Bivalent heating controls

[RussJ]

I'm thinking of installing a high temp ASHP and buffer tank to provide hot water and central heating to existing rads. I'd like to retain my oil boiler for use on really cold days and would like advice on suitable control systems please... Russ

[StationHouse]

A few things spring to mind...

You will not qualify for any RHI premium payments.

A properly installed ASHP both low and high temp will work just fine in cold weather.

All your existing rads and pipework may need to changed anyway.

Why a buffer tank?

ASHP systems tend to packaged with appropriate controls already.

Interested to see other responses... 

[RussJ]

A high temp ASHP allows retention of existing rads and pipework which keeps the total installation cost down.
Most ASHP don't work well/efficiently at low temps (less than 2/3C) so I'd prefer to have a backup.
Much easier to integrate multiple heating sources if you use a buffer tank...
Just need to sort out how best to control it.
I can't see anywhere that prohibits the use of bivalent systems with RHI payments - if I've missed something could you point me in the right direction?
Russ

[StationHouse]

Ahhh a thermal store, I see...

Certainly we needed to loose the microbore and go to 15mm for the higher flow rates some ASHP require. The manual for whatever ASHP you prefer will tell you. We had installers wanting to install to existing rads/pipework which would never have worked but that's another story...

Before we went ASHP we visited friends with a leading brand ASHP/UFH at -16c, no issues and house nice and warm. Recommend you visit reference sites done by various installers unless already done so. We visited several and found it a big help in formulating a plan of what exactly was required.

Perhaps I wrong about hooking up two systems, I did have a read but is not clear.

Very cheap to run and never looked back.

Cheers

[JonG]

Bivalent systems are OK but there are many theories of the best way of doing it so be sure that you follow specific guidelines from the manufacturer before you go down this route. Post December 2011 the new version of the MCS Standards require that heat pumps are sized to cover 100% of the load at 99% of the heating days based on CIBSE data extrapolated from MET office survey so any MCS approved installer should be advising you of this imminently.

Unless your load is excessive it makes sense to avoid the complication of bivalent systems not least because all the anecdotal evidence suggests that any RHI would be compromised if fossil fuel (gas,oil,LPG) still contributes to the mix.

You need to consider your heat loss, watts per square meter, existing rads and circuit and any potential improvements you can make to the insulation or air tightness before you make a decision.

Always go for a buffer despite what Daikin, Mitsubishi and other inverter driven unit manufacturers suggest and go for simple, preferably fixed speed scandinavian options such as NIBE, Danfoss, Dimplex.

The high temps lose efficiency over a low temp and compromise expensive cascade technology that can be expensive to repair in the event of an issue, the higher flow temp will also mean a low star rating to comply with the new version of the standards referred to above.

You also mention a thermal store to combine other technology, I have contributed to another thread on here where I explain the pitfalls of this approach that would be worth reading through before you make a call.

Hope this helps.

JG

[Solal]

Always go for a buffer despite what Daikin, Mitsubishi and other inverter driven unit manufacturers suggest and go for simple, preferably fixed speed scandinavian options such as NIBE, Danfoss, Dimplex

What is the purpose  of the buffer? To stop  it  cycling?
Buffer store won't stop it cycling.

What happens is, the heat pump heats the store. The heating comes on and starts to empty the store. The cold water from the heating system goes to the bottom of the store and is immediately cycled through the heat pump. Which turns the compressor on. Or, the heat pump is controlled via a thermostat on the bottom of the store, which will signal heat is needed as soon as the cold water from the heating system gets to it and the returning water from the heating system is immediately cycled through the heat pump.

Whilst both are running, cold/cool water returning from the heating system will go straight to the heat pump. It won't see the buffer tank at all.

So, there's little point having one.

If the heat pump is making more heat than the heating system can use, the temperature in the store starts to climb. The return to the heat pump becomes a blend of the cool water returning from the heating system and the warmer water in the tank. So the heat pump works at an increasingly high temperature. Which is bad for its COP.

If the water from the heating system goes straight to the heat pump unmoderated, it will always be the coolest possible water that could be returned to the heat pump, which will work most efficiently.

If the heat pump is working at the capacity of the heating system, then there's no need for a buffer tank.

Whether or not the heat pump is on all the time depends on how its controlled.

[JonG]

 
What happens is, the heat pump heats the store. The heating comes on and starts to empty the store. The cold water from the heating system goes to the bottom of the store and is immediately cycled through the heat pump. Which turns the compressor on. Or, the heat pump is controlled via a thermostat on the bottom of the store, (Scandinavian designs position the thermistor centrally to combat this issue to delay the next on period of the HP) which will signal heat is needed as soon as the cold water from the heating system gets to it and the returning water from the heating system is immediately cycled through the heat pump.

Whilst both are running, cold/cool water returning from the heating system will go straight to the heat pump. It won't see the buffer tank at all. (This is fine with a full heat demand and to be expected)

So, there's little point having one.

If the heat pump is making more heat than the heating system can use, the temperature in the store starts to climb. The return to the heat pump becomes a blend of the cool water returning from the heating system and the warmer water in the tank. So the heat pump works at an increasingly high temperature. Which is bad for its COP. (Again a centrally placed thermistor deals with this issue and provided the house and heat pump are not electronically interlocked, the heat pump will be driven by the thermistor and will have nothing circulating through it until the buffer decays until the point it re-fires the heat pump)

If the water from the heating system goes straight to the heat pump unmoderated, it will always be the coolest possible water that could be returned to the heat pump, which will work most efficiently. (Not if your central heating system is preventing the heat pump from creating the required max flow rate or correct delta t, in which case it will never work efficiently. Daikin 16kw units need 43lpm and a delta t of 5, which is difficult to achieve through a rad circuit, especially in a retro fit, short legs and big bore pipework to a buffer mean you can control this variable much easier and use the hydraulic separation created by the buffer to dampen the effect of a different flow rate and delta t)

If the heat pump is working at the capacity of the heating system, then there's no need for a buffer tank. (Very true but the buffer comes into its own in the shoulder periods of the heating season when the house cannot absorb the design output, Even an inverter unit will have a min output, on Daikin's it's 50% and on Mitsubishi's it's 30%. Throw some zoning in and its very easy to create a demand lower than the min step, at which point without the absorption of the buffer the units cycle and wear out the compressor motor, this is likely to be a bigger issue now that MIS3005v3 requires 100% sizing at design temps equivalent to 99% of annual hours. Bigger heat pumps mean more potential for over capacity in warmer months, if you don't deal with the output the HP will cycle. Using calcs provided by Mitsubishi many of the projects we have sized start to hit min step at around 10 degrees ambient, with a gap in their product range between the 8.5 and 14 this will always be an issue, because a bigger unit has more to be absorbed even though the heat loss may only be 9kw for example at -3 but is covered by a 14kw unit)

Whether or not the heat pump is on all the time depends on how its controlled.

(Don't ovelook defrost either, if the heating circuit is too small, restricted by zones, or even turned down low whilst on holiday or in a hoiday home for example, there may be insufficient heat in the system to effect a full defrost and the HP turns into an ice cube. A buffer gives the system a constantly available reservoir of heat energy to draw against when defrosting, irrespective of internal climate if the HP and house controls are not interlocked. Strangely enough a Mits senior manager has a buffer installed because he didn't appreciate his rads cooling down during defrost.)

[RussJ]

Good grief - I thought it was all so simple!
Heat pump during the relatively mild days and oil boiler only when it turns cold...
Looks like I'm going to need some professional help in designing and sizing the system
Thanks very much for your help and valuable comments.
Russ