How to calculate cost to fill hot water cylinder using Gas Conventional Boiler - confused!

Lfc1969

I’ve tried searching for this info but I’m not finding what I’m looking for so can anyone help please?

We have a detached four bedroom house with a Worcester 40 CDI conventional boiler. A hot water cylinder with immersion heater. On Intelligent Octopus tariff. Using Hive.

I’m trying to calculate how much we spend on gas to heat up the water in the cylinder. Currently we use Hive “hot water” option to run the boiler for one hour per day and this seems to give us enough each day for our needs.. sometimes it runs out but usually it’s fine so I think it’s about right. 

The bit I’m confused by is how much gas (= cost) does the Worcester 40CDI use to only heat water for an hour? I’m assuming (hoping) that it’s not consuming 40KWh per hour just to heat water..  I.e. does it use much less than it would to run the central heating? I don’t really understand how this works for hot water only. If it is the same as running the full central heating then obviously that’s quite expensive! I have range rated the boiler to 70% so it’s running at around 30 KWh for the CH which was fine last winter.

The reason I’m asking is because it may be better to use the immersion heater instead on the cheap overnight Octopus 7.5p rate for two hours.

Second question is that in the winter when the CH is on, does this also feed the hot water cylinder (apparently we have a diverter valve)? We’ve only had the house for a short while and last winter we found we still had to run the Hive “hot water” setting for an hour a day in addition to the CH… but we may have been doing something wrong.

I guess the last question may be very specific to our set up so I’m really looking for some information on the first question about how much gas the Worcester would use to just heat water.

Thank you!

brook_heather

What is your gas unit rate?  Your gas boiler will be less than 100% efficient so assume say 90% efficiency - divide your gas kWh unit rate by 0.9 and if that is more than 7.5p then electricity will be cheaper for those two hours.

mmmmikey

Hi - the boiler will "modulate" meaning that the gas usage is variable. Think of it being like the ring on a gas hob - you turn the heat up and down by adjusting the size of the flame, i.e. by using more or less gas. 

As far as the amount of energy needed, do you have anything that tells you the temperature of the hot water before and after you've run the boiler? If you do, and you know the capacity of the cylinder, you can calculate the amount of energy needed and by making some sensible assumptions on efficiency get to a reasonably good estimate of the cost. 

Another way to do it is to simply take a gas meter reading before and after the boiler has run, making sure that nothing else is using gas at the same time (which often isn't difficult outside the heating season). 

Or better still use both methods. If you think either of these will work for you, if you post the info I mention above here then we can work through the figures and give you an estimate. Hope this helps, Mike.

mmmmikey

Forgot to add - here's an online calculator I use....

https://bloglocation.com/art/water-heating-calculator-for-time-energy-power

Netexporter

I carried out this exercise, at my previous house. 

Let the hot water cylinder go cold. Take a meter reading to as many decimal places as possible (my meter went to 3DP). Switch off heating and run the boiler in hot water mode. Take another meter reading when the boiler finishes firing. Subtract the two readings and then do the arithmetic to change it to money, using the information on your bill.

QrizB

Lfc1969 said:

We have a detached four bedroom house with a Worcester 40 CDI conventional boiler.

That's a beefy boiler.

Lfc1969 said:

The bit I’m confused by is how much gas (= cost) does the Worcester 40CDI use to only heat water for an hour?

You'd need to need the volume of the tank, the starting temperature, the ending temperature, and then make an educted guess at the efficiency of the boiler.

You haven't provided any of these details.

However they might be moot, because of the next bit.

Lfc1969 said:

The reason I’m asking is because it may be better to use the immersion heater instead on the cheap overnight Octopus 7.5p rate for two hours.

Which gas tariff do tyou have?

If your gas is on Flexible Octopus, at about 7p/kWh, electricity is almost certain to be cheaper.

If on Tracker, currently around 5p/kWh, gas is likely to be cheaper.

[Deleted User]

If you use the immersion heater you can then run the system boiler at a low flow temperature for maximum heating efficiency. I ran mine at 55C last Winter. This would not have been possible if I was using a my boiler to reheat a cylinder. Boilers do not condense when re heating hot water cylinders. FWiW, I find with pipe run losses that 8kWh of gas reheating equates to under 4kWh of electricity (same max cylinder temperature).

Lfc1969

Thanks all, really helpful.

We are paying something like 7p for gas and 30p for electricity but obviously we have the IO overnight 7.5p electric rate which is when we charge the EV and now also do the laundry!

Last winter I did do a lot of calculations on taking before and after gas meter readings mainly to calculate CH costs and tweak a few things like the boiler range rating and flow rate which made a difference. What I didn’t measure was HW costs. Unfortunately since moving to Intelligent Octopus a few months ago our gas meter LCD display isn’t working properly and the Octopus app is also having an issue reading the gas usage.. waiting for them to resolve it.

I’ll have a read up of the modulating you mention Mikey. In the meantime I’ll give the immersion method a go.. trying a 60 min boost, then a 90 and then a 120 min boost to see which works best. Obviously at the 7.5p IO rate that would be somewhere between 22p and 45p per night based on the 3Kw element.

lohr500

Something else to consider when comparing gas vs electricity is the length and/or position of the immersion heater element.

If only a short element has been fitted, it won't heat the full tank of water, wheres the heating coil from the gas boiler is likely to cover the full height of the hot water tank. Energy consumption comparisons may not therefore be comparable.

[Deleted User]

Unfortunately since moving to Intelligent Octopus a few months ago our gas meter LCD display isn’t working properly and the Octopus app is also having an issue reading the gas usage.. waiting for them to resolve it.

Unlikely to be anything to do with a tariff switch: it sounds like a possible comms hub issue that might need a DCC remote reboot or an engineer hard reboot.

Lfc1969

lohr500 said:

Something else to consider when comparing gas vs electricity is the length and/or position of the immersion heater element.

If only a short element has been fitted, it won't heat the full tank of water, wheres the heating coil from the gas boiler is likely to cover the full height of the hot water tank. Energy consumption comparisons may not therefore be comparable.

Good point. The immersion element goes into the middle of the tank, a little above half way. It’s an 11” element so not one of the 27” ones that goes in at the top.

No idea about the heating coil.