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Discussion Starter #1 (Edited)
Looking to get a new heating system installed in my house and I'm thinking of going with a Vaillant system boiler with a Megaflo tank. I'm also looking to get a combination of hot water underfloor heating system and normal room radiators. I'm also looking at getting solar PV panels installed and getting a Myenergi Eddi device to make use of excess generated energy.

My questions are:
  1. Will the Eddi work with a Megaflo boiler?
  2. Will the Eddi work with the under floor heating system and radiators (not sure how these heating systems work, do they get hot water from the tank or do they get a special feed from the system boiler)?
  3. Let's just say my water usage was very low, is the heat loss sufficiently low that heated hot water from solar is enough to keep water hot until the next day (assuming nice hot sunny UK days)?
  4. If I know the solar generation isn't going to be much (due to extra water usage or weather being miserable), can I setup the Eddi to partially heat some hot water during off peak electricity (using my cheap Octopus Go tariff) while still allowing it to use excess solar generation energy as and when it happens? (i.e. timed mode doesn't override solar generation mode).
Edit: Doh....ignore point 4. I'm getting myself confused, if solar can't heat the necessary amount of water needed then it'll be topped up using the gas from system boiler (which has a flat rate tariff).
 

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The Eddi transfers excess leccy to the immersion heater (providing it is capable of taking it temp wise) and can be set to manual boost for a certain time. It does not know about the boiler etc.

1) yes but not sure why you ask
2) The system boiler has a seperate CH loop
3) I do this all year round and only have water not hot enough for a shower on say 25days/yr Depends on usage, tank size,pipe runs and temp but does require a longer immersion heater element and the temp turned up a bit in winter.
4) Yes but the problem is you may over heat the water and thus reduce the amount of energy the tank can take if the sun does come out. Usually on the days when my water is not quite hot enough say 36 instead of 40C i just manually switch it on as it only takes a few mins to get it to 40C
 

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Looking to get a new heating system installed in my house and I'm thinking of going with a Vaillant system boiler with a Megaflo tank. I'm also looking to get a combination of hot water underfloor heating system and normal room radiators. I'm also looking at getting solar PV panels installed and getting a Myenergi Eddi device to make use of excess generated energy.

My questions are:
  1. Will the Eddi work with a Megaflo boiler?
  2. Will the Eddi work with the under floor heating system and radiators (not sure how these heating systems work, do they get hot water from the tank or do they get a special feed from the system boiler)?
  3. Let's just say my water usage was very low, is the heat loss sufficiently low that heated hot water from solar is enough to keep water hot until the next day (assuming nice hot sunny UK days)?
  4. If I know the solar generation isn't going to be much (due to extra water usage or weather being miserable), can I setup the Eddi to partially heat some hot water during off peak electricity (using my cheap Octopus Go tariff) while still allowing it to use excess solar generation energy as and when it happens? (i.e. timed mode doesn't override solar generation mode).
Not quite answering your question but I will chip in anyway.

Ten years ago we had a complete upgrade of the heating in this house which included installing a Valiant boiler (builder's choice) and a Megaflow cylinder. We had specified two pumped electric showers and the builder suggested the Megaflow system to give mains pressure hot water to get a powerful shower at less cost. So we did that.

The mains pressure here is not that high so the shower pressure is not as high as we expected but it's acceptable.

The Valiant boiler was a disaster. Virtually every part of it was replaced under warranty, one at a time. After the warranty ran out problems continued so we replaced it with a Worcester Bosch boiler as recommended by our heating man. Trouble-free for six years.

We have had solar panels installed just before the FIT was finally stopped and
that included a device that diverts excess solar power to our immersion heater rather than export it. Works fine but is not an Eddi and we are not playing around with off-peak power so I cannot comment on that aspect.

I wish you luck with optimising your system and hope you can benefit from my experience.
 

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Cant answer the OPs questions, but I would hope that the person responsible for the design of the DHW can provide an assurance that Legionnaires risk has been properly accounted for.
 

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If your H/W usage is low then the first thing is a very well insulated tank. The water needs to go above 60 C to be Legionnaire safe. It is possible to wire a modern boiler so the water temperature is higher when there is a demand for H/W. I run H/W heating into a Megaflow tank with stratification for 15 minutes a day and that is sufficient hot water for the next 24 hrs. The temperature of the circulating water when powering radiators alone is much lower. It is automatically adjusted by temperature compensation. A thermometer measures the outdoor temperature and the circulation water temperature is raised the colder it is outside. This helps the boiler run with less short cycling and keeps it in condensing mode. The circulating temperature you would need for underfloor heating is much lower than for normal radiators. If you wanted to run at one lower temperature you might need to oversize the radiators to compensate.

A lot depends of the ratio of your underfloor heating load to normal radiator load and just how well your house is insulated so you can trickle slowly rather than pump heat into the rooms.

