Boiler or Immersion

Happy_Sloth

I know gas has always been considered the cheapest method of heating water,  but we've recently established that it takes along time for water to get from our boiler to our tank through unlagged and unreachable pipework that takes it on about a 60ft journey.  

We've done some tests and the water coming out of the boiler is much hotter than the water that ever reaches our tank.   Therefore in order to get water hot enough for dishes or a bath we need to have the hot water set to 70 degrees on the boiler...  setting it anything lower results in water thats not warm enough for a comfortable bath.  

We had an old immersion heater on the tank,  but it's been broken for a while.  I'm wondering if it would be financially worth replacing it? Obviously because it doesn't work means we can't run tests to see the difference in cost between the two before deciding.  

Could it be cheaper for us to heat the water using the immersion? is it worth replacing do you think? 

markin

Unreachable pipework by you or anyone at all, Or could it be replaced by pex insulated before being fed though the Unreachable place.

Gas it probably still cheaper, even at this less efficient set up.

QrizB

According to this table:

https://www.engineeringtoolbox.com/copper-pipes-heat-loss-d_51.html

22mm copper tube at 75C, in a 20C house, will lose about 60 watts per metre of length. 60 feet is roughly 20 metres so you're losing 1200 watts from the tube while the boiler is running.

I don't recall the size of your boiler but if it's 12kW (a small boiler by most standards) you're only losing 10% of your energy between the boiler and the tank.

With gas at 7.5p/kWh it's still going to be much cheaper than electricity at 28p/kWh. You'd need to be losing almost 75% for the immersion to cost less.

Even in the winter, with potential prices of 11p vs 40p, gas will still be cheaper.

Happy_Sloth

markin said:

Unreachable pipework by you or anyone at all, Or could it be replaced by pex insulated before being fed though the Unreachable place.

Gas it probably still cheaper, even at this less efficient set up.

it's hard to get to without structural work.  It's all trapped behind false walls and ceiling,  getting to it would involve removing part of the kitchen in order to reach it, so not totally undoable but not without serious cost/chaos.  

Happy_Sloth

QrizB said:

According to this table:

https://www.engineeringtoolbox.com/copper-pipes-heat-loss-d_51.html

22mm copper tube at 75C, in a 20C house, will lose about 60 watts per metre of length. 60 feet is roughly 20 metres so you're losing 1200 watts from the tube while the boiler is running.

I don't recall the size of your boiler but if it's 12kW (a small boiler by most standards) you're only losing 10% of your energy between the boiler and the tank.

With gas at 7.5p/kWh it's still going to be much cheaper than electricity at 28p/kWh. You'd need to be losing almost 75% for the immersion to cost less.

Even in the winter, with potential prices of 11p vs 40p, gas will still be cheaper.

Its a 40 KHW boiler (As it needs to heat 18 radiators in winter)... it's a big house with 12ft ceilings, the boiler is on the ground floor and the cylinder is on the opposite side of the house on the 2nd floor.  obviously in the winter we use a lot of gas. 

Does that make a difference? 

QrizB

Happy_Sloth said:

QrizB said:

According to this table:

https://www.engineeringtoolbox.com/copper-pipes-heat-loss-d_51.html

22mm copper tube at 75C, in a 20C house, will lose about 60 watts per metre of length. 60 feet is roughly 20 metres so you're losing 1200 watts from the tube while the boiler is running.

I don't recall the size of your boiler but if it's 12kW (a small boiler by most standards) you're only losing 10% of your energy between the boiler and the tank.

With gas at 7.5p/kWh it's still going to be much cheaper than electricity at 28p/kWh. You'd need to be losing almost 75% for the immersion to cost less.

Even in the winter, with potential prices of 11p vs 40p, gas will still be cheaper.

Its a 40 KHW boiler (As it needs to heat 18 radiators in winter)... it's a big house with 12ft ceilings, the boiler is on the ground floor and the cylinder is on the opposite side of the house on the 2nd floor.  obviously in the winter we use a lot of gas. 
Does that make a difference?

It means you'll lose a smaller fraction of your heat from the pipes, and a stronger case for using gas rather than electricity.

Le me put it another way.

