Use of immersion heater in summer v central heating for DHW?

[ericmark]

I was always lead to believe since oil and gas costs less than electric, there was no debate, oil or gas is cheaper for heating DHW.

However when I got solar panels, I also go an iboost+ and moved over to heating DHW with surplus electric before it is exported, and this time of year I would normally check oil level which was normally below half full, i.e. below the bracing bar by a few inches, this year around the same amount above the bar. This does not prove anything, but it did do something dangerous, it made me start thinking.

Winter I can't stop the oil heating the DHW, and would not want to, as there is no real surplus of sun, and any heat escaping from pipes, is heating the house, so not wasted. Debate is summer only.

So on a C Plan with I think 22 mm pipe work, boiler is two floors lower than hot water tank, pipe work not lagged, and no access to lag it, no thermostat in/on tank so water heated by running boiler 4 times a week for ½ hour each time, it actually runs for 20 minutes before the return water temperature turns off the boiler.

So it would seem 19 kW x 4 / 7 / 3 = 3.6 kWh per day average to keep DHW hot with oil, I looked a few minutes ago and the iboost+ says saved 0.24 kWh today, since not midday (1 pm) yet I would assume around 0.5 kWh per day, so it seems I am using around 7 times the energy to heat the DHW with oil as to with electric. This seems to be where the argument oil/gas v electric falls over.

So electric heats around top 9" of water in the tank, and the oil heats the whole tank, but unless having a bath, the 9" is good enough. If I used gas my smart meter would tell me exactly how much gas used, but with oil, I can't really measure accurately enough. But it seems clear it is not a case of saying off peak electric costs 8.95p per kWh and oil costs 5p per kWh as more oil is used to heat up more of the tank plus the pipework.

So 7 x 5 = 35p per kWh v 31.31p per kWh for peak electric, so even without having an off peak supply the maths seem to show heating with resistive electric is likely no more expensive to using gas or oil.

However I remember around 15 years ago now, my daughter turned down the water temperature for DHW in her house, to save on gas used, this resulted in gas temp being lower than electric temp set, so electric heated her DHW resulting in a huge bill. However did not look at how much gas bill went down. But this seems to show electric DHW costs more than gas DHW.

Clearly not every house has the DHW tank three floors higher than the boiler, but even my dads house, the boiler was in kitchen back of house, and DHW tank in space formed by stairs at front of the house, and the distance resulted in 32 mm pipe work. When replaced with a combi boiler then distance kitchen tap to boiler very much reduced, from the kitchen tap to hot water tank, so I am sure gas in that case was cheaper, and once a combi boiler fitted no real option but use gas/oil. I am talking about a system boiler with a hot coil heating the DHW tank or an immersion heater in same tank.

So if one has a C Plan boiler system, is it worth running it in summer?

 

[SUNRAY]

Oh dear.
Previosuly I've made comments about the cost of an instant electric shower Vs the cost of gas heated from a combi and found them to be comparable.
All of those claiming I'm wrong habitually refuse to consider the ineffficiencies in the remote boiler set-up, it's good to see someone else addressing those ineficiencies.

Yes I would expect the cost of heating a tank of water to be significantly higher with an electric immersion heater than gas or oil immersion heater.

 

[JohnW2]

Oh dear. .... Yes I would expect the cost of heating a tank of water to be significantly higher with an electric immersion heater than gas or oil immersion heater.

In terms of grid-supplied electricity, that's obviously correct.

However, if the immersion is powered by ('free') solar-generated electricity, then the only effective 'cost' is presumably the lost payment for exporting the electricity and/or the cost of grid electricity used for something that could have been supplied by solar had it not been supplying the immersion - so the calculation is not straightforward.

 

[SUNRAY]

In terms of grid-supplied electricity, that's obviously correct.

However, if the immersion is powered by ('free') solar-generated electricity, then the only effective 'cost' is presumably the lost payment for exporting the electricity and/or the cost of grid electricity used for something that could have been supplied by solar had it not been supplying the immersion - so the calculation is not straightforward.

Not anywhere near what I was commenting on. Purely responding to Erics post to do with imported costs.

 

[JohnW2]

Not anywhere near what I was commenting on. Purely responding to Erics post to do with imported costs.

