9kw Ferroli Electric Boiler

[nightmareflat]

Hi,

Under the advice of my builder I replaced the gas combi boiler in my flat with an electric 9kw ferroli boiler + an immersion heater tank.

It's a 2 bed 900sqfoot flat with 3 large radiators and 2 small ones.

As expected, the electric bills are ghastly. I've had double glazing installed and I've had reflective foil installed behind all of the radiators. Even still, I'm finding the heating bills to be crazy in the winter, sometimes £15, £20, £25, £30 per day of electricity being used.

In my old new build flat, similar size, similar number of radiators, there was a 4kw potterton gold vertical boiler which never had any problems and the flat heated fine.

Would it be a bad idea to remove the 9KW boiler and replace it with a 4KW one?

My understanding was that the boiler shouldn't always be drawing on the full 9KW load when in use but it doesn't seem to be the case which is why I'm inclined to switch to one that will use less electric (though it might take a little longer to heat up the radiators).

Is it also worthwhile getting one of those little booster things installed at the same time ?

Thanks.

 

[ericmark]

Size of boiler will not help, water storage size may, assuming some off-peak supply, and swapping type of off-peak supply.

I use an off-peak supply to charge the batteries got for the solar panels, and was surprised to see such huge variation in off-peak rates. With the EV tariff being a lot cheaper to the economy 7 tariff. There is also economy 10, and huge difference between suppliers, not only on price, but what they will let you have.

British Gas gave me an EV tariff, 5 hours at 8.95p per kWh, looked at Octopus and the tariff is far better, but would not allow me to have it without an EV, and each area is also different.

Basic idea is in real terms likely twin tanks, but the idea is multi-fuels can be used, and it stores some heat, one I looked at had two massive 80 gallon tanks. This could store around 26 kWh of power, which is unlikely to be enough on really cold days, to span 19 hours. So one works on an average, but you have to work out when to use what.

I have the same problem with my simple solar panels, put tumble drier on, but at moment still exporting 3 kW which is not what I want to do, had I used less off-peak than would not be exporting now.

So easy to become a slave to the solar panels, and it is the same with any stored energy, when to store, when to use, and is all the faff around worth it, we can look at how much energy from heat pump, at what point to stop using heat pump and use immersion heater, etc, etc. But add installation cost, and specially the control system, not to mention the room or any reinforcing of the floor to take the weight, and you see how the heating and ventilation engineer earns his money.

I will admit my errors, many will not, last three homes, I made mistakes, in the main selection of thermostat, there are two ways to heat the home, 24/7 or only as and when required. Heat pumps with 24/7 work well, as does underfloor heating, but as required, recovery time is the buzz word, the fastest way to heat the home is inferred, but that has control issues, so next is fan assisted radiators, there is a noise issue, but the new ivector with 5 speed fans gets rid of most of that, but then also the installation cost. So most of us use near enough engineering, and the simple panel radiator, these lend themselves to control with TRV's and linked programmable TRV heads, but at around £60 a pop, back to what is worthwhile.

So when one starts to look at cost, is it getter to get rid of electric and move to gas or oil, at 73 I think my system will pay for its self just about in my lifetime inshallah, but only if I remain here, if I go into an old peoples home, then will not pay for its self.

So the question is if it is worth it? My flat in Hong Kong had simple stand-alone air conditioning units which both heated and cooled, mainly cooled, and there is a good point in "keep it simple silly" do we really need a complex system in a simple flat?

 

[nightmareflat]

Sorry, forgot to add.

It's a leasehold flat so I can't install solar panels, ASHP nor air conditioning.

I also don't have the storage space/capacity to put any more water tanks in. The existing 150l cylinder just heats up hot water for shower/kitchen/bath taps - it is not connected to the radiator system at all. That's fine as it is and isn't the source of the eye watering bills - it's only when the heating kicks on in the winter that they begin to skyrocket.

 

[RandomGrinch]

Under the advice of my builder I replaced the gas combi boiler in my flat with an electric 9kw ferroli boiler + an immersion heater tank.

I'm afraid that was your first mistake

Even still, I'm finding the heating bills to be crazy in the winter, sometimes £15, £20, £25, £30 per day of electricity being used.

Any electric heating system (excluding heat pumps) will be virtually 100% efficient, no matter if the heat comes from a wet system, or wall mounted convector heaters.
The question then becomes - is the flat a comfortable temperature, or is it overly hot?
If you couldn't stand the flat being cooler, it would be unlikely that you would save money from any other electric heating source, unless you had off-peak charging of storage heaters etc.

Gas heating may be less efficient than electric, but it is still around 1/3 to a 1/4 of the cost per kWh.

If you want to save money and stick with electric, wrap up warm, have a blanket on the sofa, turn the thermostat/TRV's down, only heat the rooms you use and insulate the flat.

I may also question your cost per day for the heating - how are you keeping track?
At around 25p per kWh, your 9kW boiler would be running flat out for over 12 hours a day, at your upper figure.

 

[nightmareflat]

Cost per day for heating

I have a smartmeter. On days when I don't turn on the central heating at all the daily cost is around £2.50 - when the heating is on it's £17, 18, 20 etc.

Est. £15 surplus / £2.25 per hour = 6.6 hours of boiler running on full whack

The house is set to around 20 on the thermostat from 8am onwards.

