Heat Pump Sizing?

michaels

Reed_Richards said:

I think what you describe is fairly standard for a thermostat or certainly for the traditional design.  But these days you tend to have a wireless thermostat that communicates with a "base unit" that does the switching.   

It is just the thermostats I have come across before have been 'volt free' in the 'call for heat' circuit - ie low voltage circuit close to signal heat on, open to signal heat off.  SO I am a bit worried at the idea of sending 230v to the signal terminal.... However it seems pretty clear that it is not part of opening/closing a low voltage circuit as the other two wires are L and N supply.

michaels

So now a really basic question.  I am using water temp AI (=external air temp weather compensation) control on my heat LG heat pump.  Does anyone know if the pump software makes control decisions based on the flow temp or the return temp?

Eg water temp is set to 55C at -10C air temp and 30C at 15C air temp with a uniform slope 'curve' between the two.  Last night when the air temp was 5C that gives a flow temp of 40C.  I also have a +4 / -6 'hysteresis' to prevent short cycling.  I wonder if it is the flow or return temp sensor that will trigger on at 34C and off at 44C?

The unit does support 'water temp + load' compensation but only if you have the LG room temp sensor that I don't have.

matelodave

I'd guess its flow temp rather than return unless there's an element of load compensation in the control algorithm., although I'm happy to be corrected.

Whereas I should think that basic "load compensation" would use the difference between flow and return to work out how heat is being used.

In the end the only way to find out is to ask LG and tell them what controls you've got.

Reed_Richards

The primary control is based on the flow temperature (or Leaving Water Temperature) but once the flow temperature target is achieved then the heat pump will turn itself off for a while if the return temperature gets too close to the flow temperature.  Do you have the User Manual?  

Load Compensation is based around the difference between the target and actual room ("Air") temperatures and might override the Weather Compensation if there is a big difference between the two or to maintain a steady room temperature.

michaels

I think at the moment we are still in the situation where even at minimum modulation (1.3kw draw) the LWT is going above the max AI+4C after the unit is run for a while so the unit then turns off until this drops to AI-6C.  I get on cycles from 20 mins on, 40 mins off at +15 air temp to about 35 mins on, 15 mins off at +5C air temp.

Another question - is there any way to have the unit restart rather than going to standby and retain settings after a power outage - I am thinking otherwise it would be a problem if the power went out while I was away on holiday.

Reed_Richards

I'm not aware that my LG Therma V has a "standby".  Or maybe my third party controller overrides it?  It always comes back on after a power outage.

michaels

Reed_Richards said:

I'm not aware that my LG Therma V has a "standby".  Or maybe my third party controller overrides it?  It always comes back on after a power outage.

So when ours comes on after a power off the clock has reset to Sunday midnight, there are random holiday programmes enabled and it is in an 'off' condition, needing an on button to be pressed to start.  From what I can read, this seems to be wrong.  I am wondering if there is a bios battery somewhere either in the controller or the heat pump itself that needs replacing.

Reed_Richards

My LG Therma V does not do holiday programmes.  It's a real pain as I have to reprogramme the DHW schedule to set all the start dates to the date I return every time we go away. 

The only mentions of "battery" in the installation manual are in the context of an accessory to control the mode of operation according to the Energy Storage State of a home battery.   

matelodave

my Daikin resets the clock to zero and day to Sunday and then restarts so although the time gets reset, non of the other schedule programming alters. 

There is an "auto restart" setting in the engineering menu which can be enabled or disabled - mine is enabled

Its easy to reset the clock day and time, but its a bit irritating because we get random small power outages every couple of weeks - usually not everso long but enough to reset the heating and reboot the broadband router

And just to prove the point, we've just had a 30 second drop-out. Which could have been really irritating as I was just finished a 2 hour run on my 3D printer. Although it does have a power recover feature the print gets marred and weakened at the point where it gets restarted 

michaels

So at the moment all is going to plan with DIY heat pump

12kw LG, 6+ years old bought second hand for 400 plus the controller separately for 25, both from eBay.

Plumbed into central heating circuit by teeing off the tails of a rad that is next to an external wall and drilling straight through the wall.  Heat pump out is to rad 'in' and water is forced around the circuit rather than through old not-working boiler by closing off valves by the boiler.   Power supply via some SWR from a rcbo box added in the garage and then linked to the garage consumer unit.  Second circulation pump (that I had unused) added to improve the flow as it is immediately constrained by the 15mm tails and then the rest of the circuit (approx 14 rads) is half 22m, half 15mm.

Hot water is by immersion so not enabled on heat pump and all secondary/back up element heating within the heat pump is disabled.

Now outside temp is below 4C, heat pump is running constantly drawing about 1.6kw and delivering about 5kw with a flow of 45 and return of 40, temp set using 'AI' weather comp with -10C 57C flow, 15C, 32C flow.  House temp maintained between 21 and 22 both now and when it was running part time when temps were higher with cycles of about 15+ mins on and similar off.  By my calcs max house heating demand is about 7kw at 55C at -5C outside temp.  Currently running 24/7 in 'night' mode - ie capped output and sound, heat pump rated output down to -5 is 12kw so given house heating demand so far this is not a problem.

Outstanding issues: 
Unit losses all settings if power is removed - 'bios' battery?
No way to automatically apply a night time set back - could use controller to set on/off times but better to connect thermostat to enable this
Heat meter temp probes not giving reliable readings (1 in pocket, one taped to pipe) so need to estimate output based on heat pump reported flow and return temp and heat meter reported flow rate rather than having actual readings
Next summer plan to replace about half the rads to give a 45C flow temp at -5

Total cost including van to collect heat pump about £550 plus who knows what value of my time.

Running costs not bad as currently 95% of electricity is night rate at 7p per unit using car battery V2H to store from the 7 hour night rate window for all daytime house use, heat pump daytime use and also a reheat of the hot water immersion in the evening.