Eddi Vs Gas

I'm on Octopus intelligent flux and I also have an Eddi to heat my hot water using solar. On paper, it appears better for me not to use the Eddi and export the solar as the export price is far higher than the unit price of gas (approx 25p for export, 7.5p per kWh/h for gas). However, I presume heating the hot water with gas is not 100% efficient (I won't be getting 1Kwh worth of hot water for 7.5p) - does anyone have know how inefficient this is and whether I'm better off using the Eddi?

_Con_
I’ve wondered the same, but one of the best tips I’ve taken from this forum was to move to Octopus Tracker for gas. At approx 4p per kWh it makes the decision seem pretty clear no matter what the efficiency.

_Con_ It depends on your boiler's age and maintenance status but a rule of thumb is to use 90% efficiency in calculations for comparison.

An old (25 years or so) non-condensing boiler could have an efficiency as low as 60-70%

When you are comparing costs don't forget the standing charges you need to pay for gas and electricity.

killythebid I think 90% is optimistic - you only get that if it's fully condensing. When heating water, that's probably not the case.

Do condensing boilers condense while heating hot water in a cylinder? I think the return flow temperature would be too high.

I was using Solar to heat the HW tank but have switched back to gas. I'm on the Oct gas tracker tariff, and as mentioned above it is more economical to sell any surplus electricity at 15p than use it to heat water.

I tend to treat standing charge as a tax - you pay them regardless of which you use to heat the water. It's only relevant if getting rid of gas completely is an option. Otherwise, I don't think they're relevant to comparing costs.

DD Not really for a modern boiler. An up-to-the-minute unit could exceed 90% by another 4% or so. Poorly maintained or older boilers could indeed do less than 90% efficiency.

However, 90% is not a bad yardstick if the boiler is fairly modern.

DD For simplicity standing charge can be ignored but there is no escaping it is part of your annual gas or electric bill just now. It is also true that the standing charge for gas is not the same as the standing charge for electric supply. So if you want to correctly compare the cost of a kWh or gas to and kWh of electricity cost you need to factor in the standing charge.

The simplist way of doing that is to take an annual avg of kWh demand say 3000 of electricity price then add 365 days of standing charge then divide the result by the original 3000 this will get a smoothed full kWH cost and then do the same with the gas. This will get to the total cost of supply.

If you are comparing oil v electricity it is more important to factor electricity standing charge in as there is none for oil .

killythebid sorry, I disagree.
If I use exactly one unit of gas more, it will increase my annual spend by exactly the unit price of gas.

Standing charge for dual fuel on Octopus (lowest standing charges) equates to over 1800kWh of export at 15p/unit.

5 units a day, 365 days a year.

Just saying...

DD You don't have to agree of course but if anyone wants to do a comparison that is reasonably accurate taking into account the standing charge is required. This is particularly true in areas that have a high standing charge. In London electricity standing charge can be around 35p a day but not far off double that in other areas.

killythebid EONs standing charge for this area (1 April) is 99p/day or so if you have dual-fuel.

Its beyond ridiculous in terms of regressive taxing.

In doing the comparison of excess solar vs gas heating of your hot water, I don’t know why you would include standing charge in the comparison, it makes no sense to me.

Firstly if you need to have gas in your home for cooking or for domestic heating then you’re going to have to pay the standing charge for the whole year anyway, so whether you heat your hot water by gas or solar makes no difference to you having to pay the gas standing charge anyway.

Secondly, assuming you don’t use gas for cooking or domestic heating, and you only ever use gas for heating your hot water, the problem you then run into is that in winter and on cloudy days there won’t be enough excess solar to heat your hot water. So either use the immersion (which now turns this into a 3 way evaluation), or you use your gas, in which case you’re paying the standing charge for the whole year regardless of whether you use it to heat your hot water 1 day or 100 days.

geoffreycoan It is because it is part of the cost of your energy supply. If they took away the standing charge their just would be a corresponding rise in the per kWh amount. In fact in the past some suppliers used to offer the customer an adjustable standing charge the more you paid the cheaper the rate and of course the less the standing charge the higher the tariff.

To ignore the standing charge is to miss out a chunk of the cost of supplying one's energy and when comparing fuels it is best to include it, especially when comparing oil costs vs an energy supply with a standing charge.

