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“I can’t imagine a whole monobloc install would be cost effective compared to just a HPHW tank.”
Photo Credit: iStock
A U.K. cabin builder caught in a conundrum while choosing a domestic hot water system for a small, well-insulated cabin took to Reddit for answers.
The debate in the Reddit thread about whether a ducted heat pump water heater or a split system made more sense offers a useful lesson for homeowners comparing a heat pump water heater with a simpler tank-based option.
On the Reddit forum r/ukheatpumps, the poster described a “small, well insulated cabin” that already relied on two air-to-air heat pumps for space heating.
For homeowners replacing electric resistance hot water, the appeal of a heat pump water heater is fairly simple. A heat pump can use far less electricity to deliver the same amount of hot water, helping reduce energy bills over time. In the Reddit discussion, the original poster put that logic plainly, saying it “pains me to turn electricity directly into heat when I could be getting 3x the heat with a pump.”
They were trying to decide whether a ducted Vaillant aroSTOR 71-gallon (270-liter) hot water unit costing about £2,500 (around $3,300) made the most sense, or whether a split setup with an outdoor condenser could deliver a better coefficient of performance (COP).
Most of the feedback framed the pursuit of a better hot-water COP as not worth the added hassle. Commenters said a split system would likely add expense and complexity without producing enough real-world savings to justify it.
One commenter wrote: “Hot water COPs are poor because of the high temps involved. I can’t imagine a whole monobloc install would be cost effective compared to just a HPHW tank.” 
Another point raised repeatedly was that dishwashers and washing machines often contribute less to hot water demand than buyers expect. Commenters also warned that a large tank can become a drawback when use is light or irregular, citing oversized cylinders and the need for legionella cycles if hot water sits unused for extended periods.
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Producing hot water is an entirely different job for a heat pump than space heating. Because these systems usually operate most efficiently at lower temperatures, making hotter water for taps and showers typically reduces efficiency compared with normal heating use.
With only two occupants in the cabin, some commenters suggested simpler options such as a smaller unvented cylinder or an immersion-based setup. Others said that if the cabin will only be occupied occasionally, easier maintenance and lower upfront cost may matter more than a marginally better COP on paper.
More storage is not automatically better if a household’s hot water use is relatively low. A larger tank can mean extra wasted energy, more floor space devoted to equipment, and more oversight than the system actually needs.
By the end of the thread, the original poster appeared to be leaning toward the simpler route, with ducting for a dedicated hot water unit looking easier than a more elaborate installation.
For shoppers weighing water-heating options, it often makes more sense to start with real-life usage than with headline efficiency claims. Map out occupant count, number of showers, occupancy patterns, pipe runs, electricity rates, and whether solar or batteries are part of the setup. Those details can matter more than marketing claims about peak efficiency.
For households that do want a smarter heat-pump-based option, Cala is one company worth knowing. Cala makes smart heat pump water heaters designed to respond to household demand patterns rather than heating a big tank the same way all day. Its customizable smart heat pump water heaters help homeowners decrease their energy bills by heating water exactly when it’s needed.
That kind of control could be especially helpful for people trying to balance hot water production with rooftop solar, battery storage, or time-of-use electricity pricing. Buyers looking for that kind of approach may want to keep an eye on Cala alongside more conventional tank and split-system options.
The original poster seemed to reach a similar conclusion: “Yeah I think the cost/complexity of a split system vs cutting two vent holes makes it an easy choice.”
If you’re weighing similar options, the articles below look at the same tradeoffs around water-heating efficiency, upfront cost, and installation complexity. They cover heat pump water heater pricing, a U.K. heat pump journey, and the benefits of tankless systems.
• For homeowners weighing upgrades, heat pump water heater costs often decide whether efficiency pays off.
• In the U.K., one homeowner chased a 400%-efficient heat pump before the installation tradeoffs piled up.
• For some households, a tankless water heater can cut standby losses and trim bills.
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© 2025 THE COOL DOWN COMPANY. All Rights Reserved. Do not sell or share my personal information. Reach us at hello@thecooldown.com.

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