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Water "Battery" 20kwh+ of extra storage

  • 19-06-2022 10:05pm
    Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭

    A few of you know what I've been up to as a side project/experiment (with the bonus of burning less stuff and exporting less)

    Is to put a buffer tank in my heating system. A 500L one, plumbed (and wired) in as if it was a oil boiler.

    Wired in such a way that if the tank is cold it can fall back to oil.

    I prefer to maintain a temperature in the house than let it cool and then blast it but that's by the by. I also have a tado heating controls installed so only the rooms that need heat will call for heat. all rads except bathroom & ensuite are on smart trvs.

    Now for the fun bit some pics and graphs

    500L joule stainless buffer tank, there is dual immersions but unfortunately the bottom temp probe is below the immersion.

    Got the tank heated to 55c, used about 21kwh, (still haven't the full monitoring built for it yet)

    A call for heat occured at 9am and (actually I think continued for the rest of the day, there was a trv complaining about a low battery and might be full open)

    But it's interesting how quickly the tank mixes and not all of the heat is taken out of the water before it returns.

    This was the rest of the day. About 16kwh of heat has been put into it.

    Still learning and working out the ins and outs of it, may put a flow meter on it, then I could work out the kwh out of it.

    Its been a fun project and this is just the start of it.

    Also as a side bar, my night rate is currently 5c.. so that's a 1/3 of the price of oil currently! (Its 1.50 currently at the pump and I don't have a new boiler never mind a condensing one!.) We shall see what rate I change to in August.

    Got a bunch more temperature probes so should be able to read the temperature of the flow and return water, and maybe get one in that dry pocket above the bottom immersion.



  • Registered Users Posts: 6,175 ✭✭✭Ubbquittious

    How do you heat it? Just electric? Is it something that you could heat using solar tubes as well?

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Immersions. But yeah if you got a tank with a coil you could use solar tubes.

    But I could stick up another 8kwp SW facing without much hassle. But about 100m away from the tank.

  • Registered Users Posts: 6,175 ✭✭✭Ubbquittious

    that is a good bit away. otherwise you'd probably get more bang per m2 with tubes. I like the idea, its something u could combine with solid fuel stove or heat pump. Do u have a means of transferring from the buffer tank to the hot water tank for the taps?

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    I have plumbed it that I can charge it from the solid fuel stove in the house. But didn't want to complicate the main post with that. This tank is in the garage and reusing the existing heating pipes, I can "back feed it" but that automation is still to be built/figured out but the function/hardware is there. - we still have turbatory rights too

    As for a heatpump, yep I should be able to connect one directly to the tank, HP heats tank, and then house uses it as needed. That's the theory anyway.

    House when running on the stove generally runs best at a lower temperature rather that cycling on and off when hitting 65 or something, - controlled now via automation. A sharp rise in temperature is enough to trigger it and it stays on for longer than a normal pipe stat would. (Pipe stat still is there as a failsafe)

    As for heating the DHW, yes I could but I just run the immersion in the DHW (which is also heated via the solar tubes and the stove). Currently not connected but it is possible.

  • Registered Users Posts: 182 ✭✭Fantana2

    Interesting project! In winter how long do you think the a full tank would keep the heating running for? Was it a TRV issue or have you the heat on this time of year? We wouldn’t normally have the heat on at all from May to end September.

    6.96kwp South facing

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  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Heat is "on" mainly for testing but oil is switched off.

    No idea how long it will last.. depends on how cold the house is I suppose. I'll find out when I actually need heat!

  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    I'd love to do something similar with a heat pump, use it with a large buffer tank to pre heat water on the night rate for space heating during the day

    With a reversible pump it could also pre chill the tank in summer using excess solar to help cool the house in the evening

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 1,102 ✭✭✭manonboard

    Very interested project. Thanks for sharing!

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk


    Just came across this :

    Basically it's a thermal battery 40kwh.

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    A bit more digging

    Charge rate of up to 9.3kw

    Cost, gbp : 6k, + install of 1.5-2. so about £187/kwh

    3.6kwh pylontech 1300 - €333/kwh

    aliexpress cells 32, looking like about 3k now, 20kwh €150/kwh, + bms + building costs

    Hot water tank, 500L about 20kwh. coming in around 2k (inc eddi) €100/kwh + my time

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  • Registered Users Posts: 653 ✭✭✭bunderoon

    Would you mind sharing the links to the temp probes and circuit board you have? I was planning on using my old pi3B, a '1-wire' probe along with

    I have just a basic digital display with wirey temp probe stuck into the top sensor. I see that yours has a white plastic grommet/connector of some sort.