An expensive alternative would be to go to a heat pump that feeds the underfloor heating and a small boiler to do H/W and top-up heating in very cold weather.

It would be worth getting a good C/H engineer in to design you a system.

If your H/W use is low, perhaps do away with a H/W tank, using a Combi instead of a system boiler and put excess solar PV into a battery instead or sell it back to your energy supplier. Electricity is high grade energy so to turn it into heat which is low grade energy is rather wasteful as it is better to use it as high grade energy.

I would echo avoiding a Valiant. Worcester Bosch is fine but a lot of the trade would recommend an Ideal boiler.
My excellent but expensive plumber who does beautiful neat work replaced a worn out boiler with an Ideal and it has been very reliable and sturdy. He would not fit anything else saying this is the brand that gives the minimum of call backs for him and for installers provides the best back up support.

Lots to think about.
 

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Discussion Starter #6
Thanks for the replies. I've removed point 4 as it was written in haste.

I'm still a little confused with how the hot water system works for heating. Say for example, the hot water tank has plenty of hot water. If the heating turns on, will the system use water from the tank that is already hot or will it heat up other water that is already in the central heating system (e.g. using water in a closed loop system)?
 

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If you're on mains gas then you'd be wasting money installing anything other than a gas combi boiler. An Eddi or equivalent would cost you money as your surplus generation (which would earn you 5.5p/kWh if exported) would be offsetting gas which only costs 2.5p/kWh.

Don't get sucked in by the claimed savings based on the hypothetical cost of the electricity an Eddi might divert; you wouldn't heat water with imported day rate electricity so it's not a valid calculation. Our gas bill for the last 12 months is around £250 including heating, hot water and cooking with our new combi boiler.
 

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If you're on mains gas then you'd be wasting money installing anything other than a gas combi boiler. An Eddi or equivalent would cost you money as your surplus generation (which would earn you 5.5p/kWh if exported) would be offsetting gas which only costs 2.5p/kWh.

Don't get sucked in by the claimed savings based on the hypothetical cost of the electricity an Eddi might divert; you wouldn't heat water with imported day rate electricity so it's not a valid calculation. Our gas bill for the last 12 months is around £250 including heating, hot water and cooking with our new combi boiler.
In the post FITS world that is correct but if i may add
1) It does not take account of efficiences and that because of gas buring efficiency,stop/start,pipe runs in the non CH period it is difficult to achieve greater than 50% energyin v energy out for small users. It does however favour bath users over shower users.
2) the PV is not about HW but providing leccy for the house. The HW is an aside. Need to consider the overall picture.
4) when you have the EV the available excess leccy in winter is very low so again we are considering small usage case.
5) For 2 of us our HW usage is small and amounts to c900kwh/yr
6) Nice to be green even if it costs a little more.:)
 

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Discussion Starter #9
Just to add some more information. We are a two car household (one is an EV the other is an ICE). I hope in the not to distant future to switch the ICE to an EV too. Due to a recent change, neither I nor my wife use a car for work commute (although this may change in the future). Although I have missed out on the FIT payment scheme, I'm not sure the SEG (Solar Export Guarantee) scheme is useful. This is because you have to sign up to a specific tariff which means I will lose out on the Octopus Go super cheap off peak rate which is very useful to charge up the EV.

So, right now it seems like having solar PV panels would be appealing because my EV would be at home during the day and I can use that to take the generated power. However, if I had an Eddi as well, then I could get some free heated hot water too (or so that's my thinking). If the generated solar isn't enough to charge my EV to my desired level, then I have my super cheap electricity rate to top it off. I'm also, quite happy to use my washing machine, tumble dryer and dishwasher on a timer to make use of the cheap rate too (unless I know there will be enough excess solar in the day, in which case I could set it off to go during the day).

Therefore, overall I think going on a SEG tariff would end up worse off than sticking with Octopus Go tariff.
 

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I'm still a little confused with how the hot water system works for heating. Say for example, the hot water tank has plenty of hot water. If the heating turns on, will the system use water from the tank that is already hot or will it heat up other water that is already in the central heating system (e.g. using water in a closed loop system)?
The water in your hot water cylinder (it's not a tank or a boiler) is heated indirectly: The water in the boiler and central heating is called your primary loop and is pumped around by a pump. There will be 2 motorised valves, one of which will allow water to flow to your radiators, and the other will allow water to flow to the hot water cylinder. They are controlled by thermostats and a programmer, so open when needed, and it's the act of either of these valves opening which tells the boiler to fire. The hot water cylinder has a pipework coil inside it, and the water from the primary goes through this coil, which indirectly heats the rest of the water in the cylinder. The hot water which comes out of your hot tap has not been through your boiler.