I agree you have to pay it anyway but that just means the fewer kWh one uses the more of an impact the standing charge has. It is part of the total cost of supply.

killythebid I'm sorry I disagree. If it's a comparison of consumption of gas vs excess solar consumption then the fixed standing charge makes no difference for the reasons I outlined on the presumption that you are keeping both. You have to pay the standing charge regardless so adding it in to the figures would distort the comparison.

If it's a question of whether to keep gas and you are weighing up changing heating source and/or cooking source, then yes of course it has to be included in the calculation.

But the original question started off with should I heat my hot water with excess solar or gas, for which it depends on your boiler efficiency but if gas is 4p/kWh and your export is 15p then it's likely to be cheaper to use gas.
Or overnight cheap rate electricity.

Gas at 7p and 80% efficiency is still only 10.5p, which is less than a unit of exported solar.

Whilst prices remain high for export there’s no point in an eddi imho.

If you are maxing out your export and your solar begins to limit then an extra draw is useful to consume "lost" solar. The difficulty would be managing that

Hopeful Agreed. I was lucky (?) enough to not get an export limit imposed, but this can be a real issue for some people. Or if they have a very early FIT array on super-high deemed export rates you might not want to get a measured export tariff.

Its worth turning down the battery charge rate during the day to reduce the impact of solar clipping & export limits - trickle charging the battery and letting more go out to export every hour

geoffreycoan It does not matter if you are giving up an energy source or not the standing charge is part of the cost of supply and if you are comparing energy sources it is more accurate if one includes it.

I am interested in what people think the standing charge is part of if it is not part of the cost of supply.

It is a tax to pay for green energy and incompetent supply companies that went bust. It doesn't cost anymore to supply power to my home that it did in the past but the standing charge has rocketed.

Unless you are pricing up getting rid of one of the supplies you can't avoid it, so it doesn't figure in calculation.

killythebid I am interested in what people think the standing charge is part of if it is not part of the cost of supply.

Obviously standing charges are part of the cost of supply.

But equally obviously they are irrelevant to the question that was asked. Which was - is it cheaper at the moment to heat water using an existing Eddi or using an existing gas boiler? To include standing charges in that determination is not sensible, as these have already been committed to and will be paid whatever the decision. Any water heating is now a marginal cost.

Anyway - the answer is gas. And switching to Tracker will improve the saving even more.

The 90% quoted efficiency above is a good rate for a condensing boiler whilst it is condensing but with the high return temperature a better figure would perhaps be 85% in non condensing mode. Then you need to factor in the pipe losses between the boiler and the tank. Our boiler is in the loft with the tank in the airing cupboard so I'm reducing the efficiency to 75% because of that. There is also some lost in the transfer of heat from the primary pipework to the water in the tank which might be worse than the losses between the electric element and the hot water (conduction along the pipework etc.). So I'm actually assuming a poor 66% efficiency of gas water heating against electric. With gas unit rates at 6p from today I will heat by gas if the electric rates are 9p or higher (on the Agile tariff) or electric otherwise but only if there is no solar as I get 16.5p per unit export with the Eon Seg so I try to export all Solar.

PianSom Except the cost of a standing charge for electric is different to the standing charge of gas. The two standing charges do not cancel each other out in other words and need to be taken into account.

There are no losses in energy between electric element and the water other than perhaps conduction to the outside of the tank where it is mounted and then heat getting past the tank insulation. Poor thermal conduction doesn't loose any energy, it just slows its flow. That would make the element hotter as all the electrical energy has to be converted to heat, which has to go into the water. The higher temperature would restore the heat flow into the water to equal the electrical power.

Windy Miller With gas unit rates at 6p from today I will heat by gas if the electric rates are 9p or higher (on the Agile tariff) or electric otherwise but only if there is no solar as I get 16.5p per unit export with the Eon Seg so I try to export all Solar.

But the question is asked by someone using Intelligent Flux.

