oil boiler or electric immersion to heat water?

claire_parker

Hold on - you seem to all just to be taking into consideration the boiler efficiency. Three major things which are incorrect with this when comparing gas / oil domestic hot water heating costs with an electric immersion heater:

1. There are heat losses from the pipework circulation system. These are relatively considerable if you run the boiler during the summer period and particularly if the pipework between the boiler and cylinder are not well insulated. Typically for a twenty year old boiler you may be talking about an overall heating system efficiency of around 20 to 40%, and even less if the cylinder is on a grivity circuit. With a modern high efficiency condensing boiler with well insulated pipework the overall heating system efficeicy during the summer would perhaps be 60 to 70%. During the winter the effective system efficeincy is higher because the heat loss from the pipework provides useful space heating.

2.The overall boiler effieciency reduces with cycling (cutting in and out during opeartion). The efficiencies above take that into consideration.

3.Boilers without correct water treatment and in any event old boilers decreases as the result of scale build up which reduces heat transfer. Again the efficiencies indicated in 1. take that into consideration.

Also there are associated electricity costs with the running of a boiler system. Some condensing boilers have fairly powerful fans and also there is the pump motor power consumption. Some old pressure jet oil boilers also have relatively fairly powerful burner motors. Electricity costs are not considered in 1. above.

And also, a further consideration is the maintenance costs. Boilers with fans or pressure jet burners and also pumps have rotating internal components which will wear out, requiring replacement. An electric immersion heater theorectically will burn out quicker when in use but in practice this is only marginal.

A further consideration is that electric immersion heater systems are not 100% efficient. Voltage drop occurs along the cable which results in energy loss as heat is given off. Also there will be a some conductive / convective heat loss from the head of the immersion heater All these will result in an immersion heater system efficiency of perhaps 95 to 97%.

malc_b

The boiler type has been given and it is fairly new so it won't be gravity fed for starters. Also, if you look at http://www.engineeringtoolbox.com/copper-pipe-heat-loss-d_19.html you'll see that for 70C water, 55C above room temp of 15C (cool summer :-), the loss for a 1" pipe is under 80W/m. Let's guess a run of 5m which I think is pretty huge. So 2 pipes, 5m, 80W/m is 800W. A run time of 45min has been given so that is a 0.6kWh loss. Given that 16kWh is the supposed boiler output this loss doesn't account for the shortfall.

And if the pipes are insulated the loss fall to about 20W/m so even less significant.

Again as it is a fairly new boiler it should be protected with a scale and corrosion inhibiter.

Cycling could be an issue that is why I asked the question is the 45min a continuous burn. I suspect it isn't or that the boiler is modulating (i.e. burning continuously but at 50% power).

I wouldn't agree with you on wear out. I think is it far more reliable to run boilers a little each day than turn them off for 3-6 months when they will rust and seize up. So I think you would have more maintenance issues with it run less.

The heat loss from the electric heat head would be the same whether you heated the water with gas or electric since the temperature would be same. The element is in contact with the water so the head temperature is the water temperature. Hence this affects both the same and for comparisons can be ignored.

The main problem with the figures given so far is they don't stack up. 2kWh of electric is not going be the same water heating as 16kWh of gas no matter how good or bad the gas boiler is. 12.5% efficiency is just not sensible. Either the figures are wrong, the amount of water heated different, or the radiators are also being heated.

claire_parker

malc_b - my comments were for the whole topic posts where it was evident overall system efficiencies were not being taken into account. I had not looked at James 123 post in any detail or your respsonse to that but have done now. I do disagree with what you are saying though, in so far as I think the figures could be correct but a like for like comparison on the quantity of hot wtaer being generered is not being made. Assumptions are made to illistrate my points as factual information is not available.

1.5 m3 of gas equates to 58500 kJ assuming a gas gross calorific value of 39 MJ/m3.

Allowing say an overall heating system efficiency of 70% for the domestic hot water circuit, the actual heat being delivered to the cylinder would be 40950 kJ.