So to answer your question above, hot water system doesn't do anything for heating. No heat will be taken from your hot water cylinder to heat your house.

Hot water cylinders usually have an immersion heater element as well, and these sit directly in the larger volume of water in the cylinder and really have nothing to do with the boiler at all. Eddi would normally be wired to this immersion heater. The immersion heater has it's own thermostat and this is again nothing to do with the other thermostat which controls the boiler fired hot water system.

Eddi can also be used in conjunction with heating but this is a bit unusual. You can get cylinders which can take an immersion heater and be plumbed into your heating primary. But in the UK there's not a great deal of point; The time when the sun is shining and you have a surplus of PV is not really the time you need heating, and when you need heating, the sun is hiding behind clouds.
 

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So, right now it seems like having solar PV panels would be appealing because my EV would be at home during the day and I can use that to take the generated power. However, if I had an Eddi as well, then I could get some free heated hot water too (or so that's my thinking). If the generated solar isn't enough to charge my EV to my desired level, then I have my super cheap electricity rate to top it off. I'm also, quite happy to use my washing machine, tumble dryer and dishwasher on a timer to make use of the cheap rate too (unless I know there will be enough excess solar in the day, in which case I could set it off to go during the day).
My PV was self installed and so I don't get FIT at all so I'm pretty motivated to self consume. Hence EV, zappi and eddi. I do pretty well and often program the dishwasher and dryer to run in the night on octopus go.
I've also bought the relay board expansion for eddi which I've wired to a microcontroller so that my home automation stuff can know when I have a certain amount of export and switch some other stuff on automatically too.
 

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In the post FITS world that is correct but if i may add
1) It does not take account of efficiences and that because of gas buring efficiency,stop/start,pipe runs in the non CH period it is difficult to achieve greater than 50% energyin v energy out for small users. It does however favour bath users over shower users.
2) the PV is not about HW but providing leccy for the house. The HW is an aside. Need to consider the overall picture.
4) when you have the EV the available excess leccy in winter is very low so again we are considering small usage case.
5) For 2 of us our HW usage is small and amounts to c900kwh/yr
6) Nice to be green even if it costs a little more.:)
  1. Ok, but at 50% efficiency you still lose 0.5p/kWh compared to export.
  2. PV is fine (if marginal post FIT), an Eddi is not.
  3. ?
  4. ...
  5. So, with low HW usage, there is even less possibility that the Eddi could ever pay for its self (not to mention the associated plumbing and electrical work, tank etc).
  6. It's objectively greener to export the electricity rather allow the demand to be met by burning gas at a power station.
No matter how you spin it, there is no viable use case for installing an Eddi with a new (post FIT) PV installation for anyone with mains gas.
 

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No matter how you spin it, there is no viable use case for installing an Eddi with a new (post FIT) PV installation for anyone with mains gas.
I don't really understand that? If you have surplus PV generation, and no feed in tariff, then you either give it away for free, or use it to heat water.
 

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Discussion Starter #14
The hot water which comes out of your hot tap has not been through your boiler.
Thank you so much for the detailed reply. I understood everything except this bit above. So forgetting the solar PV and Eddi for a minute, are you saying the hot water from the hot tap is heated via the immersion heater in the hot water cylinder?

I thought the system boiler heated the water (via gas) and sent it to the hot water tank.

#SoConfused
 

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I don't really understand that? If you have surplus PV generation, and no feed in tariff, then you either give it away for free, or use it to heat water.
Why would you give it away for free when you can earn 5.5p/kWh from export payments?

Maybe you don't realise that FIT payments pay you for generation and deemed exports. It (vaguely) makes sense to self-consume as much as possible while you're still being paid the export tariff regardless. However, a new PV system will only earn for metered export.

It would be pretty ludicrous to install PV in the UK in 2020 without having an export meter (i.e. any new smart meter).
 

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Why would you give it away for free when you can earn 5.5p/kWh from export payments?
As I understand, the time when you're generating solar, the price of electricity is going to be at normal day rate which is around 15p/kWh. So it's not that you'd give it away for free, it's that you'd try to maximise self consumption as much as possible.

Additionally, you only get export payments if you join a specific export tariff. e.g. you can't get export payments on the Octopus Go tariff.
 

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We have 8.4kw of panels, a battery, and an Eddi (plus a Leaf of course).

Our battery fills itself up first with any excess (up to 3.6kw of power). The Eddi will divert anything that would be exported into the immersion heater (up to 3kw of power). Seems to be ~5kwh a day, with one shower and one bath each day. Once the max temperature is reached the display says "max temp reached" and the rest then goes off to the grid. Our gas boiler is set to come on for half an hour from 17:30 to 18:00, making sure there's hot water for the evening if it's been a cloudy day. Since September it's sent 640kwh into the tank. As Petriix mentions if you have mains gas you are better off financially exporting your generation and burning gas, but if you're into reducing carbon (or being smug about self usage) it's worth it.

