We are all familiar with open loop system when fitting ground source heat pumps, in fact it has been suggested to be the most efficient option when fitting a ground source heat pumps. We will be discussing if Open loop systems would be possible on Air Source Heat Pumps and will this be the future for way in which we install Heat Pumps all together.
As a country, we are seeking to reduce our output of greenhouse gasses by increasing renewable energy sources. As a company, it is important that we are offering the best support to our installer base to ensure this transition happens as smoothly and easily as possible. Exploring ways to make the installation easier for the installer is forefront to Freedom Heat Pumps.
How does an open loop system work?
The idea behind an open loop heating system is to try and make sure the whole house is being maintained at roughly the same temperature, and to have as few zones (or microzones) as possible. In an ideal scenario, this would mean there’s only one room temperature controller for your whole house, rather than splitting the house up into “zones” using zone valves, UFH actuator heads, and TRVs. As we are all aware, the lower the flow temperature you run your heat pump at, the better the efficiency (COP).
And the way we achieve the lowest flow temperature is by maximizing the surface areas of our heat emitters. If we take this example of a 4 room “house”, with heat loss in each room of 1kW, 4kW total heat loss at -2C maintaining 21C in all four rooms. Using DT25 radiators to heat these rooms. 4000W heat load total with a temperature differential of 23K between inside and outside. This means that the amount of heat lost per 1k difference in outside temp is 174W/K.
If you set 2 rooms back by 3C each, to 18C. The mean temperature of the house is now 19.5C which will make our new temperature difference between inside and outside 21.5k. 174W/K multiplied by our new DT of 21.5, means that our total heat load is now 3740W, and energy requirement reduction of around 6.5%. This is only applicable IF your “turned down” rooms actually drop to 18C, which is highly unlikely unless you have little-to-no external wall insulation, and amazing internal wall insulation and sealed internal doors.
This would mean that all of the heat from the rooms will be leaked outside instead of between the rooms of different temperatures also. So let’s say that the two rooms that we have set back only drop to 19C. Our new mean temperature is 20C, which gives us a temperature differential of 22K.
174W/K multiplied by 22K = 3828W, which is actually an energy requirement reduction of 4.3%.
Because of the heat loss between the rooms, your heat loss isn’t as low as you were targeting when you turned those two rooms down. The radiators in those two may be off, but the radiators in the other two rooms are having to work harder to maintain the mean house temperature because of the heat loss between the rooms. Now we’ll revisit the other scenario, where the rooms did in fact drop to 18C. And work out how much heat we’re losing into cooler rooms. Assuming the internal walls were 2.3m by 7m in length with a U value of 1.5W/ m2/K U-value. And the internal doors were 2m2 with a U-value of 4W/m2K.
2.3 x 14 = 32.2 Subtract the 2m2 for the door and you have 30.2 30.2m2 x 2W/m2/K = 60.4W/K 38.4 x 3K temperature differential = 181.2W 2m2 door x 6W/m2/K = 12W/K 24W/K x 3K temperature differential = 72W 181.2W + 72W = 253.2W additional heat loss on each of the two rooms at 21C
So now, our 1000W radiators need to be producing 1253.2W in the two rooms at 21C. So for our total heat loss of 3740W, where we know the two radiators in the room at 21C are producing 1253.2W each, 2506.4W between the two of them. The remaining heat requirement of 1233.6W must be split between the two remaining radiators.
16.8W each That loading could either be reduced by a TRV/actuator restricting the flow, or a zone valve/thermostat bringing the room on and off intermittently. For the 1000W radiator at DT25 to produce our new requirement of 1253.2W, we must increase that heat emitter temperature. But by how much? 1253.2 / 1000 – 1.253. That’s 25% more power required from the radiator. As a rule of thumb, we use 1.3 as a logarithmic curve for radiator conversion factors. So, we must use the reciprocal when converting power increase to DT difference.
1.253 to the power of 0.77 = 1.19 Your radiator must have a 19% higher DT to output 1253.2W. DT25 x 1.19 = 29.75
With a 21C indoor temp, our DT radiators would have had to be running at 46C flow temp to output the 1000W we needed.
Now, it must run a 50.75C flow. this increase in flow temp requirement will lower the COP and therefore, increase the running costs of your system. Let’s put some heat pump numbers in. As an example, I’ll use the smallest R32 Hitachi unit available, the 2.0HP Yutaki S-Split.
If we run the whole house at 21C, we need a flow temperature of 46C which at -2 ambient has a COP of around 2.61. 4000W divided by a COP of 2.61 means 1532W of electrical energy consumed by the heat pump. If we take our house with the two rooms on a setback, running our new required flow temp of 50.75, the COP is roughly 2.22 at –2C ambient conditions. 3740 divided by a COP of 2.2 means 1684.7W of electrical energy used by our heat pump. This means that by reducing our energy requirements by that 6.5%, we’ve increased our electricity consumed and our fuel bill by nearly 10%.
This disparity in run cost would be even worse if the two rooms were shut off completely, with stats or TRVs set to the frost settings. And none of these calculations takes into account the fact that the heat pump could be cycling on/off more frequently if there isn’t enough circulating water volume. Cycling can also lead to higher electricity consumption.
OPEN LOOP HEATING SYSTEMS WILL BE AN OPTIMAL WAY TO RUN SOME HEATING SYSTEMS IN THE RIGHT SCENARIOS, ANYTHING WE CAN DO TO REDUCE RUNNING COSTS AND CARBON EMISSIONS IS ALWAYS A BIG PLUS. HAVING SAID THAT, OPEN LOOP HEATING SYSTEMS ARE NOT THE BE-ALL/END-ALL OF HOW A HEATING SYSTEM SHOULD BE DESIGNED. ZONING WILL STILL BE REQUIRED IN MANY APPLICATIONS, FOR EXAMPLE BUILDINGS WITH WEIRD LAYOUTS/SHAPES, OR IN BUILDINGS WITH MULTIPLE DIFFERENT BUILDING FABRICS OR WITH DIFFERENT INSULATION LEVELS THROUGHOUT THE PROPERTY. MY ADVICE WOULD BE TO ALWAYS DO THE MATHS, ALWAYS ENGINEER THE RIGHT SOLUTION ON A CASE-BY-CASE BASIS.
SAMUEL POWELL
