Heat emmitters Flashcards
What are the advantages and disadvantages of radiant and convective heat
Advantages: Radiant heat is less prone to heat losses from ventilation. Radiant heat warms first by radiation and then converts to convective heat. Radiant heat does not suffer from thermal stratification so is good in tall spaces.
Radiant heat is unevenly distributed within a space, radiant heat can give headaches, especially short wave infra red radiation.
What are the advantages and disadvantages of underfloor heating systems?
Advantages: reduces thermal stratification. Creates radiant heat that is even accross the space. Low return water temperatures promotes system efficiency. Low noise, low maintenance.
Disadvantages: Expensive to install, very unresponsive makes it unsuitable for intermittantly used spaces. cannot be used with carpets so limits interior design options. Requires insulation underneath so can require floor buildup. Furniture and rugs will reduce output of system. To prevent thrombosis there is a 27 degree limit on finished floor temperature. This means that the temperature cannot be increased to maximise output on a very cold day.
What are the advantages and disadvantages of wall hung radiators?
Low upfront cost. Quiet. Low maintenance. More responsive than underfloor heating (although less responsive than fan convectors). 70-90% convective output.
Disadvantages: Relatively larger for a given heat output than fan convectors, can accumulate dust in finned radiators, purely natural convection promotes thermal stratification. If unbalanced can heat different rooms unevenly.
What are the advantages and disadvantages of fan convectors.
Advantages: Smaller size for a given heat output than natural convectors. Easy to control output by fan speed. Good zone control. Low return temperatures promote system efficiency. Very responsive so good for intermittently used spaces. Can be mounted on the wall on the ceiling. The same hydronic fan coil unit can be used for heating and cooling of a space.
Disadvantages: Loud, require more maintenance than natural convectors. The air movement affects the operative temperature.
What are the advantages and disadvantages of water filled radiant panels.
Advantages: They save space from being located outside the occupied space. higher proportion of radiant heat means lower ventilation losses. Radiant heat is better for tall spaces. Radiant heat is more responsive. Water content of radiant panels is lower than with wall hung radiators so system has lower startup time. Radiant heat is self regulating since more radiation will be absorbed by cooler surfaces. Disadvantages:
What does it mean to say that a heating system is out of balance?
This means that the volume flow rate to some emmiters is higher than the design flow rate and the volume flow rate to other emmitters is lower than the design flow rate. This will result in some zones being too hot and others being too cold. This can also result in higher than design return temperatures and short cycling of the boiler if water is passing through the emmitters faster than the emitters can transfer the heat the to space. This is solved by increasing the resistance to some emitters using pressure resistriction valves.
What is cavitation in a heating system
Cavitation is when the low pressure at a point in the system causes the boiling point to fall below the temperature of the system at that point. This normally occurs just before the pump. This results in bubbles rapidly expanding and collapsing. This creates noise and damages the system.
What information is required for sizing of an expansion vessel?
System volume (this can be estimated from heat output if neccessary). Maximum temperature. For an LWHW system this is 83 degrees.
What are the consequences of incomplete combustion?
Carbon monoxide and reduced efficiency.
Explain the difference between variable volume and variable temperature circuit control.
Variable volume control, means adjusting the output of the heating system by adjusting the flow through the system. This can be done using TRVs which stop the flow through individual radiators.
What is the empirical formula for heat output from a radiator
Heat output = Q1/Q2 = (DeltaT1 / DeltaT2)1.3
Q reffers to the heat output and DeltaT the difference between the room temperatures and the ambient temperature. In manufacturers information they will normally list a heat output in watts @ 60 degrees kelvin DeltaT. This formula allows you to calculate based on lower radiator temperature differences.
What is the typical water velocity in an LTHW system?
0.75-1.15 M/s
What is the formula for heat output from under floor heating?
Floor area * (11.7W/K)*DeltaT
Max underfloor heating temp is 27 degrees so DeltaT is normally 27-design temperature. 11.7/K comes from 6 degrees radiant heat and 5.7 degrees convective heat.