OK - but, as far as I can see, eric was talking about the cost of heating the top 9" of water in the cylinder with an immersion heater compard with with heating the entire tank of water with oil or gas - and I can well believe that, even at today's import prices, the former may be cheaper - but only because a lot less water was being heated

 

[ericmark]

Yes 9" and whole tank, that's the way it is set up, no way to change it. However I think losses are more due to pipework between boiler and hot water tank, and the boiler its self.

At the moment electric is clearly the cheapest, not sorted out payment for export, but even when that is sorted out, the iboost+ shows amount saved which is how much energy it has used each day to maintain the water temperature, and the boiler does not modulate, so run time shows the energy used.

Oh dear.
Previosuly I've made comments about the cost of an instant electric shower Vs the cost of gas heated from a combi and found them to be comparable.
All of those claiming I'm wrong habitually refuse to consider the ineffficiencies in the remote boiler set-up, it's good to see someone else addressing those ineficiencies.

Yes I would expect the cost of heating a tank of water to be significantly higher with an electric immersion heater than gas or oil immersion heater.

That also needs to account for how much the electric shower actually uses from the grid supply, I note often we are only importing 6 kW when having a shower, the solar and battery is supplying the rest.

I know my mothers house changed from a power shower, to a direct fed from boiler shower, the time for the water temperature to stabilise would mean around twice as much water used, it would start cold of course, get hot, run out of boiler reservoir, go cold again, then warm up second time, and you could then step under it. If compared with instant electric then even more water wasted.

I wanted a gas smart meter, they refused to fit one, it was near impossible to see if the boiler was modulating or not, I wanted the gas equivalent to a clamp on meter, but it seems there is no such think, but the boiler would not fire up in eco mode unless one turned the taps on full, so I would think the shower was far more than 10 kW, likely using more like 20 kW so yes likely the cost of electric v gas is about the same in power used, but gas wastes a lot more water.

 

[SUNRAY]

One of my rental properties has PP meters so it was easy to experiment while there sorting it between tenents. There is a 10.5KW electric shower over the bath and mixertap, from: https://www.diynot.com/diy/threads/are-electric-showers-expensive-to-run.596162/ in August 2022

I automatically fit an electric shower in my properties. If nothing else, as Harry says it's a plan B for hot water.

There are numerous threads on here offering conflicting advice however an electric shower IS almost instant and wasted water in minimal. For me a shower is about 2 minutes and owning a rental property with an electric shower and a combi with prepay meters it was very easy to do comparitive tests AT PREPAY PRICES with a Worcester combi. About spring last year so not current prices:
Starting electric from total cold and directing to a bucket took about 3-4 seconds to be warm enough to get under. Filling 2 buckets with water (including warming up) took 2min 15 seconds. =6p
Shower mixer tap took 30 seconds to get hot and best part of 20 seconds to set the temperature, that was a whole bucket then 2 more buckets took 1 min 40 seconds = 7p

An hour or so later did a complete repeat, gas was quicker to regulate temperature so not quite a wasted bucket full =6p, electric =7p

Another hour gas = 6p, electric = 5p. Continuing for a repeat straight away G = 4p, E= 6p.

Totals for 3 showers from cold G = 19p, E=18p. Including the 4th shower G = 23p, E = 24p.

Times for filling were not measured for subsequent tests.

What I didn't think to check was the cost of gas powered shower for 2 mins 10 sec which will have to be more than my readings for 1 min 40 sec.

The resolution on the meters is to 1p so I can't be more accurate than that and hopefully explains variations, however bear in mind the electricity cost for running the boiler is not excluded from the reading for electric shower. On no occasion did it register as a cost but of course usage was metered.

My conclusion? from cold electric makes economical sense for my ablution habits of ~2 minutes, added to that less pollution and less water consumed. Also my own usage at home is a 10.5KW heater but I don't like turning into a lobster and it is extremely rare I use it on high power so my costs will be for a lower powered device, something like 7KW possibly.

Anyone spending longer in the shower (someone on here recently mentioned 20 minutes IIRC), gas appears to be more economical, more polution and more water used.

I haven't had a bath for maybe 20 years.

Additionally I have never understood the idea of heating water then cooling it with cold water in a mixer tap... how silly to waste the gas?

Those prices are from Spring time 2021 so very much not current, I believe the trend is gas cost getting even closer to electric than the 4:1 ratio we used to enjoy.

