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internal temperature conditions 4
- Thread starter skyhooks1
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- #4
what i am after exactly is a document from ashrae, cibse etc that states that if my design control temperature is 24+-2degC the load calculation is based on 24 and the thermostatic control is +-2, i am in a legal situation where a client is suing a developer because the a/c is unable to maintain these conditions.
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- #6
That seems to be purely a specification interpretation issue, and if the developer can implement an a/c with 26°C +0°C/-4°C and unless the exact wording of the specification precludes that as a design solution, then they meet the specification.
You still haven't indicated what the exact wording of the specification is.
TTFN
You still haven't indicated what the exact wording of the specification is.
TTFN
This can't be answered in a sound byte. If this is going to litigation, questions that remain unanswered are, 1) Where does the system need to meet the criteria, in the supply duct? Exhaust duct? At the thermostat? 2) When does the system need to meet the conditions, during design days? Which, 0.4%? 1%? 3%? There are unknowns that involve research...
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- #9
the wording of the spec is 24degC +-2degC, and as IRstuf says this could also be 26 +0-4, the external design condition is 35degC, the problems are that we are getting an internal condition above the design condition on days that are under 35degC, this is mainly due to direct solar gain as on overcast days of 38 degC we are reading temperatures below 24.
i need to prove to a court that the design of an a/c system is operating incorrectly and to prove what a design condition means, i am looking for a document that can help me prove this from an institution, does anyone know of any?
i need to prove to a court that the design of an a/c system is operating incorrectly and to prove what a design condition means, i am looking for a document that can help me prove this from an institution, does anyone know of any?
Seem to me that one question is whether or not the designer should have anticipated the solar load.
Even on military systems, we assume that ambient conditions are aggravated by solar load for thermal analysis.
Since the system, in fact, fails to meet its requirements to keep the temperature below 26ºC when the external ambient is below spec value, the question about what is really the design point is essentially irrelevant.
TTFN
Even on military systems, we assume that ambient conditions are aggravated by solar load for thermal analysis.
Since the system, in fact, fails to meet its requirements to keep the temperature below 26ºC when the external ambient is below spec value, the question about what is really the design point is essentially irrelevant.
TTFN
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You would also need to check if there is an energy conservation code applicable to the building. Depending on the Code referenced standard year or issue, the reuirement may be to design the system for 78°F (25.55°C) indoor and 2 1/2% occurance outdoor design condition. The outdoor condition (and the indoor design temperature of 78°F) then would be exceeded 2 1/2% of the summer hours.
Standard thermostat on/off control of unitary AC is +/- 2°F. The AC unit compressor will continue to run till the setpoint temperature is reached. When the space temperature climbs up to 2°F above the setpoint, the compressor runs again till the setpoit is reached. If the space temperature drops 2°F below the setpoint and if the AC is in the heating mode, the heater will run untill the setpoint space temperature is reached. The +/- 2°F is called deadband. The ANSI/ASHRAE/IESNA 90.1-1999 standard now do not dictate indoor. However it tabulates the outdoor design condition (Appendix D-1 to D-3) and calls for +/- 5°F deadband (6.2.3.1.2) except for special occupancy (hospital, retirement home, museaums, process etc) and for thermostats that require manual changeover between heating & cooling mode. Unitary AC with heating or cooling switch fall within this exception. How often & when is the setpoint exceeded?
Standard thermostat on/off control of unitary AC is +/- 2°F. The AC unit compressor will continue to run till the setpoint temperature is reached. When the space temperature climbs up to 2°F above the setpoint, the compressor runs again till the setpoit is reached. If the space temperature drops 2°F below the setpoint and if the AC is in the heating mode, the heater will run untill the setpoint space temperature is reached. The +/- 2°F is called deadband. The ANSI/ASHRAE/IESNA 90.1-1999 standard now do not dictate indoor. However it tabulates the outdoor design condition (Appendix D-1 to D-3) and calls for +/- 5°F deadband (6.2.3.1.2) except for special occupancy (hospital, retirement home, museaums, process etc) and for thermostats that require manual changeover between heating & cooling mode. Unitary AC with heating or cooling switch fall within this exception. How often & when is the setpoint exceeded?
+/- 2[°]C is a wide range. Not to cloud the issue, but you also have to consider the effect of relative humidity. 24[°]C at 50% RH will feel different than 26[°]C and 50% RH. Also, is this temperature range appropriate for the type of clothing being worn in the space? 24[°]C (and 50% RH) will be comfortable for people wearing short sleeve shirts and trousers/skirts, but a man in a 3-piece suit will be uncomfortable. Just adding some more issues to consider.
