Hot water management in an extra tank

[andrewellis]

Hi,I’d love some advice if possible. I have a NIBE 1255 which only has a 180liter internal tank.  If running on eco, it takes too long to reheat the water after the kids baths before us adults can have a bath in the evening.  The pump will give 30 minutes to the central heating and 30 mins to the DHW.  If i change the ratios it can get too cold in the house.  To that end I have got myself a 4 year old 250l steel unvented cylinder.  This will allow me to buffer more hot water in the afternoon whilst we are not at home off the economy 10.  As a bonus I can use the 5kw solar pv to save money during the day as well.

I have come up with a few plumbing schemes but the problem I have come across is the control valves.  Ideally I’d like to use some central heating valves but my research has come up with the following problems.  

1. How important is the WRAS certification?  We do use the water to preheat the kettle and cooking in the pans.  None of the central heating ones seem to have the certification.

2.  Corrosion in the drinking water circuit seems to be more of a thing. Ie can’t use central heating pumps for Destratification.  Would using 3 port valves be an issue on this front if not a problem for point 1.

I have links to three diagrams.  They have the tanks in serial with extra plumbing to allow shifting the water from one tank to the other.  I haven’t drawn in the mixer valve to the house.  In order of preference I have

Setup 1. https://ibb.co/rxGNWFT.  This uses central heating three port valves to direct the water in either direction. Ie in winter hot water from the top of the nibe tank to the top of the 250l. In the summer hot water from the 250l tank could be shunted to the NIBE internal tank.  Allows maintaining stratification in either direction.  

Setup 2. https://ibb.co/yqgWrkX Uses expensive solenoid wras valves to shut of pipe work not required outside of pumping.  This allows efficient moving of water when heating using the NIBE.  I can’t seem to find any reasonably priced motorised valves with WRAS certification.

Setup 3.  https://ibb.co/SRc0Htp No valves. Uses a hot water circulator.  The problem with this is I wouldn’t be able to maintain stratification and I would end up heating warmer water at the bottom of the NIBE resulting in reduced COP.


To head off the inevitable points, I’ve left out fittings required for unvented cyclinders as I am focussing on the control side.  I am also having a qualified plumber doing the install as I am not qualified.

[Countrypaul]

I could see option 3 not giving what you want as you say, when you push hot water from the Nibe to the 250L tank it would cause mixig of the Hot and whatever is in the tank since the hot is entering at the bottom. Put a small amount of hot in to a cold tank and you end up with a whole tank just slightly warm.

I can't see what you are trying to achieve in option 1 with 2 x 3way valves bypassing the pump. The use of a check valve in Options 2 & 3 seems much simpler.

Whilst you say you haven't drawn the mixer valve for the house, it might be very relevant to do so. If your 250L tank had only luke warm water, but the Nibe had Hot water that needed mixing (to prevent scalding) then mixing it with luke warm rather than cold water could save you a significant portion of the hot water in the Nibe.  You would need 2 TMVs one mixing cold and water from the 250L tank, the second mixing the results of the first with Hot from the Nibe. If the 250L was entirely hot then no hot would be drawn from the Nibe.

[rogeriko]

We have had this problem with Nibe internal tanks and the problem is Nibe is made in Sweden where they do not have baths. They shower. You cannot expect to have 4 baths out of a 180l tank of warm water. Nibe heat pumps are for showers.The tank only reaches 50 degrees at the top the bottom will be cooler so you probably only have 100l of warm water. We put a destratification pump on one tank to help but it just increased the electric usage.

[andrewellis]

I can't see what you are trying to achieve in option 1 with 2 x 3way valves bypassing the pump. The use of a check valve in Options 2 & 3 seems much simpler.

With this i intend to allow the pump to run either one way or the other.  The valves would all shuttle and each would power the next until the pump turns on.  I could then send water in either direction to save the problem of mixing to lower temperatures.  I was heating with the NIBE then I would shuttle 50 liters or so at a time across to the buffer tank.  This would maintain some hot water in the nibe for lighter use.  Vice versa with hotter temperatures generate from the solar immersion I could shuttle into the NIBE thereby increasing the size of the storage for the pv.
https://ibb.co/sHsfhs2
https://ibb.co/mTTqqcH

Whilst you say you haven't drawn the mixer valve for the house, it might be very relevant to do so. If your 250L tank had only luke warm water, but the Nibe had Hot water that needed mixing (to prevent scalding) then mixing it with luke warm rather than cold water could save you a significant portion of the hot water in the Nibe.  You would need 2 TMVs one mixing cold and water from the 250L tank, the second mixing the results of the first with Hot from the Nibe. If the 250L was entirely hot then no hot would be drawn from the Nibe.

