Modern Heating Strategy: Rule of thumb Heat Loss Estimate
Consider the heating of a house; the size and fabric of the house is measured (height, width, breadth) and using appropriate U-values (watts per metre square per degree Celsius) then given a total heat loss for the building in kilowatts.
Example: An 8 metre wide house which is also 8 metres long and has two floors – 8x8=64, 64x2 = 128m3
128x0.05 (kilowatts per square metre for <20 year old house) = 6.4kw heat loss, this may come as a surprise to many experienced installers and this is a big problem – oversized boilers.
Now this specific heat loss is only true at the design parameters used, (e.g -3°c outside and +21°c inside). Whenever one of these parameters changes the calculation needs to be changed as well. If the outside temperature is +5°c for instance, then the output from the emitter may be too high and will lead to over-heating of the house.
Old fashioned on/off controls cope with this by turning off the heating when the thermostat is satisfied, waiting for the house temperature to fall to below the comfort temperature and then switching on again. The temperature rises to above the set temperature and the cycle repeats. This rise and fall is not only uncomfortable but wasteful, so to compensate most home-owners run the comfort temperature a little higher.
How can we control the temperature more effectively?
Well to put it simply - Weather compensation
Heat output from a radiator or a floor system is determined by several factors, these are the size (typically volume), the temperature of the emitter and the air temperature so what can we change? We can't vary the volume of the radiators or underfloor heating and we are trying to control the air temperature, only emitter temperature remains as a manipulable factor.
Let's consider this; if a radiator for example needs to be at 74 degrees to give out 1kw to a room at 21 degrees if we reduced the emitter temperature we reduce the output, then if we can relate the radiator output to the heat requirement consistently we can have a consistent internal temperature.
Now as we discussed earlier our heat loss calculation included outside temperature, as the outside temperature rises our heat requirement from our emitter reduces. Using this outside temperature to adjust the radiator or floor temperature gives us far more control. Constant temperature creates better comfort and lower air temperatures are now comfortable.
Heating to a lower air temperature by just one degree can reduce gas consumption by ten percent. Using this control method uses low temperature output from our boiler so most of the year this flow temperature is below dew point forcing the boiler to condense leading to highest efficiency gains, up to 90% of the latent heat can be recovered from the water vapour which is exactly what condensing boilers are supposed to do!
By maintaining a further reduced temperature when the building is unoccupied (setback) we reduce the energy required to raise the temperature in the building when re-occupied. Only a small shift in temperature of the emitters is required for a short period. This rise in temperature is also likely to still be in condensing temperature range.
In practice weather compensation systems are often very accurate and effective that no internal reference point is required, this means no thermostat and no sensor inside. It is worth making clear that this is only achievable with advanced weather optimised systems, Viessmann and Vaillant will manage it but many other domestic brands will not. It can also be counterproductive to add an internal reference point as it only represents one space within the building and can cause the flow temperatures to be based on corrupted data such as an open window or solar gain.
TRV’s are adequate to prevent any overheat in any space effected by an additional heat source and the opening of windows becomes an uncomfortable option and so is avoided.
So can we make a system even more effective? Yes. Running at below dew point at all times leads to constant condensing so if we correctly size the radiators we can also, even at peak output, still use flow temperature below the dew point. If the system only requires <60°c flow to maintain a 21°c internal room temperature even when the outside temperature has dipped to -3°c then the boiler will always condense and the maximum gas efficiency will be met.
Underfloor heating without weather compensation controls is an engineering travesty. With a non weather compensated boiler we heat water over the dew point only to blend it down again to a temperature below the dew point.
Underfloor heating with weather compensation
First consider the underfloor system. When designing underfloor the same process as sizing a radiator is undertaken. The average temperature of the floor multiplied by its area is used to calculate its heat output. Dt across flow and returns is calculated based on the maximum heat output required. Most UK systems come to about dt 7 / 10. If the flow is restricted a higher dt will result (measured at full load) this will reduce the average temperature and result in a lower heat output. As with radiators we can control flow temperature via weather compensation controls, providing a perfectly matched heat input to the underfloor to achieve a perfectly matched heat output from the floor.
Standard underfloor systems rely on the use of high temperature water produced without the benefit of condensing and thus at a lower efficiency than a weather compensated design. This high temperature water is blended back down to the maximum design temperature for the system. This then, at almost all times, raises the floor temperature to over the required heat output and is then turned off by a room stat. After cooling the overheating process is repeated.
We need to consider water pipe velocity and pipe sizing accordingly if we wish to use weather compensated systems. If the underfloor system requires a flow rate that causes a dt of 7c then we need to consider our boiler flow rates and pump settings.
Example.