- Thread starter
- #13
If an air conditioning design brief nominates the design internal temperature as 24 deg.C DB +/- 2 deg.C, what is the internal temperature used for the cooling load calc. 24 deg.C ?? 26 deg.C ???
Are there any ASHRAE, CIBSE guides that reference the above.
Are there any ASHRAE, CIBSE guides that reference the above.
Skyhooks1,
Carrier E20 load calculation programme witch uses the ASHRAE method, has an input data setction which asks for design room temperature setpoint and the throttling range. If the room is to be designed at 24+/-2 deg C then the room setpoint is entered as 22deg C and the throttling range is specified as 4 degrees C. I am almost certain that the programme, when sizing the plant allows the room temperature to float to 26Deg C.
If you want more info on that i can email the section in the help menu that explains all that stuff to you.... it is very good and could explain the situation to you better
Regards
Billyq
Carrier E20 load calculation programme witch uses the ASHRAE method, has an input data setction which asks for design room temperature setpoint and the throttling range. If the room is to be designed at 24+/-2 deg C then the room setpoint is entered as 22deg C and the throttling range is specified as 4 degrees C. I am almost certain that the programme, when sizing the plant allows the room temperature to float to 26Deg C.
If you want more info on that i can email the section in the help menu that explains all that stuff to you.... it is very good and could explain the situation to you better
Regards
Billyq
Skyhooks1 what type building & what function is the building used for. What is the total airconditioned floor area. What type airconditioning system is used? Is it chilled water or DX. Are there special areas rwith high heat gains or large amout of outdoor air. Did the contract spdecify the design outdoor air condition? Did the contract specify lighting level, equipment loading & number of people in the space? Is there extensive glass areas. Are there blinds. How often and when is design condition not reached. Does the shortage in capacity occur only in spots or throughout. Balancing and/or adittion of separately zoned thermostat controls may solve the problem. Are there simmilar buildings nearby? If so see if you can find out their tons/AC area square feet capacity and compare it to your building.
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To my mind there should never be a tolerance used on figures given for design conditions. The design condition numbers are used solely for the purposes of load and system sizing calculations.
The 24C +/- 2C belongs in the "desired internal conditions" section, where it should clearly state that this is the desired set point and acceptable limits of control. (+/- 2C is really too much by the way.)
You are not providing much information, which is understandable given the situation. If you are attempting to base your entire case on the +/- thing you seem to be in a pretty weak position.
Have you obtained the contractor's design calculations yet? You lawyer should get these on discovery, but if he hasn't, should subpoena for them. That is where you are most likely to find something to make your case.
Given the apparent lack of consideration of solar gains you might be able to demonstrate that the design calculations did not comply with accepted practice.
The 24C +/- 2C belongs in the "desired internal conditions" section, where it should clearly state that this is the desired set point and acceptable limits of control. (+/- 2C is really too much by the way.)
You are not providing much information, which is understandable given the situation. If you are attempting to base your entire case on the +/- thing you seem to be in a pretty weak position.
Have you obtained the contractor's design calculations yet? You lawyer should get these on discovery, but if he hasn't, should subpoena for them. That is where you are most likely to find something to make your case.
Given the apparent lack of consideration of solar gains you might be able to demonstrate that the design calculations did not comply with accepted practice.
- Thread starter
- #17
Skyhook1 you stated: "the situation is that the developer claims the design temp of 24+-2 means the a/c is sized to 26degC, we say this is incorect and that it should be designed to meet 24degC with a 2 degC control band width".
If the design documents state that 24*C is design set point then that's what the system should do under all conditions. Set point is always allowed to drift up and down from set pt(deadband/throttleing range)because perfect set point is really impossible to achieve even with DDC Controls. Control point: The actual value of the controlled variable (setpoint plus or minus offset).This from Honeywell
Gray Manual
Roger
If the design documents state that 24*C is design set point then that's what the system should do under all conditions. Set point is always allowed to drift up and down from set pt(deadband/throttleing range)because perfect set point is really impossible to achieve even with DDC Controls. Control point: The actual value of the controlled variable (setpoint plus or minus offset).This from Honeywell
Gray Manual
Roger
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hi,
Design criteria:
Internal and external are basically the
designer's choice, subject to local code.
Probably (very good idea) chosen from CIBSE /
ASHRAE / other guides.
unless specified in your local code, there is
no black and white requirement. However in some
countries there is a minimum temp. covered my
labour laws.
If I chose a particular internal condition
temp. say 24 deg C, and prep calcs, they are
based on a set of external conditions.
Effectively, I am promising the owner that the
internal temperature, will be *maintained* ONLY
when the external conditions are within my
described conditions.
If unexpected weather conditions drive the
external conditions beyond the design range,
the internal conditions will vary also.
variance, since automatic control systems can't
maintain a single point without excessive
hunting - the designer would specify a
control range say -+/-1 deg C.
