What is the cheapest way to heat water?


If you’re refurbing or building new, what is the cheapest way to produce hot water?

There have been a few discussions of various technologies individually, but it strikes me that there must be a “best” answer for a typical house in the Irish climate.

Air source heat pump?
Ground source heat pump?
Solar water heater?
Solar PV + immersion?
Grid + immersion? (Surely not!)
Something else?

By “cheapest” I mean TCO over the life of the equipment, including energy inputs to run the system (pumps, etc). Ignore time-value of money. Assume a typical house with a grid connection etc.


If you have easy access to timber or turf then I’d say a stove. It doesn’t suit every home. After that Solar collectors. The most cost effective solution is a solid fuel stove, solar collectors and an immersion.

A modern well insulated house will also have enough space heating from that combination.


Thanks. Questions:

Solar collectors = solar hot water (not PV)?

The immersion runs from the grid and is a backup presumably?

And if you don’t want to run a stove, is solar + immersion still the best combo?

Assuming the above, why do people bother with heat pumps?



This is a topic I’m interested in.

Current set up in my gaff is an aged gas boiler (‘Ideal’ brand, late 90s model) heating c. 2000 sq ft (single zone), and an immersion providing hot water for the taps, (pumped) shower etc. Currently these are timed with basic mechanical one-day clocks.

Anyone any experience/thoughts on whether or not a modern combi boiler setup would provide efficiency in terms of gas usage on the heating, and over the immersion for hot water?

Or would I be better doing some draft proofing (there’s quite a bit of exposure at the moment through external French doors etc) and insulation upgrades in the interim and examining the bills over the next winter?


Large vacuum flask and bring it home from work. After the initial capital cost, it’s entirely free (if sneaky). :smiling_imp:


I looked extensively at this for our big fat renovation.

Our solution

  1. Solar water heating
  2. A big 300l insulated water tank
  3. A combi boiler
  4. Pressurise the ststem.

Hot water for showers and the hot tap is 40 degrees. During all but the coldest winter months or persistently dark days we are expecting the solar alone to keep the water tank at or above 40 degrees. If it drops below the combi boiler steps in to bring it up to the required temp on demand.
By pressurising the system the 3 showers in the house and taps should have good pressurised hot water all of the time.

We will not have an immersion heater.
While we will have a wood burning stove it will not have a back boiler as we donot antisapate it being required. It also obviously sucks a lot of heat energy from the stove which will be vented from the water tank if it’s excessive.

I’ll let you know how we get on.


Thanks Coles, really useful. Just to clarify, when you say gas do you mean just gas, or solar + gas? I presume the latter.


Thanks TI, but what do you mean buy “pressurise the system”? A opposed to what?




For hot water? Anywhere I’ve lived the hot water tank has been lower than the shower heads.


We went with this combo in our new build constructed last year. December was really dull with minimal solar gain so most of the time the water was being heated from the gas boiler but averaging out the 9 months so far our gas bills, which include underfloor heating, have been very modest. Really happy we went with this type of set up as it means we are not letting the hot tank cool down at all and with the pressurised system we have instant hot water on demand across the whole of the house. We do have an electric emersion aswell but purely as a belt and braces backup.

And because the hot water is always there it has even motivated me to reduce use of dishwasher and do more hand washing which has proved surprisingly therapeutic.


The head height from the tank in the attic is acting on the hot water cylinder to pressurise it


What he said.

The advantage of pressurizing your system is that you get a good pressure through the house regardless of the height or location of your showers relative to the vertical location of the hot water tank.

It’s the type of system they use in large buildings like hotels etc


not just large buildings. lived in a flat in england that had one. very popular in europe (sometimes called megaflow) but less so in uk/ireland because it’s more expensive to fit / maintain and builders always choose the cheapest option.

its basically an unvented system so the hot water runs at the same pressure as the input main (an expansion vessel maintains pressure as the water heats) which means you do need good mains pressure (so don’t forget to pay the water bill…) but it’s utterly fantastic - no need for noisy expensive pumps, instant hot-water at every tap / shower etc.



@Terra, That’s an ideal system for an urban dwelling. With all my projects I use pressurised systems.


Out of curiosity what would it cost to retrofit solar + gas as per TI’s system, ballpark?


I’d guess €6-7k incl VAT depending on size of solar collector. It might also be necessary to modify a room/hotpress to take the cylinder.


Less than I thought.


The reason unvented systems are rare in Ireland and the UK is because what you have described is illegal. That’s not to say it’s not done, or its necessarily wrong to do.
It’s required to install a break tank, and to put a pressurizing pump at the outlet - giving around 2 bar to both cold water feed and feeding into an unvented domestic water tank.
On the continent you can plumb directly into the mains as long as you install a check valve. If you are going this way better to use a stainless steel tank rather than enamel.
The Irish / British ‘fix’ is to put a shower pump at the outlet of the domestic hot water tank to pressurize both hot and cold water simultaneously via twin impeller. These pumps are noisy, epensive and less reliable, so the other solution is better, if you have space for a break tank (1m3).