FWIW at home I use my 10.5KW electric shower on the low setting which I assume is 2/3 of the power.

 

[Harry Bloomfield]

However when I got solar panels, I also go an iboost+ and moved over to heating DHW with surplus electric before it is exported, and this time of year I would normally check oil level which was normally below half full, i.e. below the bracing bar by a few inches, this year around the same amount above the bar. This does not prove anything, but it did do something dangerous, it made me start thinking.

I know nothing about oil boilers, but....

Can you not time the run time of the pump, as a measure of the flow, then use that as a measure of daily consumption?

 

[Why Not Indeed]

I have temperature probes on the pipes of interest on my boiler, without [for now] any other way to know remotely if it's running.

 

[Harry Bloomfield]

One of my rental properties has PP meters so it was easy to experiment while there sorting it between tenents. There is a 10.5KW electric shower over the bath and mixertap, from: https://www.diynot.com/diy/threads/are-electric-showers-expensive-to-run.596162/ in August 2022

Those prices are from Spring time 2021 so very much not current, I believe the trend is gas cost getting even closer to electric than the 4:1 ratio we used to enjoy.

FWIW at home I use my 10.5KW electric shower on the low setting which I assume is 2/3 of the power.

Average here, at the moment at quick glance, seems to be around the 16p and 4.65p mark, via Tracker tariff.

 

[JohnW2]

Yes 9" and whole tank, that's the way it is set up, no way to change it.

I'm not so sure about that. You're talking about a deficiency of your system and if you did as I do, I think most of that issue would go away.

The heating of water in my DHW cylinder by the boiler is controlled by an external ('clip on') thermostat which switches a motorised valve in the feed to the coil from the boiler which, in turn, switches on the boiler and pump (if it's not already on because of CH).

I have that thermostat clipped on to the cylinder fairly low down, so it allows water from boiler to the coil until the cylinder is more-or-less full of hot water. However, if I clipped it on "9 inches from the top" of the cylinder, then I imagine it would do much the same as your ('top') immersion does - and, if you wanted, you could have a switch to over-rise that if you wanted the entire tank of water to be heated.

f both immersion and oil/gas were only heating the top 9", then (at least for grid-supplied electricity), the immersion would clearly be a lot more expensive to run.

However I think losses are more due to pipework between boiler and hot water tank, and the boiler its self.

As often discussed, such 'losses' are not really relevant in Winter, since the heat is not wasted. If you were worried about losses from pipework during Summer, I imagine that some decent lagging ion the pipes would do a lot to reduce that. As for losses from the boiler itself, the outer casings do not seem to get very hot, so I doubt that such losses are very great.

 

[DetlefSchmitz]

However, if I clipped it on "9 inches from the top" of the cylinder, then I imagine it would do much the same as your ('top') immersion does

I imagine that your heating coil is also near the bottom of the tank, so moving the thermostat would have very little effect.

 

[JohnW2]

I imagine that your heating coil is also near the bottom of the tank, so moving the thermostat would have very little effect.

The coil is centred not much below the middle of the cylinder, but it's fairly 'long' so the bottom of the coil is fairly low.

However, as recently discussed, just as with a 'bottom' immersion, with even a low coil, the great majority of the heated water would rise to the top, so I would expect a thermostat near the top to 'open' long before all of the water in the cylinder (particularly the bottom) tank was particularly hot. If (from 'cold') I switch on the bottom immersion for just a short period of time, very hot water can be drawn from the top, although the water temp would be very much lower if the heated water was distributed evenly throughout the entire cylinder.

 

[ericmark]

There is no tank thermostat when using oil to heat DHW, the boiler turns off when thermo syphon starts returning warm water so the boiler turns off, that takes around 20 minutes, and the tank is not hot just warm, but warm enough to wash hands.

Any thermostat would have to be wireless. It seems not that readily available as they were. Not looked into how to wire up, as now no real need.

I have no reason now to want to heat DHW in the summer with oil, but the reduction in oil use was not what I had expected to see.

 

[JohnW2]

There is no tank thermostat when using oil to heat DHW,...

As I wrote, that's why you can't emulate the immersion by "just heating the top 9" of the tank" with your boiler.

Any thermostat would have to be wireless.

Why? 'Wire' is freely available