That’s an interesting idea. I still have the problem of heating the water in the 250l tank in the winter. I don’t want to use the immersion unless there is spare pv.  However it would allow option 2 & 3 mixing warmer temperatures in the tank. I shall have a look at that.

[Countrypaul]

I can't see what you are trying to achieve in option 1 with 2 x 3way valves bypassing the pump. The use of a check valve in Options 2 & 3 seems much simpler.

With this i intend to allow the pump to run either one way or the other.  The valves would all shuttle and each would power the next until the pump turns on.  I could then send water in either direction to save the problem of mixing to lower temperatures.  I was heating with the NIBE then I would shuttle 50 liters or so at a time across to the buffer tank.  This would maintain some hot water in the nibe for lighter use.  Vice versa with hotter temperatures generate from the solar immersion I could shuttle into the NIBE thereby increasing the size of the storage for the pv.
https://ibb.co/sHsfhs2
https://ibb.co/mTTqqcH

Maybe I am misreading things, but if the flow is as given in your diagrams why would the draw anything from the Nibe or put anything into the 250L when it could simply circulate in a very tight loop (anti clockwise)? You might want to drop one of the 3 ways and add some NRVs.

Whilst you say you haven't drawn the mixer valve for the house, it might be very relevant to do so. If your 250L tank had only luke warm water, but the Nibe had Hot water that needed mixing (to prevent scalding) then mixing it with luke warm rather than cold water could save you a significant portion of the hot water in the Nibe.  You would need 2 TMVs one mixing cold and water from the 250L tank, the second mixing the results of the first with Hot from the Nibe. If the 250L was entirely hot then no hot would be drawn from the Nibe.

That’s an interesting idea. I still have the problem of heating the water in the 250l tank in the winter. I don’t want to use the immersion unless there is spare pv.  However it would allow option 2 & 3 mixing warmer temperatures in the tank. I shall have a look at that.

You could still shuttle water from the Nibe to the 250L which would save on the immersion (assuming the Nibe is much more efficient).

Is there anyway to trigger the Nibe (no idea how much power it requires) when there is plenty of PV, that wayyou would be making much more of the PV than a simple immersion would give you. I know you could not drive it directly from your PV diverter as that would require a resistive load, but turning on the Nibe when the PV is prodcing 3kW and "off" when it drops below 2 would be a possibility (exact figures would be for you to determine). The off would have to be such that the Nibe could run for its minimum run time unless the PV turned it on during that period  so not straightforward.

[andrewellis]

Maybe I am misreading things, but if the flow is as given in your diagrams why would the draw anything from the Nibe or put anything into the 250L when it could simply circulate in a very tight loop (anti clockwise)? You might want to drop one of the 3 ways and add some NRVs.

May be I am calling the valves by the wrong name. I saw some that allow you to direct the input to either output but not both at the same time. By controlling the valves I could achieve pumping in either direction. Oh, I copied it wrong from my original. The right and 3 way should be on the next junction to the left.  However this is all moot if I can’t use them in a system with drinking water. Do you know if they would be safe to use as they don’t have a wras certification?

I have got some software ready to control the heating as you describe I hope to be able to control the compressor and immersions independently. I am ordering an eddi to run the immersion at the bottom of the 250. . The cop for a tank with 48 at the top and 44 at the bottom was about 2.8 in the summer. It is a tricky problem as I don’t want to short cycle the pump following solar production. It would ask be good to reduce the hours on the compressor if the weather forecast allows it. I shall be trying to tie the forecast into it. However none of them seem to be particularly reliable round us. Batteries would make it all work but I am not ready to dive into that yet.


I have been thinking about the double mixer setup you mentioned. The only time the nibe tank is greater than the requirement to mix is if the 250 has been warmed up to a greater temp and the hot water has been shunted to the nibe.  When water is drawn off, it is mixed with cold and the bottom of the nibe tank is replaced with the warm water from the top of the 250. Does it gain me anything?

[Countrypaul]

Have you checked the wras site? https://www.wras.co.uk/search/products/section-2050

This implies they are available to me, but I haven't looked in depth.

Yep, I can now see it, with the RH 3 way one position to the left.

Has your 250L got 2 immersions or only 1? If 2, then use the one at the top first and the lower one second that way you get HW at the top earlier rather than lots of luke warm water.