What is the cheapest way to heat water?


Lower temp => longer steeping time


Just re-reading this thread and this struck me:

I thought that was a dangerous temperature to hold water at?

From hse.gov.uk/healthservices/legionella.htm


for geothermal etc you typically store it at lowish temps but briefly raise the temperature once a week for the reason you mentioned


Have solar hot water tubes for last 5 years or so, they work great about 4-5 months of the year (and odd time here and there thruout the rest of year) with no lack of hot water for baths, showers etc

I think they paid themselves off by now, i need to sit down and do the maths, in that time only 1 of the 40 or so tubes broke (lost vacuum) and was replaced for free by installer.


Do you have any data on their performance in the winter? i.e. is there some kind of a smart controller that gives you numbers?

Also how do you know if a tube fails? Do you have to inspect them or does something alarm?


Yeh there’s a controller attached to the wall in kitchen that came with it, shows temperature at about 5 different points in system (collector, bottom of 300l tank, top, somewhere else i cant remember), I haven’t really got to play with it to be honest, there’s few settings on it like holiday mode etc

You know a tube has failed at a quick glance since they are usually deep blue in color, and one was silvery/white few years back, installer came in for free and replaced the tube, apparently they replace up to 2-3 for free but forgot to mention during the sale :smiley: anyways hes a good guy and a reputation to protect to which I can relate. Price back then was €4-4.5K I think but that included a whole load of unrelated plumbing, pressure system, expansion tank and the 300L 3pipe cylinder. I presume costs have come down since seeing how these are manufactured in China for feck all.

So far it has fared much better than I expected being located on a very windy hilltop location in the west with incredible gales at times

Tho’ in winter we still have to use oil for home heating and heating up the water most of the time. It works fine on overcast days, its more the misty constant drizzle we get here in West that makes it stop working, i presume someone in Dublin should expect much better performance seeing how yee get half the rain of the west coast, you get a few surprises in winter with the water being hot lol

Currently thinking of putting in a another stove to heat the water in winter since we have no lack of turf and wood, might be the most cost efficient way, tho heating oil has come down thankfully in price lately. The house is well insulated so do not need much to heat house and water thruout the year.

I haven’t looked into geothermal, there was no room for underfloor heating during the build and the ground is very rocky, if anything a bloody windmill might do wonders here if they weren’t so expensive!




Go for a stove with backboiler & have a look at otherpower.com/.


Has anyone looked into these, or had any experience with them? Thinking of putting one in a new kitchen. The reviews are certainly positive…

watersave.ie/product/eht10-e … p/#reviews


I installed a Solar - Stove heating system after moving in to my place three years ago.

the house had a decent enough 5 zone underfloor heating system in place, oil fired and open fires.

The first winter was an eye opener, we used approx 3000l of oil to keep the place warm and hot water. 3200sqf house

The system we installed incorporated:

  • 90x Kingspan solar tubes
  • Heatmister mainfold for the underfloor with digital thermostats in all rooms
  • Inis Meain MKII Boiler Stove
  • Firebird Heat Exchanger (since replaced with a Heat Genie)
  • 300l Joule Hotwater tank
    -800l Buffer tank
  • tripling the insulation in the attic

It took about 9 months of tweeting to get the system working properly as we uncovered other plumbing issues and when under pressure small leaks etc.

The first full winter with it running properly our oil consumption feel to less than 400l and used about two pallet loads of logs in the stove, giving a fairly decent saving.

It was worth persevering with the plumber to get the set up correct and now it works without much thought.

During the winter the underfloor is kept on consistently, this will draw from the buffer at much lower temps and on a bright winters day the solar will heat enough for this, the stove in the evening tops up both tanks and only if there isn’t enough hot water in the system from the stove or solar will the oil kick in. (we also have an electric emersion but never use it)

At the moment we have almost permanently 300l of water at 60+ and the buffer between 40 & 60.

I will be replacing all the window in the house this autumn and new external doors.

It wasn’t cheap to get set up, and I don’t have real data to go by (I do have an SD card recording everything but haven’t looked at it - the plumber uses that); the only gauge I have is that it is saving me approx 2600 litres of oil per year.


Fantastic thread!

Im in the middle of some renovations at the moment and looking to upgrade my hot water systems to get away from the current electric showers in place.

Any feedback on this proposal?

Existing gas boiler
300L joule triple coil tank (solar a future upgrade)
400L break tank and pump on mains supply
Stove with back boiler - heating rads and HW tank

What are the energy costs to keep hot water without having solar and infrequently using the stove compared with electric showers? Better to heat for an hour in mornings and evenings?

Feedback appreciated!


Sadly I have been told that solar hot water does not make sense for my new place, due to a combination of awkward plumbing routes and lack of space for a big cylinder. Looks like we’re going to go the other way and ditch the cylinder altogether for a combo boiler.