When the Agile rates go negative i will set the boost timer on the Eddi and it will come at that time, but in reality it's only replacing the energy from one evening bath, so can only get a couple of kwh in there.

I have been considering getting a Zappi (Zappi and Eddi will talk to each other) but in reality, with Agile (or Go if you can't avoid the 4PM-7PM high price), it's cheaper to charge in the night and export in the day. Again, if you want to reduce carbon or be smug it's ok, but will not save you money. Today I plug in the EV when it's sunny on the granny lead, and even if a cloud goes over the house battery can output 3.6kw so I don't end up drawing from the grid. Once "normality" returns I'll go back to night charging ready for the morning school run.
 

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Why would you give it away for free when you can earn 5.5p/kWh from export payments?
Additionally, you only get export payments if you join a specific export tariff. e.g. you can't get export payments on the Octopus Go tariff
I hadn't realised FIT had been replaced with something else.

I get neither; anything I export is truly given away.
 

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As I understand, the time when you're generating solar, the price of electricity is going to be at normal day rate which is around 15p/kWh. So it's not that you'd give it away for free, it's that you'd try to maximise self consumption as much as possible.

Additionally, you only get export payments if you join a specific export tariff. e.g. you can't get export payments on the Octopus Go tariff.
I'm not sure why you would maximise self consumption. You would want to minimise import costs; but that is something entirely different. If you can shift your normal usage to when you are generating or to when import costs are low then you will maximise your savings.

I'm not fully aware of the limitations with the Octopus Go and Agile tariffs; I was under the impression that you could shop around for an export tariff but maybe that's wrong, in which case the sums are more complicated as you have to weigh up both the import and export tariff.

In any case, creating extra consumption for the sake of it is not a real saving.

...As Petriix mentions if you have mains gas you are better off financially exporting your generation and burning gas, but if you're into reducing carbon (or being smug about self usage) it's worth it....
I can understand why you think like that, but I absolutely contest the idea that self consuming electricity can possibly reduce carbon - unless you're in one of those rare times when total renewable generation is exceeding the total grid demand. Almost all short term demand on the UK power grid is met by burning gas at a power station (at around 56% efficiency) so, every kWh you export is directly preventing at least 1.6kWh of gas consumption.

It possibly makes sense (although it's arguably pretty selfish) to use an Eddi or similar if you qualify for deemed export payments. However, the cost vs benefit analysis should be entirely based on the cost of the fuel it is preventing you from importing. I've seen countless claims that an Eddi can save you £400 per year; but, if you're only paying £50 per year on heating water (like me), and you understand that an Eddi can't possibly meet all of that demand, and you factor in the entire cost of installation including buying the unit plus plumbing and electrical work, then the case doesn't really add up.

I'm not saying people with an Eddi shouldn't use them; just that the vast majority of people would be wasting money by installing one.
 

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I'm not sure why you would maximise self consumption. You would want to minimise import costs; but that is something entirely different. If you can shift your normal usage to when you are generating or to when import costs are low then you will maximise your savings.

I'm not fully aware of the limitations with the Octopus Go and Agile tariffs; I was under the impression that you could shop around for an export tariff but maybe that's wrong, in which case the sums are more complicated as you have to weigh up both the import and export tariff.

In any case, creating extra consumption for the sake of it is not a real saving.



I can understand why you think like that, but I absolutely contest the idea that self consuming electricity can possibly reduce carbon - unless you're in one of those rare times when total renewable generation is exceeding the total grid demand. Almost all short term demand on the UK power grid is met by burning gas at a power station (at around 56% efficiency) so, every kWh you export is directly preventing at least 1.6kWh of gas consumption.

It possibly makes sense (although it's arguably pretty selfish) to use an Eddi or similar if you qualify for deemed export payments. However, the cost vs benefit analysis should be entirely based on the cost of the fuel it is preventing you from importing. I've seen countless claims that an Eddi can save you £400 per year; but, if you're only paying £50 per year on heating water (like me), and you understand that an Eddi can't possibly meet all of that demand, and you factor in the entire cost of installation including buying the unit plus plumbing and electrical work, then the case doesn't really add up.

I'm not saying people with an Eddi shouldn't use them; just that the vast majority of people would be wasting money by installing one.
Yes if I were installing again I don't think i'd get an Eddi, but I had mine installed after FIT had ended and before SEG had begun. But I do get to be smug about my hot water being solar powered.

It might not reduce net carbon, but it does reduce my carbon. Is that good or bad? We're getting into an area where it's really not that important in the context of national emissions, it's really a psychological and emotional gain for the buyer, and there's no sense arguing about that - after all what's the payback period on a playstation?
 
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