Even if they are - like you - suffering 66% losses the answer for the person asking the question is clearly gas.

nophead That's true of the element but there are some losses in the control circuit. The Eddi varies the element power to balance the available input power I believe so most likely incorporates a Triac. I use a 16amp power control module from CPC which also incorporates a Triac and estimate it loses around 1% of the load power as heat in the controller making it around 99% efficient as this energy is not imparted to the water. Somewhat off topic though now and I agree the OP would be better off using gas to heat their water (as I hope my calculation showed why).

Ive mentioned it before in these debates but don't forget to factor in cost of replacing element every 2-3 years if you switch to sole electric water heating approx 9p - 14p a day (£100 - £150).

DreadorUK Presumably that is only in hard water areas? I would expect an element to last longer than me in a soft water.

DreadorUK
I had to have my Megaflow element replaced last Nov. £360! Had to do a full drain down and refill, of course.

Another reason to use gas! 😄

_Con_, the cost calculations also need to take account of what proportion of the hot water tank is heated by the electric immersion element compared with the coil served by the gas boiler system, along with how much hot water you use each day. There is not much point heating a full tank using the gas boiler if you are only using half the tank that day, as you also increase the standing heat losses from the tank.

I use the electric immersion for water heating as it probably cost neutral with using the gas boiler, but only on my setup and tariffs at the moment.

The electric immersion only heats the top half of the traditional tank, so 1/2 the required kWh compared with the gas boiler heating the full tank. This suits two of us for showers and washing up for 6 of the 7 days a week. On typically 1 day per week, the system boiler gets turned up to 70-75 C flow set-point for around 2 hours whilst heating the full tank for a bath. The rest of the time time the boiler is turned down to 50-55 C for higher efficiency used just for heating the house.

Using an Eddi, my typical consumption is 1.5 to 2.5 kWh on cheap overnight rate along with 1-2 kWh solar top-up during the day, average is probably around 4 kWh/day for 6 days/week.

The economics all depend on the relative costs of the electric import and export and gas, along with the relative efficiencies of each system.

DreadorUK That's a very good point. I wonder if the life of the element depends on max. power, total power, hardness of water or some other factor (a combination perhaps). Do people think running the element below it's rating will help? I'm not sure but it may be possible to limit the Eddi to 2Kw into a 3Kw element but would it make a significant difference to the life of the element? It's strengthening the case for only using gas for heating water (even though that is the higher carbon source).

Simon_C The electric immersion only heats the top half of the traditional tank

Yes in a traditional tank where the element is inserted from the top but the gas thermostat is attached to the side usually about half way down the tank.

If you have a Megaflo tank the element switch and gas thermostat are side by side towards the bottom of the tank so probably allow the same amount of energy to be stored in the water when set to the same temperature. I actually have mine set with the element slightly higher than the gas stat. so that when I've heated it with electric I don't get a small top up with gas as that would be very inefficient as it would need to heat up all the pipework to get the tank that little bit hotter. I only switch the gas on at 6am when I haven't topped up with electric overnight.

Windy Miller hardness of water

This, basically.

New cylinders usually require fitting a magnetic filter on the cold water feed to the cylinder.

Magnetic filters to reduce water hardness are basically useless. The only way to reduce hardness is a full blown salt based softener. If your 'plumber' or heating 'engineer' suggests a magnetic filter tell him to unhitch his horse and bugger off. Disclaimer - my opinion only ....

Slartibartfast What's on ours seems pretty effective - hot and cold upstairs are filtered and the amount of limescale on showerglass etc is minimal compared to what you'd expect (and we've seen before) in this area (very hard water).

I tend to agree with you & we'll see how it goes but as I say its specced on the cylinder manufacturer datasheet so that's why its there.

We live in a very hard water area. Our ( convention Y plan ) HW tank failed after 20+ years with a pinprick leak. We replaced it with a modern insulated S/S cylinder. We only ever used the gas boiler to heat the water. Out of curiosity we chopped the top off the old cylinder to check for limescale and were really surprised to find it pristine inside, with just a very fine milky fluid lying on the bottom of the tank. A friend of mine who used the immersion element exclusively had major issues with limescale and when the tank finally failed there was about a washing up bowl full of limescale particles in volume lying at the bottom.

Vestas Windy Miller hardness of water

This, basically

So the build up of limescale effectively reduces the transfer of energy to the water. The element runs a lot hotter than it should and prematurely fails?