@ Wellboy - I have the same 300L Joule Triple coil Cylinder, note if you want the ECO version (Extra few mm of insulation built in to reduce heat loss) then they are made to order - so allow some additional time in your schedule for that.

If your using your Stove back boiler to heat water and rads and thinking of integrating it with your existing Gas boiler setup you will also need to consider adding in a Heat Genie / System Zone from Syslink (Syslink.ie) and maybe add an additional heat bleed radiator as you need some method of controlling the heated water coming off the boiler.

I also went with Stelrad Radical energy saving radiators when re-fitting my home - they are apparently the latest and greatest in rad design (yet look almost the same as a standard rad), they suggest they are 10.5% more efficient, produce more radiant heat than traditional radiators etc, also the flow in/flow out are located in the centre of the rad, look neater, and if you decide at a later date to change the rad to a shorter/ longer rad, you don’t have to move any pipes, you just swap the rad over. stelrad.com/plumbers-install … s/radical/


Thanks for the info Gleo - I have a danfoss zoned temperature and water controller that i’m hoping to re-use to save costs. Hope this is feasible?

Where did you source the Stelrad radiators in the end? Any ideas on cost?


resuscitating this thread after a bit, @Mantissa what did you go with in the end? (and are you happy with it?)

in renovation mode atm and appear to be stuck in gray area where a combi won’t work (water pressure too low, demand potentially too high) but don’t really want to go with a traditional vented cylinder + boiler.

is a pressurised system with a storage combi (no cylinder) a practical / cost-effective solution?


Sorry for the delay, don’t log in much any more.

I love the combi boiler. It feels to me like this is how hot water should be done; you want hot water, you turn on the hot tap. You don’t want any more, you turn off the hot tap – it “just works”. The idea of going back to immersion would feel a bit like going back to cooking over an open fire :smiley:

The fact that we were able to ditch the cylinder and reclaim the hot press as a much-needed coat closet is also a definite bonus. Boiler was sized by the plumber based on the number of bathrooms, etc. We still have a cold water tank that feeds the boiler (I presume?) and a pump for both hot and cold water so no water pressure issues.

We’ve been in for over a year now; only maintenance has been that I needed to re-pressurise the system once (took 5 mins once I watched the instructional YouTube video) and it’s now due the annual boiler service. It’s also hooked up to my Nest thermostat for the central heating.

I suspect this is not the cheapest way to heat water, but it certainly provides an excellent user experience.

tl;dr: Combi boilers are the shizzle.


I totally agree. Modern combis get the water up to temp really quickly (there used to be quite a lag: tepid, warm, hot. Insulated or not, a storage vessel can’t but leak heat > money. Heck, you’ve an expansion pipe located on the top of them vented to outside the building or attic.

I fed mine from mains. Great pressure and no pumps to hear/worry about. Sure, if there’s a water outage - but it’s not like a tank is going to provide much by way of water to a family in need of bathing for long.

I’ve a Worchester, which, if using their Wave controls (which takes it’s outside temp data from the web rather than a probe) brings with it a 9 year parts and labour warranty. You’ve to get your boiler serviced annually by their service eng but still a good deal. There was something like 1200 euro to be had back in grants which puts a dent in the cost of installation.

It’s probably the cheapest way if a straightforward upgrade is desired.


Related question…is there such a thing as an A rated water cylinder (for use with a standard gas boiler)? I’m not finding one on Google :cry:


Anyone get this hot water heater/dispenser or other brand?

It’s not as efficient as claimed probably but if you have PV power it might be OK as doesn’t need a power surge which would dip into the grid?

Fitting a timer at night would help…

But then if you’ve an induction hob heating water only takes a jiffy anyway???


Really great thread.

Earlier there was mention of solar collectors vs PV panels for heating water. (New to this topic so forgive the basic questions). Is there a reason why PV panels can’t or shouldn’t be used for heating water? We have an electric vehicle and would ultimately like to power it, at least partly, with solar so I’m wondering if we should get PV panels that could do a bit of both (water and car)?

Asked a plumber about getting a new cylinder as we rarely seem to have hot water, except in depths of winter when heating is on a lot. The cylinder is rubbish, loses heat in an hour or two, so we use the electric shower a lot. He suggests a Mega-flo system, also mentioned earlier in this thread.

**I’m now leaning towards getting the megaflo system (unvented system, pressurised to 3 bar), and then the following year adding solar collectors. This would, I hope, give us solar-heated, high pressure water in summer and then the oil boiler would pick up the slack in winter. **

Main fear is going with solar thermal collectors and then wishing we’d gone with PV in a few years. (But I read somewhere that running an EV on solar would require a huge surface area of PVs unless the tech improves fast (which it is)).