A sensible designer would specify, in the
design brief, the "DESIGN conditions" at the
outset.
However, these are NOT 'cast iron' guarantee's
that the internal conditions will be maintained
under every possible circumstance.
If the designer specified 24 +/- 2 deg at the
outset, during specified external conditions,
and there is no other hidden local code
requirement, the designer has achieved the
intent.
One might try to argue, using "best practices",
etc. however legally the designer is probably
safe, depending on documentation
my 2 cents
cheers,
Design criteria:
Internal and external are basically the
designer's choice, subject to local code.
Probably (very good idea) chosen from CIBSE /
ASHRAE / other guides.
unless specified in your local code, there is
no black and white requirement. However in some
countries there is a minimum temp. covered my
labour laws.
If I chose a particular internal condition
temp. say 24 deg C, and prep calcs, they are
based on a set of external conditions.
Effectively, I am promising the owner that the
internal temperature, will be *maintained* ONLY
when the external conditions are within my
described conditions.
If unexpected weather conditions drive the
external conditions beyond the design range,
the internal conditions will vary also.
variance, since automatic control systems can't
maintain a single point without excessive
hunting - the designer would specify a
control range say -+/-1 deg C.
A sensible designer would specify, in the
design brief, the "DESIGN conditions" at the
outset.
However, these are NOT 'cast iron' guarantee's
that the internal conditions will be maintained
under every possible circumstance.
If the designer specified 24 +/- 2 deg at the
outset, during specified external conditions,
and there is no other hidden local code
requirement, the designer has achieved the
intent.
One might try to argue, using "best practices",
etc. however legally the designer is probably
safe, depending on documentation
my 2 cents
cheers,
- Thread starter
- #20
thank you everyone for the comments but i think this is getting a little side tracked. i am a very experienced designer and have many years experience in the application of a/c systems to various building types. the building in question is an apartment building and the client in question owns the penthouse (a very nice place i can assure you). the situation is the developer has bought a spec from a consultant which states the a/c is to be designed to 24+/-2degC and the external design condition is 35degC, the mech contractor has taken this on and designed the system in a design and construct contract. the owner has purchased the apartment with this in the contract.
the problems associated with the a/c system is that it is not coping with the solar gain, it works fine during overcast days even though the external temp is as high as 40degC, but when the external temp is 32degC and a clear day with direct solar radiation entering the space it is not maintaining the internal design temp.
now the issues i see that i must put forward for my argument is,
a) the design calcs did not take into consideration direct solar radiation, to which i am informed that there are no heat load calculations. (or they don't want to submit them at this stage)
b) the glazing spec was not considered
c) the developer reduced the quality of construction with the view to reduce cost and did not inform the sub contractor,
d) a risk assessment was taken with the cost of installing a lesser system and taking the chance of no tenants taking action
e) the power supply to the building was not sufficient and a probability of consumption taken.
the question i am asking of you is, is there a document that can support my argument that an internal design condition with a tolerance of +/- XdegC means the design calculation is based on X and a tolerance is allowed for the fluctuations in the air system thermostat. i know as well as the rest of you what this means but when i put this forward to a court i must prove my statements which can be done through a document from an institution like cibse or ashrae etc, or through an industry standard calculation methods such as the Carrier method.
i am reviewing the suggestions made from imok2, mintjulip and billyq, hopefully i can put a good case forward.
anymore suggestions are most appreciated, thank you all for your help.
the problems associated with the a/c system is that it is not coping with the solar gain, it works fine during overcast days even though the external temp is as high as 40degC, but when the external temp is 32degC and a clear day with direct solar radiation entering the space it is not maintaining the internal design temp.
now the issues i see that i must put forward for my argument is,
a) the design calcs did not take into consideration direct solar radiation, to which i am informed that there are no heat load calculations. (or they don't want to submit them at this stage)
b) the glazing spec was not considered
c) the developer reduced the quality of construction with the view to reduce cost and did not inform the sub contractor,
d) a risk assessment was taken with the cost of installing a lesser system and taking the chance of no tenants taking action
e) the power supply to the building was not sufficient and a probability of consumption taken.
the question i am asking of you is, is there a document that can support my argument that an internal design condition with a tolerance of +/- XdegC means the design calculation is based on X and a tolerance is allowed for the fluctuations in the air system thermostat. i know as well as the rest of you what this means but when i put this forward to a court i must prove my statements which can be done through a document from an institution like cibse or ashrae etc, or through an industry standard calculation methods such as the Carrier method.
i am reviewing the suggestions made from imok2, mintjulip and billyq, hopefully i can put a good case forward.
anymore suggestions are most appreciated, thank you all for your help.
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