My understanding was that the boiler might start at 9kwh but then once the flow temperature is reasonable it would taper down to just being on a few minutes per hour, or a lower KWH usage - which is why I'm confused that it's still costing so much. So, it's not as if I'm saving anything with the system being at 9KW, so maybe I should just get a 4KW electric flow boiler and the cost can't be as much?

 

[Madrab]

For the price it'll cost you to buy and install a 4Kw boiler, I'd be throwing a 24Kw gas combi in!! If you were saving say £15 a day, you'd make the cost of the cheapest combi back in as little as 2 months!! You could also get an indirect HW cylinder and run that off of the combi too if you so wished.

It's a no brainer IMO.

 

[stem]

Cost per day for heating

My understanding was that the boiler might start at 9kwh but then once the flow temperature is reasonable it would taper down......... maybe I should just get a 4KW electric flow boiler and the cost can't be as much?

The boiler will control its output depending on the amount of heat needed to heat your flat to the temperature you desire.

Assuming that a 4kW boiler would be sufficient to heat your flat, it will take it 55% longer than a 9kW to do so, therefore it would be running for much longer meaning the energy used will be pretty similar.

Just as an example, because I don't know your actual figures, if you require 9kWh of heat per day to heat your property, to provide that:

1. A 9kW boiler will be running for 1 hour over the heating period

2. A 4kW boiler will be running for 2.25 hours over the heating period

So whilst the smaller boiler will be using less energy, it will be operating over twice as long to provide the same heat.

The general rule is that electricity usually costs 2 to 3 times as much as gas for the same amount of heat. Therefore, I would expect the heating portion of your bill with electricity to have increased by a similar amount.

 

[nightmareflat]

Thanks.

I think where I'm confused is that if my heating is only on for c. 10 hours a day, and the boiler is currently using 6.5 hours of full whack electric per day, then surely if my 4KW was running for 9 hours - it still wouldn't be costing as much?

6.5 x 9 = 58.5 KWH
10 x 4 = 40 KWH

 

[RandomGrinch]

then surely if my 4KW was running for 9 hours - it still wouldn't be costing as much?

You're correct. It wouldn't cost as much.
But your flat would be colder.

 

[nightmareflat]

So with the 9KW, the flow temperature gets up to 70c within about 30 mins on a ~5C day. So, presumably that would take 2.25 x long (so ~70 mins with a 4KW boiler).

But once it gets to that temperature it then keeps it there , so would it not just be cooler for the first 70 mins (rather than 30m at the moment) but then once the system is at 70C it surely regulates and keeps the temperature around that level ?

 

[ericmark]

I do have off peak, and to work out what to run when, seems to be not as easy as one first may think, but as far as heating goes, yes to heat home a bit more during off peak, and the turn down the heat during peak times, may save money, but it is not the time when you want the home warm, so in real terms no help.

But £17 - £2.50 = £14.50 / 0.3131 = 46 kWh so a 9 kW boiler would run for 5 hours. If I was working, then that would be around the limit of my use.

But at 70ºC no wonder it costs so much, my house 22ºC at tops.

 

[RandomGrinch]

But once it gets to that temperature it then keeps it there , so would it not just be cooler for the first 70 mins (rather than 30m at the moment) but then once the system is at 70C it surely regulates and keeps the temperature around that level ?

Ignoring other factors, and assuming constant heat loss from the flat, if the 9kW boiler were to maintain the flat temperature at 20C, it may run for say 20 minutes in the hour. A 4kW boiler would run for 44 minutes.
If your 9kW boiler really was running flat out, to maintain a comfortable temperature; it would be very difficult for a 4kW boiler to ever get the flat up to that temperature in the first place.

 

[stem]

I think where I'm confused is that if my heating is only on for c. 10 hours a day, and the boiler is currently using 6.5 hours of full whack electric per day, then surely if my 4KW was running for 9 hours - it still wouldn't be costing as much?

6.5 x 9 = 58.5 KWH
10 x 4 = 40 KWH

That would assume the 9kW boiler was running flat out when it was on. If that is the quantity of heat you need to heat your flat then there is no way a 4kW boiler would heat your home satisfactorily. Usually the boiler element will cycle on and off during the heating period to control the water temperature leaving the boiler. And be off for longer periods when the room stat was satisfied.

Using your figures, and assuming that the boiler heating element is actually on for 2.5 hours in total during the 6.5 hour period then:

2.5hrs x 9kW = 22.5kWh
With a 4 kW boiler to provide the same heat then its heating element would be on for 5.6 hours
5.6 hours x 4kW = 22.5kWh

It's all to do with the heat emitted by the radiators. Hot water leaves the boiler, circulates around the radiators which emit heat therefore the water cools and goes back to the boiler to be reheated to replace the heat that has been lost by the radiators. The the boiler will control that by switching its heating element on and off as required.

To replace the heat, and as I said before if a 4kW boiler could provide enough heat for your flat (and quite likely it won't) it would be running for over twice as long so the electricity used would be the same.

Remember it will be replacing the heat emitted by the radiators which won't change regardless of the boiler attached to it

To illustrate: imagine filling a 90 gallon tank with water, and you have two buckets one contains 9 gallons [big bucket ] and the other 4. To fill the tank it would take 10 fills from the 9 gallon bucket, or 22 from the 4 gallon bucket. One will take over twice as many fills as the other to transfer the required volume of water.

Likewise to provide the same quantity of heat the heating element inside the smaller boiler will be running for over twice as long as the larger one.