Heat pumps, weather compensation and temperature modulation.
Heat pumps, weather compensation and temperature modulation.
It is early February 2024 as I write this post as I am sitting in my 250 year old semi-detached cottage. This is in Scotland with a heat pump that has been installed, replacing a gas boiler, about two and a half years ago. It is worth saying that my home is now more comfortable and costs much less to keep warm than if I had stayed with the gas boiler. I didn't install expensive wall insulation but did do basic DIY loft insulation and draught proofing.
So I have practical experience that dispels the common myths about heat pumps not working in old properties or cold weather.
However I do read about people having genuine problems on how to use their heat pumps as this is a relatively new way of heating even with many installers.
As a retired electrical engineer and physics teacher with now the practical experience of having a heat pump I see two general ways in which problems arise. Step 1 is knowing that your heat pump will run better if run low and slow as explained later.
Step 2 is knowing how to use the many features that the manufacturers have enabled us to use.
OK if you don't have the time to get to understand the features it is probably better if you have a prescriptive set up and then you “don't fiddle”. However good engineering is about getting the best and adjusting knowledgeably where necessary and not relying on over prescriptive procedures.
Heat pump features:-
The big advantage of a heat pump is that a typical heat pump can have an efficiency of somewhere between 200 and 500 percent in a typical UK environment because it can obtain renewable energy from the environment be it air, water or the ground.
How efficient that will be will depend largely on how much it has to raise the temperature of the central heating water. This means that your heat pump will have the lower efficiencies in the colder weather.
However how you use your heat pump controls can also greatly affect the efficiency.
In a typical gas boiler the radiator water will likely be somewhere around 60 to even 70C, the flow temperature. When you ask for heat your room will quickly heat up to your demand temperature. Your thermostat will then switch off the boiler and the radiator water temperature will fall until your room temperature falls back hopefully only about 0.5 degree. Your radiator water temperature fluctuates greatly keeping your air room temperature within the half degree of your desired temperature (ignoring passive heat gain/loss). We are mostly all familiar with this thermostatic way of heating our homes.
With a heat pump, however, as well as decreasing the life of your system this would lower your efficiency as the central heating water would have to be raised to a high level above the outside environment temperature (as measured by an outside temperature sensor near your heat pump).
Weather compensation.
Clearly if you central heating water was just at the right temperature to meet your demand temperature your house would be losing heat at the same rate as the heat pump is heating the water. Instead of fluctuating over extreme temperatures the flow temperature of the water would be maintained by the heat pump within a narrower range creating a more stable room temperature. This means the maximum temperature of the central heating water is minimised, maximising the efficiency of the energy transfer. So what flow temperature will the central heating water be? Clearly the colder it is outside and the higher the room temperature asked for then the higher the temperature of the radiator water needed.
You want to find this flow temperature when there is no external passive heat gain from for example solar gain through the windows or from cookers or tumble dryers etc.
Once you find out this temperature needed for any situation when your home is heated solely by the heat pump, by selecting an appropriate heat curve, then weather compensation ensures that as the outside temperature or your demand changes then your flow temperature will automatically change.
When you use weather compensation you do not nescessarily need to set your controls to thermostatic (thermostatic is described as expanded in Vaillant heat pumps)
The heat curve.
Selecting an appropriate heat curve is a relatively straightforward procedure in situ that can be found by trial and error.
If you are lucky your installer may have already deduced this prior to installation by heat calculations but it is a tall order to expect this to be precise. The better the insulation of your home and the larger the size of your heat emitters (or radiators) then the lower the heat curve and the higher your overall efficiency will be.
If you set your heat curve too low then your room will not reach its desired temperature unless you have other heating or passive gain from other sources.
If you set your heat curve too high and you have thermostatic control deselected then you will overshoot your set temperature with or without external heat gain.
So set your heat curve at a low level depending on how quickly you want your room to heat. The higher the heat curve the more responsive your heating will be but you pay the price by lower efficiency.
Temperature modulation active or inactive?
Inactive:-
You can set your control settings so that temperature modulation is inactive, whereby the flow tempperature is purely based on weather compensation, that is on the difference of your demand temperature and the outside temperature and not on what your room temperature actually is.This has a number of advantages:-
1. You will know if you have set your heat curve too high because your room temperature will exceed your demand without external heat gain. So you know you are minimising the flow temperature and maximising the efficiency.
2. Your control/sensor doesn't need to be in your main living area. (although there can be advantages if it is located in your main living area for other settings.)
3. If your control/sensor is in a location that gets random external heat gain (or loss) and that area changes from the desired temperature other areas of the house are not affected and the heating there will continue as normal.
So clearly there are times when it would be beneficial to ensure you have “inactive”selected. (not temperature modulation). However there are situations where this is not the most effective setting.
