap enough electricity: if you're going to run a fan heater or oil-filled radiator anyway, then the terrible COP of Peltiers is a minor issue. Even a low COP of 1.1 means that for each kilowatt you burn to make heat, you get another 100W "free" from the water. That seems to make them somewhat more useful for heating than for cooling.

 

When considering WSHP on a boat, one must be absolutely certain that no combination of leaks or failures can result in the onboarding of that water you had been floating in, because you'll sink. For me that suggests bringing the water source into the well deck only, and then a heat exchanger or the WSHP unit itself to be outside (in the cold); but compare the risk profile of a raw water cooled engine.

 

What did I do in the end?

I already knew that under 1kW was usually plenty to keep the cabin temperature over 16°C, at least up one end. 2kW is useful when starting from cold but it's not needed. I don't think I want a 5kW output solid fuel stove.

I found that heating one end of a living space will cause all the condensation to jump to the cold end. A curtain may help, but then

I found that a 400W rotary dehumidifier can be quieter and more useful than a fan heater.

I found the body of water stabilised the boat temperature significantly against outside air temperatures, especially in late autumn. Remember that it's 4°C not far below the surface, and half the boat (including baseplate) is dunked in that.

I found I'm not so bothered by the cold, at least down to 8°C cabin. Also that the fridge stays cold by itself on those days.

I found the number of days below 8°C was actually not very many, in recent years.

I found that any sort of electric heating blanket is very effective for many fewer watts - such as https://www.ebay.co.uk/itm/397534964708 which is horribly over-priced but apparently comes with a very lovely free book. There are 12V "trucker" electric blankets too.

I found I have other options than the boat, for places to be in the cold spells.

 

I was lucky / YMMV!

 

Have I given up on heat pumps for boat heating? Practically speaking, probably yes at least for a while.

In theory, never give up! The ideal would be a very small solid fuel fire, producing a small amount of electricity and driving a vapour absorption WSHP into underfloor heating that is well insulated from the baseplate - all without using up your limited headroom or making the cabin bilge damp. Adding a decorative moon on a stick would also be nice.

I'm half joking but all these technologies have existed for decades, even the stick with the Moon on one end and a flag on the other.

The problem is they're not cheap enough to install compared to the other solutions, and they're not effective enough to pay back the difference.

 

This brings me back to the suspicion that the only reason to install such things on a boat is as a publicity stunt. One day it will be practical and sensible but I don't think we are there yet. 😞

[Up-Side-Down]

I believe that the landscape for burning anything will look very different in 10 – 20 years time. It's quite possible that burning wood or coal will have come close to being outlawed and the only obvious sustainable options will be ePropane and HVO. This will, in turn, set the above in a very different light ......

[IanD]

2 minutes ago, Up-Side-Down said:

I believe that the landscape for burning anything will look very different in 10 – 20 years time. It's quite possible that burning wood or coal will have come close to being outlawed and the only obvious sustainable options will be ePropane and HVO. This will, in turn, set the above in a very different light ......

Given the recent scientific discoveries about how damaging to health particulates (PM2.5) from burning solid fuel are, I suspect it won't take 10-20 years -- especially banning lifestyle woodburners in urban areas, which don't even have the "essential heating" excuse.

 

Even then, using heat-pumps for boat heating only makes environmental sense when most or all of the power to run them comes from shoreline connected to a "green" energy grid -- for boats not plugged in during the winter (little solar, lots of heating needed) it makes little or no sense, even with an HVO-fuelled generator.

[wakey_wake]

35 minutes ago, Up-Side-Down said:

I believe that the landscape for burning anything will look very different in 10 – 20 years time. It's quite possible that burning wood or coal will have come close to being outlawed [...]

Sadly I can't disagree, and I do see that you've avoided putting value judgement on that or ascribing any motive.

 

At risk of being sent to the Virtual Pub, I would suggest that a source of winter heat that can be piled up during summer is a wonderful form of security and independence from short/medium term outside influences.