    Are these what you used and then pushed the probe tips into them?

  • Registered Users Posts: 4,417 ✭✭✭Gerry

    Very interesting. I wish I could fit enough panels to make something like this a reality. Could I ask how you've wired the immersions for control.. do you have a contactor for the pi to trigger?

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    yeah the plastic white bit is just a normal stuffing gland/compression gland that screws into the thermopocket.

    Im using the ds18b20 temperature sensors. but need to cut the heatshrink off to fit into it. the gland stops it from coming out. the probes fit prefectly into it.

    The circuit board is just some protoboard with some headers and a 4.7kohm resistor on it. Its running on an ESP8266 wemos d1 board with a0.66" oled screen. The code is something that i threw together.

    Simply, an eddi runs the immersions.

  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    @graememk I'd love to hear an update on this project. Have you had need for the heating yet and did the heat battery provide much heat before you needed to switch to oil?

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Haven't had much need for heating just yet. But have been able to cycle it a few times so far it has been a nice boost on the bad days after the good. No proper good runs.

    Not put it to the test yet with a full house load. Although running the flow temperature at 45 c out of the tank is working well. (Testing the viability of a heatpump)

    Haven't fallen back to oil .. Well there isn't any in the tank 🙂

    I have lit the stove the odd evening and can successfully heat it to about 60c, going past that is difficult as the open rads in the bathroom leak heat. 2 bathrooms and a towel rail. Thinking of dropping the 2 bathrooms to smart TRVs and leaving just the towel rail open.

    Easy to cycle it down to 30c. Possible to go colder, but house needs to be colder.

    Stove is rated at 6(wood)-9(coal) kw to water, depending on the fuel.

    When the colder months roll in, stove is usually running 24/7 but just ticking over most of the day and closed down overnight.

    But planning on heating the tank on night rate to give a boost in the morning. (Currently 2x cheaper than oil)

    I'd say I'd need another 8kwp of panels to be able to heat the tank every day in Sept + Oct.

    TLDR no proper test yet, solar can heat it, and I'd estimate a full house load it would last 2 hrs but majority of it goes in the first hour. Winter heating will be solid fuel and night rate electric

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    I have sensors on the tank top middle, one between middle and bottom and bottom.

    You can "see" the tank depleting as the heating is running. But tends to heat all at once (if using the bottom immersion)

    Also have sensors on the flows out of the tank so I can see the flow out of the tank, the mix out and return.

    Considering adding a motorized valve to allow the bypass of the mixer valve when temps are below say 40 but unsure if it would make any difference,

  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    So I'm trying to make sure I've got this right in my head

    You've got a big buffer tank which is just a 500l direct tank with immersion heaters

    The immersion heaters heat the tank from the night rate or solar PV

    Then when your thermostats call for heat it starts the circulation pump which cycles the water from the buffer through the rads

    If the tank water is say 50C and your oil boiler is set to 45C then your boiler doesn't kick in because the water is already hot

    Then when the tank is "depleted" the oil boiler starts up

    Do you then use valves to zone out the buffer, or does it get "recharged" by the oil boiler as well when it heats the rads?

    I'm contemplating doing something similar with a heat pump at some point and just trying to understand the system

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 73 ✭✭Punchin A Keyboard

    The heat geeks do recommend a buffer tank as part of a hp install

  • Registered Users Posts: 23,246 ✭✭✭✭mickdw

    Alot of people here in mayo have reverted to turf burner furnace in shed for central heating, removing oil burners. They are though doing it without buffer so little to no control over heating.... It fire is strong, heat is strong and no heat in the morning til you get the fire going.

    A buffer tank would make that a very good system too being able to release heat to the house in the mornings and soak up that unneeded heat in late evening.

    Turf is plentiful here.

  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    True, but typically it's around 50l, not 500l like @graememk has

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

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  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    I've seen a few stove installers with recommendations to install like a 300l buffer, pretty much to absorb the excess heat and release it back slowly

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 1,341 ✭✭✭Nelbert

    Would this be doable with a heat pump with the internal unit having a 200l tank already built in?

    Is it purely a plumbing thing or would it needs it's own pump?

    Electrics I'm fine to DIY but water I don't like...

    Would be potentially interested in this as a way to avoid running the HP on day rate over winter.

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Yep its a Stainless Steel Direct buffer with 2 immersions. no coil, just has 6 1" fittings on it with a few 1/2" for thermostats/etc

    I have a tado, but yeah when there is a call for heat, That signal is sent out to the "plant room/energy room/ whatever , Its a room in the garage. So a bit of a leak wouldnt be the end of the world during setup/install.

