Heating Oil Consumption

Alfster

Hi,

I have a modern (one year old) Grant condensing oil boiler. Think it goes up to 36kW. I just wondered if anyone with a similar boiler could give me a rough idea of either how many kWh or how many litres of oil would be used for a 30 minute hot water boost. As a comparison when we had our tank changed we were without an oil line so used the immersion which is 3kW and needed to be on 2 hours to get a fully hot tank (50L) compared to the oil boiler only needing 30 minutes to achieve the same? 

Also, I’m new to oil boilers. Is it normal when I’m watching the flue outside for the exhaust gases and boiler to come on intermittently during that 30 minutes? Sometimes it’s probably only firing up and expelling exhaust gases for what seems like maybe 5-10 minutes of the 30 minute boost? I guess if it’s got heat a small tank with a large power capacity of 36kW it makes sense it wouldn’t come on continuously I guess?

thanks 

Alfster

Just seen online that 1L of heating oil is 10kWh so I guess if the cylinder takes 6kWh of energy to heat up as per immersion time and power I mentioned it would use 600ml of oil? Does that sound feasible? 

Reed_Richards

With an immersion heater you heat the water in your cylinder.  With a boiler you heat the water in your cylinder and the water in the pipes to and from the cylinder.  Afterwards these pipes will cool and the heat is lost.  This lost heat may or may not help to heat your house, depending on where the pipes are positioned and in summer you don't need your house heated so there is extra wastage.

If your boiler cycles on and off whilst heating your hot water cylinder then the return water coming back to the boiler is getting too hot.  If it just comes on for a bit then stops then that just means the hot water cylinder has reached its set temperature.

If the issue is on/off cycling, you either need a cylinder with a longer coil that will absorb heat more rapidly or you need to reduce the water temperature leaving the boiler.  I imagine your boiler has only one water temperature control so if you reduce the water temperature this will make your radiators colder.  This should improve the efficiency of your boiler, giving it more opportunity to condense, but you could find that the house gets too cold.     

QrizB

Alfster said:

As a comparison when we had our tank changed we were without an oil line so used the immersion which is 3kW and needed to be on 2 hours to get a fully hot tank (50L) compared to the oil boiler only needing 30 minutes to achieve the same?

Heating 50 kg (ie. litres) of water from 10C to 60C will take 2.91kWh per this calculator. At current prices of around 28p/kWh that's 81.5p-worth of electricity.

Alfster said:

Just seen online that 1L of heating oil is 10kWh so I guess if the cylinder takes 6kWh of energy to heat up as per immersion time and power I mentioned it would use 600ml of oil? Does that sound feasible? 

Your immersion heater is 100% efficient; all the electrical energy ends up in the water (some will then leak out to heat up your airing cupboard but that should be a small fraction and I'm ignoring it here).

Your oil boiler is not 100% efficient. When heating your hot water tank it's likely to be 85% efficient, but then as R_R says you will also be heating the circulation system. It could work out to be 60-70% efficient, depending on exactly how your system is configured and how long those pipes are.

If it's 60% efficient, the boiler will need to burn (2.91 / 0.6) 4.85kWh of oil - 485ml - to heat your water. If oil is still £1 a litre, that's 48.5p-worth.

Alfster

Reed_Richards said:

With an immersion heater you heat the water in your cylinder.  With a boiler you heat the water in your cylinder and the water in the pipes to and from the cylinder.  Afterwards these pipes will cool and the heat is lost.  This lost heat may or may not help to heat your house, depending on where the pipes are positioned and in summer you don't need your house heated so there is extra wastage.

If your boiler cycles on and off whilst heating your hot water cylinder then the return water coming back to the boiler is getting too hot.  If it just comes on for a bit then stops then that just means the hot water cylinder has reached its set temperature.

If the issue is on/off cycling, you either need a cylinder with a longer coil that will absorb heat more rapidly or you need to reduce the water temperature leaving the boiler.  I imagine your boiler has only one water temperature control so if you reduce the water temperature this will make your radiators colder.  This should improve the efficiency of your boiler, giving it more opportunity to condense, but you could find that the house gets too cold.     

Thanks for such a detailed response. So there is one temperature dial on yeh front panel of the boiler - you would turn this down? Would you also change the temperature dial on the cylinder?

Alfster

QrizB said:

Alfster said:

As a comparison when we had our tank changed we were without an oil line so used the immersion which is 3kW and needed to be on 2 hours to get a fully hot tank (50L) compared to the oil boiler only needing 30 minutes to achieve the same?

Heating 50 kg (ie. litres) of water from 10C to 60C will take 2.91kWh per this calculator. At current prices of around 28p/kWh that's 81.5p-worth of electricity.

Alfster said:

Just seen online that 1L of heating oil is 10kWh so I guess if the cylinder takes 6kWh of energy to heat up as per immersion time and power I mentioned it would use 600ml of oil? Does that sound feasible? 

Your immersion heater is 100% efficient; all the electrical energy ends up in the water (some will then leak out to heat up your airing cupboard but that should be a small fraction and I'm ignoring it here).

