Putting the pieces together: PV, V2H, ASHP, TOU Tariff, Gas Boiler

Reed_Richards

michaels said:

Reed_Richards said:

I would have thought what you want is for the heat pump to stop and the gas boiler to start once the return water gets hot enough.  So the heat pump takes the water from cold to "lukewarm" and the gas boiler takes it from lukewarm to hot as necessary.  It's not clear to me that you have the same idea.  

Nope, my thought was that the heat pump runs all the time and the gas only cuts in if the HP water temp is not high enough.  Does that make sense?  Of course it might be that the HP then starts cooling the water in which case one would want to to go off.  Otherwise it could be on all the time unless the outside temp (and thus COP) and the gas/electric prices made gas cheaper.  So basically the boiler weather comp would never turn on the boiler if the HP was outputting enough energy so no need for any other controls on the boiler.

Hot water is on a separate circuit (diverter valve in boiler) so am thinking that using HP to help with that too would be tricky to implement - and not as obviously a saving over immersion/gas due to the higher temps.

What you want to avoid is a large difference between the outside temperature and the temperature of the water leaving the heat pump.  That ensures the heat pump operates with a really good CoP and it's easy enough, you set a low maximum temperature for the leaving water, say 35 C for example.  But when the water reaches that temperature you want the gas boiler to kick-in and heat it further (whilst the heat pump will turn itself off).  I have no idea how you could do that because it's the opposite of what you normally ask a boiler to do.    

michaels

OK.  Current boiler has 'weather comp control' and a heating curve:
Outside Temp 10 degrees, boiler return temp 30 degrees
Outside temp 5 degrees, return temp 40 degrees
etc

Boiler comes on and then modulates to try and maintain this desired temp, input power is between 19kw and 5kw.  If return temp goes too high even with minimum 5kw output then boiler will go off until return temp is too low.

If for return temps of say up to 40 the heat pump maintains the return temp then the boiler will stay off, once the heat pump can't keep up then the boiler comes on.  Lets also assume this is the magic temp where the boiler becomes cheaper to run than the HP.

Now with the heat pump I suspect it probably is either an off/on design or possibly there is the option to set a water temp.  With the former it would hopefully be possible to add a smart switch into the power circuit which would turn off the HP when the external temp goes below 5 degrees (it might be that power on is not enough to restart the heat pump in which case something like a switch bot might be needed instead).  If the HP had a set max temp then this could be adjusted to the same 40 degrees then whenever the boiler was on keeping the water temp at or above 40 the HP would automatically turn off.  Magic indeed, no further controls required.

Once you add in smart swiches/switchbots then hopefully they can also know whether the house is using mains electricity or using power stored in the car battery and turn on off off the heat pump accordingly.

michaels

If I position the heat pump on the back wall of the property I could go straight through the wall and join into the in and out pipes of a radiator that is about 25% around the circuit.

Could I make this work? The trick would be to only have a small proportion of the flow though this 'shunt' so the majority of the flow still goes around the main circuit and feeds all the other rads but still be able to transfer heat quickly enough, potentially up to 16kw per hour.

michaels

michaels said:

If I position the heat pump on the back wall of the property I could go straight through the wall and join into the in and out pipes of a radiator that is about 25% around the circuit.

Could I make this work? The trick would be to only have a small proportion of the flow though this 'shunt' so the majority of the flow still goes around the main circuit and feeds all the other rads but still be able to transfer heat quickly enough, potentially up to 16kw per hour.

Some calcs:
Suppose the heating circuit flow rate is 14l per minute and about 8% goes through this rad (1/12th of the house total) that would be 0.02 x 10-6 m3 per second.
The energy transfer is up to 16kw per hour or 0.0044kw per second.
Flow rate = Heat Transfer / (4200 x Delta Temp)
So Delta Temp = Heat Transfer / (4200 x Flow rate)

Plug in the numbers and the delta T is +47 degrees - as this rad is early in the circuit the water temp may be 50 degrees already....

This will then mix with 93% of the circulating water that has gone through all the rads and is at say 40 degrees so this bit is fine but it is the high temp of this water that seems to be a problem.  If we put more flow through this shunt then there is obviously less to go to the rest of the house. perhaps increasing the total flow might be the answer to make up for the diverted bit through the shunt - not sure if the current pump would support this though.