If you've got a 250-litre hot water tank and you're heating it from 10C to 60C, you'll need to add 14.5 kilowatt-hours. The volume of 20m of 22mm tube is another 8 litres, which will take another 0.4kWh to heat, total 14.9kWh.

Let's say your boiler will run for an hour to do this (it will probably take less time) and it's 90% efficient. In that hour you'll also need to provide 1.2kWh for the heat lost from the pipes.

That's a total of 16.1kWh. If your boiler is 85% efficient you'll have to burn 18.9kWh of gas, which at 7.5p/kWh will cost you £1.42.

If, instead, you use the 100%-efficient immersion heater, you'll need 14.5kWh of electricity. At 28p/kWh that will cost you £4.06.

Happy_Sloth

QrizB said:

Happy_Sloth said:

QrizB said:

According to this table:

https://www.engineeringtoolbox.com/copper-pipes-heat-loss-d_51.html

22mm copper tube at 75C, in a 20C house, will lose about 60 watts per metre of length. 60 feet is roughly 20 metres so you're losing 1200 watts from the tube while the boiler is running.

I don't recall the size of your boiler but if it's 12kW (a small boiler by most standards) you're only losing 10% of your energy between the boiler and the tank.

With gas at 7.5p/kWh it's still going to be much cheaper than electricity at 28p/kWh. You'd need to be losing almost 75% for the immersion to cost less.

Even in the winter, with potential prices of 11p vs 40p, gas will still be cheaper.

Its a 40 KHW boiler (As it needs to heat 18 radiators in winter)... it's a big house with 12ft ceilings, the boiler is on the ground floor and the cylinder is on the opposite side of the house on the 2nd floor.  obviously in the winter we use a lot of gas. 
Does that make a difference?

It means you'll lose a smaller fraction of your heat from the pipes, and a stronger case for using gas rather than electricity.

Le me put it another way.

If you've got a 250-litre hot water tank and you're heating it from 10C to 60C, you'll need to add 14.5 kilowatt-hours. The volume of 20m of 22mm tube is another 8 litres, which will take another 0.4kWh to heat, total 14.9kWh.

Let's say your boiler will run for an hour to do this (it will probably take less time) and it's 90% efficient. In that hour you'll also need to provide 1.2kWh for the heat lost from the pipes.

That's a total of 16.1kWh. If your boiler is 85% efficient you'll have to burn 18.9kWh of gas, which at 7.5p/kWh will cost you £1.42.

If, instead, you use the 100%-efficient immersion heater, you'll need 14.5kWh of electricity. At 28p/kWh that will cost you £4.06.

Arh right that makes sense!  we did have the boiler set to come on 2x per day and it was costing just shy of £3 a day (£90 a month) ... so that is about £1.50 per ... which is pretty much spot on what you said.  

Tbh knowing thats how much it costs to heat the boiler was an eye opener and we've since changed it to just once daily.  I've got a new cylinder jacket arriving tomorrow as it's current one is pretty old. i'm hoping that will keep the water hotter for longer but with the current prices it still feels really expensive.  

I'm dreading winter when my heating comes back on... 

Magnitio

Earlier this year I switched from using the gas boiler for heating the hot water cylinder to an immersion. I did also replace the old hot water cylinder for a new one, which is better insulated and has a nice new coil. Our typical daily water heating consumption has reduced from 8kWh gas to just under 4kWh electric. I haven't tested what the gas consumption would be with the new cylinder, but judging by the thick limescale deposits on the heating coil, it would be quite a lot less.

The main reason for the change was to reduce gas consumption and use solar energy. If I looked at it from a purely financial point of view, there are many days where it would still be cheaper to burn gas and sell the excess solar energy.

wrf12345

The radical solution is not to use hot water at all, invigorating cold shower in the morning, etc.

jrawle

The guy who services my boiler said that he sees a number of seized pumps each autumn due to people using an immersion heater over the summer, meaning the boiler and pump aren't used for a few months. A callout for that and possible cost of a replacement pump could be expensive, plus there's the inconvenience of not having heating when you've finally decided it's too cold and time to put it on. Therefore if you do decide to use an immersion heater, I'd suggest using the boiler to heat the water every couple of weeks just to keep it in good working order.