Your oil boiler is not 100% efficient. When heating your hot water tank it's likely to be 85% efficient, but then as R_R says you will also be heating the circulation system. It could work out to be 60-70% efficient, depending on exactly how your system is configured and how long those pipes are.

If it's 60% efficient, the boiler will need to burn (2.91 / 0.6) 4.85kWh of oil - 485ml - to heat your water. If oil is still £1 a litre, that's 48.5p-worth.

Thanks for this. So roughly half a litre is a good estimate then. Does anyone have any idea about oil price forecast over the summer. Ours is running fairly low - should I hold out a bit to see if it drops as we get more into the summer? 

lohr500

We have a one year old Grant Vortex Blue external boiler calibrated to run at +/- 30kW.

Through the summer last year we were using an average 1.7 litres of oil per day just for hot water.

We have a 140 litre hot water tank and are heavy users of hot water. I had the boiler set to run early morning for an hour and then again late afternoon for the same time. I don't know if it needed the full hour to heat the tank. Out tank thermostat is set at 55 deg C.

Last summer I concluded it was cheaper to use the boiler to heat the hot water than our off peak electricity rate. But with oil now at +/- £1 a litre, I have reverted back to using off peak electricity for the overnight heating and just use oil for the late afternoon top up.
(We are heavy off peak electricity users as we have an electric Aga which works like a big storage heater).

There is a calculator here which allows you to work out how much energy is needed to heat a full tank of water from 15 to 60 deg, and how long it would take with a 3kw or 6kw electric immersion heater. 

https://www.sust-it.net/immersion-heater-energy-calculator.php?fu=10&cu=uk

In reality I think the energy use will be less than the calculator suggests as most of the time a full immersion tank of water won't need heating.(Especially if it is a well insulated tank). 

As Reed_Richards points out though, heating the water directly with an immersion heater will be more efficient than heating the water indirectly with a boiler.

Not sure if this helps!!

dunstonh

Heating a water cylinder doesn't take that long for a well lagged cylinder just needing bringing back up to temperature.  If your controller has separate timers for heating and water, then you probably only need 15-30 minutes for the water.

So, reduce the time the boiler is on for hot water.  Start at 30 minutes.   If you notice no reduction in temperature then reduce it to 25 minutes and repeat until you find the point where the water starts losing temp and go back to the point it doesn't.

Ours comes on for 20 minutes twice a day. And the oil use is tiny from May through to October.  it doesn't drop a bar on the Apollo meter.      Our immersion heater has no timer on it. So, it would need turning on/off manually and that risks being in longer than necessary wasting any tiny differences in price.

(We are heavy off-peak electricity users as we have an electric Aga which works like a big storage heater).

Same here. 30amp AGA.   Although we are lucky that our economy 7 timer is currently 8.56am to 3.56pm.  A few more powercuts and it will move to the evening and we will get cheap rate during peak hours.

lohr500

My gut feel says the tank thermostat setting shouldn't be higher than the boiler flow thermostat setting.
Otherwise the tank will never reach its set temperature and this will cause the boiler to cycle on and off frequently.

I run my boiler with a 60 C flow temperature and the tank set to 55 C. For a boiler to condense effectively (and so help with efficiency) the return water temperature needs to be below 54 C.  When our central heating is running, I am seeing a return temperature of between 45 and 50 C. The lower the return temperature below the magic 54 C, the more efficient the condensing process. Adjusting the flow and return temperatures involves playing around with the boiler thermostat setting, the circulation pump speed and the balancing valves on the radiators. It took me a while of trial and error to fine tune our system, but hopefully worth while to save on oil consumption. You can buy reasonable quality Brannan clip on analogue pipe temperature gauges for about £10 each and use these to check the flow and return temperatures.

Are you sure your immersion tank is only 50 litres? That sounds quite small.

On the oil price, I had hoped that prices would fall this summer on the back of some resolution to the terrible events in Ukraine. But unfortunately this seems increasingly unlikely and my own view now is that we are going to be stuck with high prices for quite some time. I bit the bullet a couple of months ago and filled our tank at £0.95 per litre including the 5% VAT. Best price in my area today is £1.03. 

lohr500

Good point dunstonh. I've reduced our afternoon oil powered hot water top-up period from 1 hour to 45 minutes. I'll reduce it further if we find we have enough hot water in the evenings for baths/showers.

Reed_Richards

lohr500 said:

Good point dunstonh. I've reduced our afternoon oil powered hot water top-up period from 1 hour to 45 minutes. I'll reduce it further if we find we have enough hot water in the evenings for baths/showers.

Seems crazy to me.  Once the water in the cylinder reaches the set temperature the boiler will be turned off (or allowed to revert to heating your radiators).  If you want the water in the cylinder to get less hot then the sensible way to achieve this is to turn down the cylinder thermostat.  Trying to make the cylinder cooler by reducing the heating period is just using an inexact inaccurate method when you have the option to use a method that is completely accurate.  If you want different hot water temperatures at different times of day, get a programmable cylinder thermostat.