Summer use of immersion heater, or boiler.

[ericmark]

I have seen many times people claiming cheaper to heat DHW with a boiler than immersion heater, and until this year have always used gas or oil, this year for the first time used the immersion heater, used with a iboost+ and sensor on the meter tails so only uses power when exporting, today at 11 pm the unit says energy saved today 0.56 kWh, in other words that's how much energy needed to maintain the DHW hot, this is less than 2 minutes (1.68) run of the oil boiler, since run every other day, so 3.5 minutes, it did run 20 minutes, so using 6 times the energy to heat will oil, so the oil would need to cost less than 5.3p per kWh to break even with electric at 31.31p per kWh. Since I buy by the litre not sure of price per kWh but seems unlikely that low, and if heated overnight or with solar, the fuel cost much lower. I simply can't see how heating DHW with oil can ever be cheaper than electric?

[stevei]

I've just done a Google search asking the average kWh per litre of heating oil in the UK. The answer givern is 10.35 kWh per litre. That would tend to show that your boiler and pipework to the hot water cylinger is hugely inefficient.

[ericmark]

I would agree, the system is not designed to be used for just 20 minutes every other day. It is designed to heat the home, and the heating of the DHW is an extra thrown in. And during the winter having hot 28 mm pipes in the walls and floors not lagged does not matter as it all goes to heating the house, does not matter if heat comes from pipes or radiators, it still heats the house.

I had heard is said so many times, as to how oil and gas were around one fifth of the price of electric, so even with losses still cheaper to use the central heating boiler for DHW in the summer, winter can't turn DHW off, so only talking about summer.

So for 4 years I have run the boiler for half hour four times a week, it does not run for half an hour, after 20 minutes the return hot water turns it off. It was not enough to get the DHW stinking hot as in the winter, but did mean washing hands in warm water. There is no tank thermostat for the boiler, so relied on time control.

When solar panels were fitted, the iboost+ was also fitted, so summer hot water now heated with solar, in hind sight, and hind sight is easy, a simple timer would have been better and heating it when we have off peak supply, as once set up, the export rate is higher than the off peak import rate, however what it did do is show me how much power used to keep DHW hot, so 11 pm last night pressed the buttons, and it says saved 0.56 kWh yesterday, i.e. that's how much it used to heat the DHW through the day.

Most other things I have used a plug in energy monitor with, but the immersion heater does not plug in, so I had not realised how little energy is required when heating with electric. Clearly if I used the DHW for a bath or shower, then there would be a different story, but I don't. The same if the cylinder was closer to the boiler, but direct heating, be it a fire tube boiler like we use a work, or an immersion heater, must have less losses to indirect heating.

A gas geezer at the kitchen sink is likely the cheapest way to heat DHW, not seen one of them for years, the combi boiler where the water store is inside the boiler must also be quite good, but the system boiler heating a remote cylinder of water simply has too many losses to compete with the immersion heater.

[ericmark]

Today a little more energy used 0.69 kWh, but still so well below the energy used to heat with oil, clearly cheaper to heat with electric.

[Neelix]

We have PV and a myenergi Eddi for our immersion heater. Last week our son was home alone and I can see that it took about 2 kWh per day to top up the tank. The tank is large so can do 2 baths plus more per day.

When we refurbished and extended our house the radiators in the en-suite, bathroom and shower room are included in the hot water tank plumbing so the energy to heat the hot tank is far more than people realise. I do not have any accurate figures for gas usage per day so can’t comment on the savings . When our daughter was at Uni and it was sunny , we could go for weeks not using the boiler for heating water.

We don’t get any FIT

[ericmark]

In summer I want to keep house cool, it is not simply the cost of heating DHW but also cost of cooling house, last thing I want is a load of hot pipes heating the house or a massive water store, so having just the top foot of water hot in the tank suits me.

However I feel I have over the years been conned into thinking gas was cheaper than electric. No option in last house, no DHW storage, this house we do have the option, and for first 4 years I have been using oil to heat the DHW in summer, only found out this year how much cheaper electric is to heat the DHW, so kicking myself.

[Neelix]

Got our bill this morning - the cost for gas in August was £3.82 plus 5% VAT plus standing charge @ £8.52 plus VAT

[dewaltdisney]

That's going to be a struggle to pay N now you have lost the winter fuel allowance.

DWD

[ericmark]

Got our bill this morning - the cost for gas in August was £3.82 plus 5% VAT plus standing charge @ £8.52 plus VAT

I can also look at my thermostat and it simply says "no usage" for each day, if your gas bill is for domestic hot water with a storage system, then £3.82/31=12.3p per day, which is clearly very low, compared with my 0.69 kWh @ zero, 8.95p, or 31.31p per day which = zero, 6.17p or 21.60p per day, clearly when the immersion heater is used will change the cost, can ignore standing charger as not real way not to pay it.

The pipework of a combi boiler to tap is the same as pipework cylinder to tap, so not really worried about that, and with a combi boiler one has no option not to use gas/oil/solid fuel for DHW, so really the question does not arise, only with system boilers has one got an option, used gas/oil/solid fuel, or electric. So only interested in the bill where people have an option, so can compare.

Most electrical items in my house plug in, or have a fixed power use, i.e. lights, the items not plugged in are immersion heater and cooker, they are the only items where energy usage is hard to measure, I know my immersion heater is 3 kW maximum, but can't time either how long this is used for, or now how much is being used, except what the iboost+ shows on the display, so until iboost+ fitted I just took it for granted that an immersion heater was expensive.

However the iboost+ now shows me day by day amount saved, it does vary day to day, 0.54 kWh to 0.69 kWh and since not getting any money for export then my bill for DHW is zero, but before the solar was fitted, I paid 29.57p per kWh so at the highest daily use, it was costing me 20.4p per day. And @Neelix is paying less than that.

However my oil boiler ran for 20 minutes 4 times a week, and is rated at 20 kW, measuring the oil used is near impossible, but if no losses at the boiler looking at around 3.8 kWh per day, a google hunt for cost of oil per kilowatt hour shows electric £0.271, electric economy £0.094, gas (mains) £0.072, Gas LPG 0.122, and oil £0.104 for a kWh, that seems reasonably up to date.

So mains gas is the cheapest, peak electric is 3.8 times the price, and off peak 1.3 times the price, yes one can argue some suppliers are over or under those figures, but losses in the boiler and pipework means off peak electric will likely always be cheaper than any of the others, only peak electric is more expensive.

So peak electric v gas it depends on how much DHW is used in the day, as the amount used goes up, so gas gets first same price and electric and then becomes cheaper, and the point where it becomes cheaper to use gas will vary depending on the losses.

We also have a second problem when comparing, in that often electric heats up less water than the central heating, however it would seem, unless one wants a bath, in which case I would have to use oil, as there would not be enough DHW with electric, but other wise electric works out cheaper. So for 4 years I have been using expensive oil heating of the DHW, where electric heating would have cost me far less.

[Neelix]

^^ bear in mind there are 4 adults in our house so hot water usage is high

[ericmark]

My hot water use is very low, 3.90 kWh for 7 days, since past midnight nothing showing for today.