Convincing a population to give that up, after giving up so many other securities and independences, might seem to make them softer to whatever future demands may be made of them.

We don't have winters here like in Ukraine or Russia, but look at what kind of weapon that is!

 

27 minutes ago, IanD said:

Given the recent scientific discoveries about how damaging to health particulates (PM2.5) from burning solid fuel are, I suspect it won't take 10-20 years

Again, sadly can't disagree that health research can and will be used to push that.

Doing health research is wonderful but the results are better taken to inform choices about total life quality given a set of constraints; else the health research can be abused for harmful purposes.

 

Look at one of the "life hacks" promoted in response though: burning some candles under a clay pot.

Can I do that on a boat? Sure. Do I need to run it past the BSS inspector? Not so much.

Will that produce PM2.5s indoors? I presume so. Will the PM2.5s get you before the CO or fire risk does? Maybe not.

Are we going to ban candles as well? 🤦‍♂️ And anything that can be used to make one, including Crisco and toilet paper? 🤡

 

Burning stuff isn't the problem, but it is difficult to make honest incentives to burn stuff cleanly and in moderation.

 

I do believe that it's unhelpful to sound off about a problem without offering something constructive.

How about two or three levels of "solid fuel stoves installed in homes with >40% median income have to reach Euro6 (or whatever) level of cleanliness" ?

Yuppies installing dirty coal fires: bad. Yuppies that had a dirty coal fire installed when they were penniless students: on borrowed time. Families below the bread line: we make an exception for them because any area can tolerate a limited amount of poorer combustion - and has done for centuries.

[Lori2025]

On 22/03/2026 at 17:17, Up-Side-Down said:

I believe that the landscape for burning anything will look very different in 10 – 20 years time. It's quite possible that burning wood or coal will have come close to being outlawed and the only obvious sustainable options will be ePropane and HVO. This will, in turn, set the above in a very different light ......

 

 

Looks like we might not have that long...

 

https://www.bbc.co.uk/news/articles/cy41d7dg1n3o

 

Thanks to all those who have responded and contributed to the discussion. I'm not really any closer to finding an installer but it's been an interesting topic

 

 

[Lori2025]

On 22/03/2026 at 16:57, wakey_wake said:

 

Putting it on the roof looks like a recipe for trouble. Periodically a boat may need the roof clearing, whether for painting or low bridges. Anything left on the roof may be subject to the brutality of trees and bridges if you allow someone less careful than yourself 😉 to take the helm. Solid fuel stoves usually have the flue on the port to avoid the trees, but anyway you can remove the sticky-uppy bit any time the fire is cold and if it does get written off by a tree the damage is not huge.

. 😞

😞

This was my main worry with roof mounting. I get that others have had positive experiences but the fact that once installed it really cannot be moved under and circumstances, it's a deal breaker. 

[wakey_wake]

6 minutes ago, Lori2025 said:

This was my main worry with roof mounting. I get that others have had positive experiences but the fact that once installed it really cannot be moved under and circumstances, it's a deal breaker. 

I only recall seeing one roof mounted split unit, on a large ?Dutch barge or something. I suspect it's perma-moored but it is also well placed for tall bridge holes across a fair stretch of water.

 

I'm not certain they "cannot be moved under any circumstances". This keel-cooled fridge unit https://beardmarine.com/marine-refrigeration/refrigeration-adler-barbour-coldmachine-kits selling in USA says

Quote

ColdMachine kits now com with self-seeing quick-connect fittings for easy attachment. Unlike the old “one-shot” connections, quick-connect fittings can be disconnected and reconnected whenever necessary without loss of refrigerant or exposing the system to air or moisture.

 

I didn't know such things existed.

I don't know how often you would want to do that before losing too much gas or letting in too much contamination. Also I don't know if they are available and legal in UK. In the last couple of years, refrigeration engineers have told me that "pre-filled aircon units" are no longer available here? Maybe I misunderstood.

