LTHW heating systems Flashcards
What are the four categories of wet heating system?
Warm 40 to 70 LTHW 70 to 100 MTHW 100 to 120 HTHW over 120
What are the three components of an indirect heating system
Heat producer, heat distributor, heat emitter.
What is the recommended maximum flow temperature of an LTHW system and why?
In systems operating at atmospheric pressure the margin temperature is 83 degrees. This is because it would be problematic if the water starts to boil in the heat exchanger. Hence there is a recommended 17 degree margin below 100 degrees. Obviously under higher pressure the boiling point will be higher.
What are the advantages and disadvantages of a high flow and return delta T in the system.
It means that heat is more effectively transferred so reduced pipe sizing and energy expended by the pump. A low return temperature results in condensing if it is below 56 degrees which increases the boiler efficiency. The only disadvantages is that a larger and usually more expensive emitter is required to create the high delta T.
What are the two ways of dealing with thermal expansion of water in a heating system?
A sealed system uses an expansion vessel to hold the expanding volume of water. A vented system uses an open tank to hold the expanded volume of water
Explain the difference between sealed and vented heating systems and the advantages of each.
A sealed system has a direct feed from the mains water supply (separated by a non return valve) whereas a vented system has the water pouring into an exposed break tank. A sealed system is cleaner because it is not exposed to oxygen which creates rust.
What are the different configurations of pipes and radiators
Single pipe series system, Single pipe paralell system, two pipe paralell system, three pipe system
What are the advantages of water over air as a heating medium.
Water has a many times higher heat capacity per unit volume. Hot water can be stored. This results in much smaller pipe sizes.
Explain gross calorific value.
This means the latent and sensible heat (measured in joules) that will be released from combusting a given quantity of a given fuel.
Explain way condensing boilers always need a fan, but not necessarily conventional boilers.
In a conventional boiler the thermal buoyancy of the products of combustion may be sufficient to create a natural convection draft that pushes the waste gases out of the boiler and sucks the fresh air into the boiler. In a condensing boiler, the waste gases are cooled so much that they are much less buoyant and so the natural convection is much weaker.
Why should long convectors be used on a variable volume control circuit.
If the volume is reduced significantly then so will the velocity be. The temperature of the water may be significantly reduced by the time the water has passed part way through the convector. Hence one half of the emitter will emit less heat.
Give two advantages of natural ventilation.
Less noisy, much lower upfront costs than mechanical ventilation, does not take up space.
Define the efficiency of a heat recovery unit.
Inside air temp-external air temp)/warmed supply air temp
