Immersion or Gas for summer hot water?

[dishman]

I have a very old but reliable Gas boiler. Let's just say it's efficiency is perhaps around the 50% mark.
Now, I was pondering if it was worth switching to an immersion for summer hot water only heating.

But from what I can see, even at 50% efficiency, it is still cheaper than electric.

Gas cost for 1 m3 unit:

Electric cost. 50% of the gas use in kwh:

What surprises me is that, that would mean that current gas tariff, taking into account a 50% efficiency penalty is still cheaper (just) than my pre-april tariff from last year.

I think I have all the tariff prices correct, but I have not included VAT etc. Just raw tariff prices.

Am I correct? Or are my calculations off.

I guess I have to balance against this the extra inefficiency of indirect heating of the tank vs a direct immersion which may heat 50% of a 100l tank. So perhaps a real world test is the best attempt to see what is potentially cheaper.

 

[ericmark]

Not quite that straight forward, as there are losses from the pipe work, I have same question, I have used the oil boiler up to now, simply timed, 20 minutes 3 or 4 times a week, so at 19 kW that is about 20 kWh, which is not enough to worry about, and ensures the boiler runs regular.

Electric will for me have a lower legionnaires risk, as uses a thermostat, so likely gets hotter, but am sure it will cost more.

 

[ianmcd]

Your boiler will be a lot more than 50% efficient, if you say what you have I will tell you its efficiency, Gas will be cheaper than using electric

 

[dishman]

Your boiler will be a lot more than 50% efficient, if you say what you have I will tell you its efficiency, Gas will be cheaper than using electric

Trust me it won't

It's a Glowworm Spacesaver MK2 circa 1975. Bulletproof reliability.....but full of bullet holes in terms of efficiency....I think I'm being optimistic at 50% efficiency to be honest.

I've been hypermiling "the beast" this winter to be as efficient as possible. I've done pretty well.

Not quite that straight forward, as there are losses from the pipe work, I have same question, I have used the oil boiler up to now, simply timed, 20 minutes 3 or 4 times a week, so at 19 kW that is about 20 kWh, which is not enough to worry about, and ensures the boiler runs regular.

Electric will for me have a lower legionnaires risk, as uses a thermostat, so likely gets hotter, but am sure it will cost more.

In my situation the hot water tank is positioned directly above the boiler, the floor above. Very little pipework between the two. It is gravity circulated though....which I guess saves on electric.

 

[ianmcd]

Trust me it won't

It's a Glowworm Spacesaver MK2 circa 1975. Bulletproof reliability.....but full of bullet holes in terms of efficiency....I think I'm being optimistic at 50% efficiency to be honest.

your boiler is 65% efficiency but that does not take into account pipework losses and as the HW cylinder is directly above the boiler , yes good chance you have Gravity HW, so minimal pipework will still be cheaper than electric

 

[dishman]

your boiler is 65% efficiency but that does not take into account pipework losses and as the HW cylinder is directly above the boiler , yes good chance you have Gravity HW, so minimal pipework will still be cheaper than electric

Ahh, yeh, It's certainly G rated. It's actually a Spacesaver 45-60, non-fan assisted. I was getting mixed up with something else.

But I think the G rating classification is 65% "and below". I think they give up trying to give you specifics at that point. But based on anecdotal comparisons and years of monitoring. I'm pretty sure it is less than 60%.

However, back to the question at hand, it seems, based on current tariffs, even if it was as low as 50% efficient, it would still be much cheaper per kwh compared to electric based on current tariffs. What surprises me is that it is still just about as cheap (give or take a few pence) as electric based on pre april electric tariffs from last year.

I may try and run an experiment, see how much the immersion costs to heat 1/4 of the tank from cold. (I think it is only an 11" immersion).

 

[ianmcd]

What do you actually use the HW for ? maybe an electric shower and a kettle for washing dishes would be your best answer

 

[robinbanks]

Surely you'll be able to test this just by using the boiler and the immersion (and work out your gas boiler efficiency to boot) for, say, a week and tracking the usage? (per appliance power meter for the immersion rather than trying to use the meter on the incomer )

Using the rule of thumb that gas is a third the price of electric, a 50% efficient boiler setup (taking into account transfer losses) is still cheaper than an immersion. At 33% efficiency they're on par

If you reason 50%, how long will it take to pay back if you buy a small second hand air source heat pump? In summer you should be able to get a COP better than 3, which means it's getting towards parity with a 100% efficient gas boiler

 

[ianmcd]

if you buy a small second hand air source heat pump?

where do you buy these ? and why are thay available ? maybe because they are absolute crap

 

[ericmark]

Surely you'll be able to test this just by using the boiler and the immersion

I have also considered that idea, but how to measure? The immersion heater is on a FCU not a plug and socket, so using a energy monitor is not really possible, I can measure the current, but not time switched on.

The same with the boiler, the drop in oil level is not enough to measure, remember need to temperature correct, maybe a little easier if using gas.

I have my wall thermostat (Nest Gen 3) set for normally 4 times a week at ½ hour each time, but it never runs for ½ hour, after around 20 minutes the boiler cuts out, and before it has auto reset, it has timed out.

I have it set Sat, Sun, Tue, Thur ½ hour each, what I have considered is fitting a timer on the immersion heater and have it come on just after the Sun boiler time to ensure hot enough to stop legionnaires, but not got around to fitting a timer yet.