A 30kw boiler is to supply an underfloor heating system with a total load of 20kw. The underfloor designer has selected a flow rate that at full load will require a dt of 7c. In this example the flow rate required by the underfloor system will exceed the maximum flow rate of the boiler by a small margin. To over come this problem we can install a larger boiler or a low loss header. We could also employ close coupled tees. A secondary circuit pump would be sized and selected to run the underfloor at its designed flow rate. No blending valves or pump would be used at the manifold/s. The pipework to the manifold/s may need to be larger than those used to deliver high temperature water so consideration to these sizing issues is important.
If in this example these system design parameters are ignored and standard manifolds and mixing valves are used, the low temperature water supplied by the boiler would never cause the blending valves to close, so in effect two pumps would power one circuit. Also if additional flow was induced in the primary circuit as a result the boilers max flow rate could be exceeded.
As a guide Viessmann state that, in any system incorporating an underfloor system greater than 30% of the boilers out put, a low loss header must be used.
An understanding of the relationship between heat load, flow rate and velocity is key to an engineers ability to understand, design and fault find a heating system. The first element is to understand heat loss calculation or heat load. Once the heat load for any circuit or system is established the engineer can start to select pipe sizes and pump sizes. From the calculated heat load (watts) a mass flow rate can be calculated (m3h) this is the volume of heat transfer fluid (water) required to convey the heat load from the source (boiler, heat pump etc) to the emitter ( radiator, underfloor etc) The next step is to select a suitable size of pipe to convey the volume of htf (heat transfer fluid) at an acceptable velocity. Pipe size determined the engineer calculates the resistance of the circuit. This is a combination of pipe length, fitting resistances, emitter resistances and heat source resistances. For pump selection the highest resistance circuit in the system is used in conjunction with the mass flow rate for the entire system. From the manufactures literature a pump (circulator) is selected that is capable of delivery of the required flow at the required pressure to overcome the resistance calculated.
Please remember that boilers with weather compensation have some extra design considerations:
To obtain higher outputs from floors a high flow rate is used. This flow rate produces a dt of usually 7c. Boilers are designed to run at a maximum flow rate so as at full output they achieve dt 20c. If the floor and the boiler are matched then we have a problem. To provide heat to the floor the boiler would need to work at dt7. This would exceed the maximum flow rate by 300 %. To solve this we need hydraulic separation in the form of close coupled tees or a low loss header. Alternatively the boiler could be oversized by 300% this would often be a bad choice unless the requirement for hot water happens to be 3 times the requirement for space heating.
With a weather comp boiler the thermostatic mixing valve may not be required and the pipe sizing from the llh will need to be calculated as 22mm will not run more than approx 15kw of ufh on weather comp.
Mass flow rate required ls = output in watts /( 4187 x temp change)
This simple equation is one of a few tools that can change your understanding of heating. Understanding the theory of heat transfer is what its all about. I like to think about heating systems as a of heat delivery system. So if I take a bucket of water, say 10 litres and heat it up by 20c and give it to you, how much heat energy have I given you? Well from the calculation above we can see that it takes 4187 joules to raise 1kg of water by 1c so it takes 10 times 4187 joules or 41870 joules to raise 10 litres 1c. I raised the water by 20c so that's 20 times 41870 joules or 83740 joules.
So joules are a measure of energy that mean nothing to us, so thanks for the bucket of water but so what? Well we all have a grip on what a kilowatt is I hope. So if the joule is a measure of energy then what is a watt. A watt is a measure of power. Power is energy over time. So its a rate of delivery of energy. In my example what if I gave you a bucket every minute? That's 60 buckets an hour. So 60 x 83740 joules. Time gives a way of measuring the rate of supply of energy and that's power. 1 joule per second is one watt.
So how many watts in my bucket of water? None. We can only talk about watts if we have a time factor. So let's say I give you a bucket every second. That's 83740 joules per second or 83740 watts or 83.740 kw. Wow that's a lot of power!
So let's think about flow rates. A bucket every second is 10 litres a second that's 600 lpm. We can't visualise this its just to fast a flow rate. We understand up to about 20lpm and can visualise this range of flow rate. So a bucket a minute is 10 lpm. So our 83740 joules is now divided by 60 seconds rather than 1 second. 83740 divided by 60 is 1396 joules per second or 1.396 kw.
Let's calculate the flow rate for a 30kw boiler. Mass flow rate required = output in watts /( 4187 x temp change)
30000w /(4187 x 20c) = .358 lps x 60 = 21.48 lpm
Once you have your flow rate you can select a suitable pipe size by calculating the velocity through the selected pipe and checking that the velocity does not exceed the maximum for the application.
Pipe velocity is the rate in m/s that water travels through a pipe. The relationship of flow and velocity is affected by the pipe diameter. The larger the pipe the lower the velocity for any one flow rate.