Temperature modulation active:-
Temperature modulation can be chosen (by selecting “active” with Vaillant heat pumps) as a very effective stabilising feedback. Clearly your outside temperature only measures temperature but there are other factors that determine your heat loss (or gain) such as from wind speed, wind direction or radiant sunshine.
If your room temperature falls below the desired temperature your flow temperature is given a boost in proportion to how much lower your room temperature is. Similarly if your room temperature exceeds the demand temperature the flow temperature is reduced by a proportional amount. This negative feedback means your weather compensated heat curve will be modified to accommodate different types of weather and random heat gain to some extent.
Advantages of temperature modulation:-.
1. Your system will cope with a wider range of conditions such as wind factors.
2. When you get external passive gain in your main living area then your flow temperature will be reduced proportionally decreasing consumption. (This is in situations when there is no concern if other area get less heat of course as described in the advantages of “inactive”)
3. If you use set back temperatures your flow temperature will immediately be reduced by more than if “inactive” is selected but as your house temperature falls then your target flow temperature will gradually increase towards the end of the setback period.
4. When you increase your room demand temperature your target flow temperature is temporarily increased making your system more responsive with minimum effect on efficiency.
Thermostatic/expanded control. Part 1.
So with weather compensation whether with or without temperature modulation is there any need for normal thermostatic control?
First it is of extreme important to discuss high efficiency versus low consumption:-
High efficiency versus low consumption:-
First an analogy:-
When driving a car does high mileage mean low consumption? Consider doing a a journey in two different ways:- a direct route of 1 mile, stopping and starting with low mileage or a steady 50 mile circular route round the bypass with twice the mileage. Few would choose the latter but just maybe some would!
Now if you are motivated by a high COP (Coefficient Of Performance which means basically efficiency) you may be tempted to run your heat pump 24/7 or even 24/7/365 and perhaps on “inactive”. It does seem that this is advocated by more people than would advocate the 50 mile journey to get a better mileage at the expense of higher fuel consumption. Clearly if you are running your heat pump more than necessary in warmer weather your average COP will increase in an analogous way to the increased car mileage, simply because you are using your heating at times when the efficiency is higher even though you don't need the heat at that time (or near that time).
Thermostatic/expanded control. Part 2.
So in the deep winter I run my heat pump using weather compensation on “inactive” when appropriate or “active” temperature modulation using a “demand curve” whereby I set back the demand room temperature at night and bring it up gradually in 0.5C steps. This is usually during cheaper off peak time at appropriate points of the day, along what i call a demand curve.
When the days are warmer I find that I can get all the advantages of temperature modulation plus a few more by using thermostatic control.(called expanded control on my Vaillant heat pump)
It must be stressed though that a danger of using thermostatic control is that you might be relying on the room thermostat to maintain your room temperature without realising that you are on an inappropriate high heat curve. This could stress all your system, in particular the heat pump itself, and reduce the efficiency as described above. However if you have already established an appropriate heat curve then this will not be the case. If your heat curve is set low enough it should only switch off with passive heat gain.
Another point to consider is that there is a limit on how low you can run your heat pump in terms of kW. A heat pump can modulate down to somewhere about 25% of it's maximum to accommodate different outside temperatures and heat demand. In warmer weather once it has reached it's lower limit the only way to reduce heating further for mider conditions (or lower temperature demands) is to cycle off and on. If installed properly your heat pump may be cycling one or two times per hour particularly in milder conditions and this is perfectly fine. Often when I select thermostatic control it is when it is mild enough for this cycling to be the norm and it is on an appropriate heat curve. My heat pump may switch an additional few times per day due to the room thermostat when the room temperature exceeds the demand curve because of the natural heat of the day. However overall daily switching is reduced considerably as there will be ever increasing times when the heat pump is off due to the daily passive heat gain until over summer it is not on at all. More importantly electrical consumption will be greatly reduced.
When the heat pump does come on, it is when the room temperature falls to the demand curve and this means during this less cold time of the year the heat pump will be coming on at its lowest modulation anyway. So there is no point in saying your heat pump wouldn't have to work as hard if it were set to "inactive". Furthermore if you have a buffer tank it will likely have a lot of stored heat. If thermostated off this heat is saved until the next switch on time. This is heat that would otherwise have been circulated when for example solar gain heated your home.
Now if you run your heat pump during the heat of the day or during summer you will get a very high COP during these times but I will use zero energy at these times meaning less overall energy even if it means a lower overall COP.
In conclusion the main points I would like to make :-
1. Don't expect a heat pump to heat your home from cold in a short period of time and switching high flow temperatures off and on.
2. You don't have to run it 24/7.
3. The goal is not to get a high COP but to get the lowest consumption ensuring comfort and minimum stress on the system.
4. Make use of the cheaper times that I referred to above if you can to further reduce your running costs. (This is an extremely important topic, which goes far beyond saving a few quid, on our path to net zero.
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