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

 

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.

 

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.

 

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.

 

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)?

 

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.

 

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.

 

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.

 

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.

 

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.

 

I suppose my point is that you can feel happy about reducing carbon when you export to the grid without feeling the need to self consume. It's not my intention to criticise people for their past decisions. I just want to help people make properly informed choices before opting to pay for expensive devices that may well not make economic sense.

 

Look at Sunamp: Sunamp Heat Batteries & Thermal Energy Storage
I installed a Uniq 12 last autumn to replace my old hot water cylinder. Installed an Eddi with it, to make use of our 9kW PV array. It provides mains-pressure (potable) hot water - no stored water, heated "on demand" - and also runs a small heating circuit (three towel rails, two small in cloakroom and ensuite and one large in the main bathroom).
Figures given by the Eddi suggest:
2.12kWh today
3.85kWh yesterday
29kWh this week
227kWh this month
953kWh this year

As you might infer, it's been unusually cloudy the last couple of days around here. The overnight Boost has been disabled for the last 2-3 months so all of our water heating (and the towel rail heating) has been supplied exclusively by solar - the heating circuit runs for 3.5 hours per day (05:30-07:00, 12:30-13:30 and 20:00-21:00 daily) and we still have hot water at full temperature, despite little input the last couple of days. The Uniq is extremely well insulated (standing losses of about 28W if properly installed, according to the specs), using vacuum insulation panels.

 

First, I agree that the cost of an energy diverter cannot now be justified when you can get as much for exporting a kWh as you spend on burning gas to heat your hot water.

However, if you are going to do it, you can safely use the boiler and the immersion heater together as follows.

Set the immersion heater thermostat to slightly higher temperature (45 c as I recall).
Set the boiler water heating thermostat to a slightly lower temperature (42?) and set the timer on the boiler system to 17:00 - 22:00 or so. The solar will heat the water as much as it can during the day and the boiler will only run in the evening if the solar gas not reached a good temperature.

This has worked perfectly for us for 6 years.

Note - check the minimum thermostat settings against the recommendations for legionella etc.
Rethink the timings if you have a variable rate tariff.

 

I had a 3.8 kW. PV system installed just before the FIT deal expired.

This year I added a new gas boiler which I just use for CH and have installed a new DD250 Megaflo (250 litre direct).

3 months ago I installed an Eddi and a hub, and the Eddi has great functionality for a twin immersion tank. Additionally I can monitor the house consumption, HW consumption, and PV consumption with the Eddi, as you can with a Zappi.

I'm pleased I don't have to run the boiler in the summer and there is no diverter valve and piping.
It does a bath and shower a day and I'm happy with it, although it has not been done to save money, it is what I wanted. I will need to top up overnight in the winter, but I will most likely then be on Octopus agile and be able to pick the cheapest and greenest top up times.

The cost of electricity is high as the electricity consumer has to pay towards subsidies for green generation and more, whereas gas has no charges against it. Hence cost wise it is not an even playing field, but the government have realised this and this will change in the future, most likely very slowly.