heating buffer with immersion

newbie80

Hi all,

ive moved into a house that has a large 800 litre buffer tank and solid fuel boiler heating it. the house has a day/night meter. im a welder so i could easily install 2 immersions down near the base of the buffer. would it be cheaper to heat the buffer on night rate with the immersions from 11pm to 8am and then supplement with solid fuel in the day if needed?

Dtp1979

I assume the buffer is insulated?

Is JohnT around? He’s the man for the equation. I can’t remember my phase 2 maths on all this stuff

cruizer101

The below compares the cost of various fuels for space heating, buffer tank is no different really, a unit of heat is a unit of heat.

According to it average night rate is 9.83c/kWh (you can probably get better than that though) and coal is just below 7, wood anywhere between 5.5 and 9, however the solid fuel won't be as efficient might be around 70%, is which case they are fairly comparable prices and it would come down to how cheap you can get them.

There is also the convenience of not having to fuel the boiler. Link might be of some use to you anyway

https://www.seai.ie/publications/Domestic-Fuel-Cost-Comparison.pdf

Dtp1979

Would 2 immersions have the legs to heat 800litres?

cruizer101

1kW will give 1kJ/s of energy, in an hour that is 3.6MJ.

Heat capacity of water is ~4200 so 3.6MJ = 800 x 4200 x deltaT which give deltaT equal to just over 1degree per hour per kW.

So say 2 x 2.5 kW heaters over 8 hours could heat 800litres by 40 degrees.

That may be enough, however you really want to check it by the number of rads and sizes. Can the daily heat demand of house be met by 2 immersions is the real question, and that depends on a lot.

Dtp1979

I think op said that they’d be using solid fuel in the day to supplement it

newbie80

yes to supplement

my current night rate is 8.71

John.G

More or less repeating what cruizer101 stated, assuming a 40deg draw down from say 80C to 40C then it will require 37.2kwh to reheat it, so, again as stated above, 2x2.5kw elements will reheat it in under 8 hours (800*40/860/5 =7.44). It will certainly supply some of your heating load, I currently burn ~ 8 litres of Kero/15hour heating day which with a SE boiler = ~ 62kwh of useful heat for CH+DHW for a 4 bedroom, 50 year old cavity block house.

8.71c/kwh is very competitive as a "fuel" price.

Dtp1979

Is there any point in the OP going to the extra hardship of installing immersions when he could install an oil boiler that costs similar to run?