3 kW of electricity for 40 minutes equates to 7200 kJ and will heat 34.5 litres of water from 10 to 60 deg.C, which could be just about enough for a small bath or for a few people to wash.

Therefore if James 123 figures are correct the quantity of hot water being heated by electricity with those figures is 17.6% compared to his gas figures.

James 123 states the electricity does not heat up all the tank. Therefore I would interpret with gas a greater quantity of hot water is being generated, which probably means the boiler is being left on for a greater period than necessary, IF the 17.6% of hot water is adequate. Perhaps the boiler is turned on by time switch and is then left to control via the cylinder thermostat.

Looking at what would be happening at the cylinder, if this had a storage capacity of 135 litres (which it may not be) and assuming 80% of that water can be effectively heated by the boiler system from 10 to 60 deg.C, the heat energy to achieve this would be 22572 kJ. Therefore if James 123 is using this scenario within the comparison then the overall net domestic hot water heating system efficiency would be 39% (and not 70% as I had assumed above), which is a bit low for a modern system but would not totally surprise me. (However, if the cylinder has a storage capacity of 180 litres the heat energy to increase 80% of the stored water from 10 to 60 deg.C would be 30096 kJ, resulting in an overall net domestic hot water heating system efficiency of 51.5%).

All this is just playing with figures and there are also so many more possible variables, such as the control domestic hot water temperature; this could quite easily be anywhere within the range of 55 to 65 deg. C, or even greater, which would affect the above figures considerably.

Overall though I do not feel James 123 figures are necessarily incorrect I just think the figures for electricity are for a lesser amount of hot water.

Incidentally, you make the point about the heat loss from the primary pipework as being 800 Watts. It is a shame that this source of heat wastage is not brought to the public’s attention as I am sure a lot of people would insulate their primary heating pipes if they realised, or at least those which are readily accessible. Mine were about twice that length so I had about twice that heat loss and yet the insulation only cost about £20.

Mazio_2

My own system gch is just as you state clair gravity feed 60' x 22mm pipes(flow and return) from boiler to hot water tank now 117L (well insulated) I seem to be using 1 unit of gas 31khw to heat my previous 140L tank per day decided to fit immersion and reduce tank size. My system is cheaper using the immersion during the summer but will depend on quantity of baths/showers (4 adults 2 children).
I have stated what you have written the 80-95% boiler efficiency has to be carried to the hot water tank this includes good control system . The only way you would get 95% efficiency is to have the boiler heat the hot water directly as any othe way you will be losing heat all the time.
But if your boiler is close to the hot water tank and boiler is working well then I think gas / oil will still be cheaper.
The only other thing is in the summer months with an immersion you could switch your boiler off which should mean less maintenance cost or boiler lasts longer. I would still switch it on for small durations though to stop pumps etc seizing up.

malc_b

I agree with you about system efficiencies that's a good point. What I was trying to show is that the current figures don't add up and until that is answered it is all guess work in this particular case.

In my house I heat for a total of 1hr/day in 10min bursts so as to get a long burn time (oil). My pipe run is about 12ft and the pipes are indoors and insulated with rigid poly split tubes. From http://www.engineeringtoolbox.com/heat-loss-insulated-pipes-d_1152.html as insulation is 0.5in which is not on metric graph and adding in the -10% for indoors and -30% for rigid poly I get a total loss of 106W. Boiler output for rated efficiency of 84% gives 8.69kW / l of oil. So the 106W loss is just 1.2%. My estimated HW demand is 9.63kWh/day (might be a bit high that). Electric is 8.9p/kWh and with Oil at 50p/l I calculate oil to be £50 cheaper for 6 mths (~£150 elec, ~£100 oil). Perhaps boiler is a bit less efficient but you would need a big error to wipe out the difference. And oil is going down in price at the moment. The actually cost difference may be less as I think the usage may be a bit high and the water rise calc is from 10C so winter water not summer water.