Boiler efficiency

ontoitarethey

Hi all hope someone can help

I have an 150L hot water cylinder with an immersion element after 1 hour it has consumed 2.69kW of electric (im not too sure but I suspect the starting temperature was approx 42.9 degrees and reached 60 degrees).

Now when I use my boiler i measure the water temperature and its 42.9 degrees. After 1 hour of boost on the control panel the water temperature has increase to 55.8 degrees so the temperature increase is 12.9 degrees. According to a digital meter I have consumed 7kW however the digital meter does not show a decimal point so I ought to deduct one unit so the usage was at least 6kW to heat my hot water tank.

From that data is it possible to calculate the efficiency rating of my boiler in comparison to using electric?

Also would there be a massive energy loss between the boiler and cylinder (the pipe is about 2 metres to the cylinder and return length approx the same) - its fully lagged.

I suspect I should use the formula:

Size of tank X 4 X temp difference / 3421 = Kw rating

150L X 4 X 12.9 / 3421 = 2.26

100 / 6kW X 2.26 = 37.6% efficiency rating

Does that make sense? it that the correct method?

Thanks in advance

CashStrapped

It is a bit more complicated than that.

Is the immersion element a bottom or top immersion?

If it is a top immersion, it is usually a much smaller "boost" immersion. Therefore it may only be heating a much smaller proportion of the tank. Maybe only 1/4. Therefore you are not comparing like for like.

The boiler fed indirect coil will be lower down and designed to heat the whole tank.

Then there is the control set-up and age of the boiler to take into account.

Then there is the tariff cost difference between gas and electric. The latter being 4x - 5x higher in cost per kwh.

What is the make and model? You can get a rough SAP estimate on-line as to the measured efficiency of the boiler.

Do you have a diverter valve which ensures you are heating the hot water tank only? Or do radiators get hot in conjunction with heating the tank?

In the end you have to decide what works for you.

With an older system, you may find, as you use the boiler to heat the radiators to keep warm, it makes sense to heat the tank at the same time. It should be more cost effective doing this as the boiler will usually be on anyway. All that happens is the diverter valve (if you have one) moves to a mid position and heats both at the same time.

However, in summer, when you do not use the radiators, you may find that the immersion is more cost effective.

This all depends on the measured boiler efficiency, your tariff, insulation level of the pipes and tank and, as mentioned, how much of the tank you actually use.

If heating a smaller portion of the tank suits your needs, using the immersion in summer, may be more cost effective.

I assume, obviously, that by efficiency you mean "most cost effective". There are, of course, lots of efficiencies to consider. It is a bit of a can of worms question when you look at it in detail.

matelodave

Don't forget as well that your 2.69kw of leccy would have cost yoy around 32p at 12p/kwh whereas your gas probably wold have heated more water for say 6- 7kwh at round 4p = 24-28p. So could well work out cheaper

You need get a better idea of how much water gets heated by the immersion or the tank coil - where is the tank stat situated on the tank then you can decide which method is the most economical.

You might be able to improve the efficiency of the boiler by insulating both the feed and return pipes between the tank and the boiler to reduce the heat loss.

The coil on our hot water tank feeds in about 25% from the floor and exits ablout 50% from the top so I guess it will heat around 75% of 200l = 150 litres, The immersion is above the coil up so will heat around 100-120 litres. The boiler takes longer and will use more energy than the immersion to heat the tank to the same temperature as it's got more water to heat - our temperature probe controls both the immersion and boiler flow.

I cant be more scientific than that because our tank is highly insulated with a steel casing so you can't measure the temperature at various heights.