During the winter the DHW is turned off as it is auto heated when central heating runs, I now have motorised valves on the central heating, as found radiators would get warm even with pumps switched off, I have two pumps, one for lower floor, and one for middle and upper floor, clearly the designer thought if you turn off the pump, the water stops flowing, that was incorrect, found water still circulates, however thermo-syphon over three floors likely more than with only two.

My main problem is when heating with oil there is no tank thermostat, the only thermostat is the one in the boiler, so until Nest Gen 3 was fitted, minimum time was 1½ hour, 7 days a week, I have been advised against fitting a motorised valve to the DHW, as that is how the boiler cools down, but the route to the cistern is through the two floors, which are sheets not floor boards, so not an easy job to run cables, only option would be wireless, and last time I looked around £150 to fit a wireless thermostat to the cistern, it would ensure the water is hot enough, but not save money, so as yet not bothered.

As to heat pumps, I have a 60 amp supply, with three story house, so likely on the edge as it is, also it would need all floors lifting and larger pipework fitting, and larger radiators, as said lifting floors is a problem, so even if a heat pump did save money, it's a non starter.

But the question is also would the saving be enough to be work all the disruption? This is the same with any improvements, to spend £10,000 on insulating the floors to save £60 per year is clearly daft, why the energy surveys ever suggest it, never mind put it on the top of the list I don't know? Maybe it is to point out how daft it is to take any notice of the survey?

My last house had two gas boilers, one for DHW the other for central heating, both fitted in the 80's, soon after the house was built, after we moved out, my son moved in, and one boiler sprang a leak, it seemed prudent to change to a modern combi boiler to replace the pair, we expected a massive drop in the gas bills, but this did not happen, no longer my house, so can't really do any measurements, but two doors were also changed around same time as boiler, and there was a draft with the old doors, so we had expected a massive improvement, the house was much smaller that this one, but the gas bill was far higher than our oil bill. Both homes fully double glazed.

The main change was old house open plan, new house has doors on all rooms. Nothing to do with how the home is heated.

 

[robinbanks]

where do you buy these ? and why are thay available ? maybe because they are absolute crap

I see them on ebay occasionally, usually having been removed because they were sold to some poor schmuck as a suitable replacement for their 30kw gas boiler feeding tiny rads at 80 degrees in a shoddily insulated house that's draughty as...

Can't call a 9kw ASHP crap for not being able to function in a setup that's way outside its operating spec

 

[robinbanks]

I can measure the current, but not time switched on.

You can measure the temp rise of the tank and assume the current flow is constant? You can buy a better power meter that tracks the consumtpion over time rather than just an instant reading?

As to heat pumps, I have a 60 amp supply, with three story house, so likely on the edge as it is

I don't understand the assertion there. An immersion heater that consumes 3kw of electricity generates 3kw of heat. An ASHP that consumes 3kw of electricyt produces 9kw of heat (say, with a COP of 3). Both appliances consume the same amount of electricity and if your house can support a 3kw immersion it can support a 9kw heat pump. When you read marketing literature that says that a heat pump is 9kw, thats the output, not the input.
This is the thing that levels ASHPs up with gas boilers to some extent; electricity is 3x the price of gas, but ASHP use a third of the electricity that a gas boiler does gas in heating the same volume of water by the same amount.

I'm also not advocating the OP rip out all their floors and do whatever work you're envisiging requires ripping out the floors (wholesale swapping the entire heated water needs of the house over to an ASHP?). The query is essentially around using a gas or electric to heat water, and I'm pointing out that a more efficient, cheap way of heating the water with electric during the summer may achieve a cost saving over persisting heating it with gas depending on various factors.

 

[Rodders53]

I have also considered that idea, but how to measure? The immersion heater is on a FCU not a plug and socket, so using a energy monitor is not really possible, I can measure the current, but not time switched on.

Kerosene / Oil boilers only way to measure HW heating use is to have a meter e.g. https://www.bes.co.uk/domestic-heating-oil-tally-meter-10mm-19438/ installed. Then you can do the same as the OP can do via his mains gas meter (and some sums wrt calorific value).

I know my kerosene use for HW only in the summer is negligible in the grand overall scheme of my boiler's consumption.

Electric clamp on energy meters are readily available and you have the skills to fit one temporarily (or in the CU on the immersion circuit)... can even get multiple clamp units to measure many circuits - e.g. oven, hob, immersion, hot tub, EV point)

Cost of such measuring devices probably will never be recovered in energy savings. Especially as we can make good estimates as to real life consumption from maker specs often enough.

Super-Lagging the existing HW tank and pipework will reduce losses for both electric and gas fired heating of the water.

This boiler install might be one where the install cost vs gas savings will have a fairly quick payback time. However, will any new boiler have the reliability and longevity?

 

[ericmark]

Yes I have a clamp on but they don't measure time, I also have plug in energy meters which do measure time, so anything on a plug is easy. I am sure if I wanted I could connect an old electric clock to the immersion heater to measure how long the built in thermostat has turned it on for, or simply measure size of tank, and use the on line calculators to see how many kWh it requires to heat the water, as loses not that much.

However to measure the oil or gas used not so easy, could connect an old electric clock to boiler, to record running time, but know it runs for approx 20 minutes as it is 19 kW so weekly power = 19/60x80 = 25 kWh per week. For electric that is around £8 per week, but less losses with electric, so less than that, and for oil/gas looking at around £3 a week. So the difference between them is something like £2 a week, is it really worth the effort to work out which is cheaper?

With my mothers house with a modulating gas boiler it was impossible to measure how much gas was used for domestic hot water, same with my old house.

 

[robinbanks]

I always wondered if those pass-through-plug power meters that do do kWh were just internally a CT clamp around the passed through live..