@editor Hi Mars,

Is your weather compensation now working?

My ASHP heating still off. It’s working fine
Put on a jumper and save

I’ll turn the heating on in October whether we need it or not

@julianc cant say I need heating at the moment. We had the Air conditioning on earlier when the back room got up to 24 all on its own.

@heacol 

Hi Brendon,

I fully agree with your statements about using weather compensation to produce the lowest leaving water temperature that meets the heat demand.

From your experience, have you noticed any detrimental effect on the effectiveness of weather compensation, if the ASHP is located in direct sunlight and could be experiencing solar gain. Dependent upon where the ambient air temperature sensor is located within the ASHP, it could be indicating higher temperatures when in direct sunlight.

Do you know if any manufacturers or models are affected by solar gain?

@derek-m, I have never seen any effect from being in the sun or shade, a single fan unit will move between 4000 and 6000 M3/hr air per hour (double will move double that). That is why  they must be installed in a place where there is free air movement and the prevailing wind does not push directly in to the fan. It is more important to prevent the air recirculating than positioning the unit in the sun or shade. Once the fan starts, you will find that the air temperature will drop quite quickly, that is one of the reasons you must try to keep the units running and not to switch them on and off. Let the inverter do it's job, like cruse control in your car.

The effectiveness of weather compensation has a lot to do with the thermal mass of the building, the higher the thermal mass, the better weather compensation works. In a new build, low thermal mass building with radiations, it does not work, load compensation (internal sensor, not a thermostat) works well. On high end (Swedish) units, you can adjust how much effect either has on the internal temperature.

Ive sold 10000 units and 9980 of them use an external thermostat. many years ago i did 20 units using the controller on the unit as the stat. It was the most horrible experience of my life. All heat pump controllers are crap and unusable. The customers hated the controllers so we gave them perfectley good room stats.

Please dont use the midea remote as the stat, its ok by other units standards but its still unusable. Using an external stat works just as well and will give exactly the same performance and cop as using the remote controller. 

@grahamh 

Hi Graham,

I appreciate it is much better from the installers perspective to prefer a 'fit and forget' thermostat, and it may have been that ASHP controllers were not very good 20 years ago. Having watched the RED video once more, I see that there was no mention of thermostats, and that temperature control of each zone was performed by measuring the flow and return temperatures and then varying the flow rate and temperature  to achieve a 5C temperature difference. Is this the system you will be, or have installed with your new RED ASHP? What level of accuracy with regard to room temperature does this achieved?

It is therefore a modified version of weather compensation, using one or more speed controlled water pumps.

I suppose in an ideal world a domestic heating control system would measure (using actual temperature sensors, not thermostats) the outside air temperature, the air temperature in each room along with the temperature of each heat emitter. The controller would then vary the temperature of each heat emitter to balance the heat loss from that room, by varying the water temperature and flow rates around the system. Because the reaction time of domestic heating systems is very slow, the outside air temperature measurement would preempt changes in heat demand before they occur.

I use a somewhat similar system to control our home at 21C +/- 0.2C.

Obviously most heating system installations are carried out during the warmer months, so are not optimised for Winter conditions, and I suspect installers are not particularly interested in returning during Winter to complete the optimisation procedure.

@heacol 

Hi Brendon,

I appreciate what you mean by low thermal mass, particularly in new build properties that probably don't have brick or block inner walls and have lower heat demand. I suppose UFH along with a thick concrete slab would be reasonably effective in increasing the thermal mass.

Do you know if anyone has tried installing a large volume (500L) buffer tank to act as a thermal store, and hence help to provide thermal mass. It would be particularly useful if the temperature of the water in the buffer tank was increased during the warmer hours of the day, when an ASHP would be operating more efficiently, so that the stored energy could be used to reduce the demand on the ASHP during the colder hours of the day. Of even greater benefit would be to power the heat pump from a solar PV system whilst heating the thermal store.

@derek-m In theory, a large buffer tank is a good idea, however, in reality, a 500 litre tank will only hold 12 Kw of heat with a rise of 20 Deg C. If your system, at 7 Deg C ambient, requires a flow temperature of 30 deg C, the COP will be approximately 5.5. Heating the same water to 50 Deg C the COP will be less than 3 (Panasonic 5 Kw unit).

The additional cost of the tank and required equipment and the associated reduction in performance of the heat pump, makes it unviable, if you heat with gas/oil or direct electric, the maths is different.

Thermal storage using water or phase change is considered the path to follow, but if you do the maths, you will find for heat pumps it is not the case.

Concrete can store nearly the same amount of heat as water per cubic meter, a 50 m2 house with a concrete floor has about 5 tons of concrete, therefore, in this house the floor holds 10 times more energy than the 500 litre tank. This is usable thermal mass, and we use it to the best advantage to reduce the heating bills. To do this, you need to use totally different heating strategies than is normally applied. Using an on-off thermostat does not allow you to benefit from the shock absorber properties of thermal mass, so we never use them.

@heacol 

Hi Brendan,

I think on the calculations that I carried out it produced something like a 3.5% reduction in energy consumption, though of course if the electrical energy to drive the heat pump during the overheating phase was provided by solar PV, then it would be productive.

Perhaps house designers should be looking at ways to use all the waste plastic created by society, to provide thermal water stores that could be incorporated into buildings to improve thermal efficiency.

@derek-m, not at the moment - the heat pump is off and hasn't been required. Given the electricity tariff, it'll be great having it off for months to go.

@heacol 

Hi Brendon,

I forgot to ask for details of the high end Swedish units you mentioned. I would like to have a look at what they offer.