    There is a thermostatic mixer valve on the outflow from the tank and the tank has its own pump.

    It is plumbed in parallel to the oil boiler with one way valves to prevent the water going the wrong way.

    Currently cannot heat the tank from oil, but with a bit of wiring and another relay I could do it, but theres no real point, better to just heat the house with it.

    There is a manual thermostat on the buffer tank so if it drops to below 20c, It switches the call for heat from the tank pump to the oil burner and its pump. It will work even if the automation crashes/fails.

    I have tado TRVS on all rads except the bathroom + ensuite, Only really need 1 open radiator for the stove. Might consider changing a valve or 2.

    I have a few other pipework/valves to allow back feeding from the stove.

    Explained any better? Tank is essentially working as a oil boiler.

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Stove + buffer would be v good, house can call for heat when required, can use it later in the evening/morning.

    Buffers with heatpumps seem to help with flow + cycling but with a cost of less efficiency but in reality it wouldn't store that much heat, 500l moved 50c - 30c is about 10kwh of heat.

    Would nearly be better to just use batteries.

  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    Gotcha, so the two are essentially like parallel heat sources. Sounds like a good setup being able to switch between the two and not have to lose a whole load of heat from the boiler to heat the buffer

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 10,769 ✭✭✭✭the_amazing_raisin

    One of the main reasons you need a buffer with a heat pump is to stop the outdoor unit from icing up

    Because it's cooling the outside air then condensation can freeze on the outdoor unit when it's cold outside

    So it heats up the buffer first and when the outdoor unit gets iced up then it switches off the heating zones and recirculates the warm water from the buffer through the outdoor unit to defrost it

    Also it helps to stop short cycling the heat pump and other advantages

    I think if I was doing the heat battery idea with a buffer tank, it'd probably need to use the immersion heaters to get the water up to a higher temp. The heat pump probably can't get the water hot enough for it to be worthwhile

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    I suppose If you were pre charging the tank with a heatpump on night rate, Even if your COP went to 2, its still twice as good as an immersion.

    Had a call for heat at 8.. when we got home, possibly left the door open!

    I think i might need to tweak the mix out, its running a bit high. Might stick another temperature sensor in the upper dry pocket. as the "middle" could be about 1/3 of the total heigt of tht tank. That was about a 20 mins run time.

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Starting to get a bit colder outside, some graphs from this evening, I am now keeping an eye on calls for heat too.

    Living room felt a little bit chilly so I bumped it the thermostat up to 21, that being said I'm still in a t-shirt and shorts!

    Stove was not lit today, but all the air is closed on it to limit heat loss.

    Not sure what "state of charge" the tank was in, as the middle temp sensor is below the halfway point. Maybe about 40%?

    State of play in the house

  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    Came across this Heat Geek Video.

    If you think my setup is unusual, they are running an experiment with

    Solar thermal, Sunamp, Buffer tank, Heat pump..

    The jist is,

    Solar PV heats the Sunamp, and DHW comes from that. When that is hot it diverts to the buffer.

    Heatpump is connected to the buffer and that heats the space heating.. I think it uses the buffer then the heatpump takes over

    If buffer gets completely hot, Its moved out to another storage in the garage, and then also can be moved back if needed to the buffer.

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  • Moderators, Home & Garden Moderators Posts: 5,727 Mod ✭✭✭✭graememk

    A further update.

    It's working really well, I can either heat it from the stove (takes about 4 hrs, keeping the fire stoked), hard to get past 60ish. It's allowing load shifting, and I'd say more effective use of turf.

    Or I've started heating it via the immersion overnight.

    Been playing with the temperature in the living room too. Currently set to 22. Usual setpoint is 20/21. Only in the evening, setback rest of the day is 18.

    Fire is lit but all closed down today. Bumped the temperature to 22 as I could still feel a very slight "chill" in the air...and I'm in shorts.

    I have new window seals on the way for the windows (approx 20 yrs old, and can feel cold breeze at the window when it's windy)

    Onto the data:

    Put 12kwh into the tank last night and topped up with solar (charging the house batteries to 100% now every day, excess can go to the tank. Started this evening at 70+c.

    The majority of the sensors are at bottom of the tank, and the "middle top" is just under the top immersion. Not sure how to monitor the top half of the tank.

    You can see the tank being used up, and the heat requirements for the rooms in the house(ensuite window was left open!)

    You can see the tado cycling the pump from the tank, only running a flow temperature out of the tank, using a mixing valve turned to 45c, this is my "virtual heatpump"

    Rough estimates are looking like the radiators are oversized, having been upgraded over the years.

    I basically have a electric boiler that runs on night rate.