 

The tip was from another thread, thanks @Mike Coombes

On 24/03/2026 at 22:56, Mike Coombes said:

Refrigeration on serious yachts is far in advance of inland waterway vessels. Most, for efficiency, are keel cooled, extremely well insulated and are top loaders so the cold air does not escape when opened. Many have remote units made by Frigo boat or Adler Barbour and others. This allows the freezer plate to be fitted in a good place in the insulated box. As one would expect when cruising in out of the way places where supplies are hard to get, keeping food for extended periods is essential.

[wakey_wake]

On 12/03/2026 at 16:37, IanD said:

Running a genny to generate electricity to drive a heat pump is about 4x more efficient (assuming COP=4) than using that generated electricity directly for heating -- however it'll use a similar amount of fuel to burning the diesel in a conventional CH boiler, so no fuel saving but at least 10x the total installation cost... 😞

The exceptions seems to be either

• when the installation cost is comparable to the cost of energy input (fuel or grid), as with my "Peltiers might get you 10% extra 'free' scheme" above, or
• when waste heat from the generator can be captured in a way comparable to an Eberspacher, rather than blown down the towpath; then into a decent WSHP

In the latter case you still have installation costs which one might try to keep under 10x, but the running numbers look better.

Burn 99ml of diesel = 1kWh total available thermal.

Lose 70% to the usual ICE inefficiencies of necessary output heat, hot exhaust gas and heat radiating off the machine. Perhaps it's practical to harvest 50% of that without noise pollution and CO risk? 70%*50% = 350 Watt hours heating.

Collect 30% electrical. Lose 10% of electrical to heat in electrical conversions, 30%*10% = 30 Wh heating if it can be done in the right place,

then into the WSHP for COP 3.5 heat = 27% * 350% =  945 Wh heating.

 

Total 1.3 kWh delivered, COP 1.3 which must be compared to any fuel-burning heater which has COP below 1 because they vent hot gas, and Eberspacher-type ones are also pointlessly warming the engine bilge.

 

It's not completely useless, but you can't do it with any petrol or diesel generator I know of. The nearest I can think of with the usual kit is to circulate domestic hot water from the calorifier through radiators and back into the calorifier cold feed (thereby polluting both DHW and cold), and do that fast enough that the thermostat to the skin tank doesn't open. 🫤

 

However the idea of sourcing bulk heat from the boat's baseplate seems unwise unless there is a flow on the waterway.

In summer for cooling it'll be fine, the hot water will rise and at worst you lose some efficiency.

In winter for heating the cold water will also try rise, but I think less strongly for any ΔT around 4°C, and it seems there is some risk of making a boat-shaped ice cube.

[flASH shabby]

Did you go for it?

[colin stone]

What about a single internal wall unit?  No split. Panasonic RAC Solo. And a bunch of solar panels on the roof. 

https://www.aircon.panasonic.eu/GB_en/product/rac-solo-r290-r32/

[MoominPapa]

1 hour ago, colin stone said:

What about a single internal wall unit?  No split. Panasonic RAC Solo. And a bunch of solar panels on the roof. 

https://www.aircon.panasonic.eu/GB_en/product/rac-solo-r290-r32/

Interesting. First disadvantage vs a split system is that the unit hanging on the wall in the living room doesn't just contain a fan. It contains two fans _and_ a compressor. That brings noise. It also helps explain why these things weigh 40kg.

 

MP.

[MtB]

3 hours ago, MoominPapa said:

Interesting. First disadvantage vs a split system is that the unit hanging on the wall in the living room doesn't just contain a fan. It contains two fans _and_ a compressor. That brings noise. It also helps explain why these things weigh 40kg.

 

MP.

 

I agree. The whole point of a split system is to have (some of) the noisy bits outside and not in the room with you. (At increased cost.)

 

Not that they are that noisy when new, but give it five years...

 

Also, in boat the 'walls' are not vertical which might mess with the operation of it. And the Panasonic design might be for walls 9" thick or more, like in a house. Best to check this.