View Full Version : How long is a piece of string?

Daddy Cool
22-12-2004, 05:08 PM
i have a question that only requires an approximate answer.

i have to price a large residential block in London, the man making the decision is a real pain, and wants to estimate the running costs. the system will be inverter driven a/c, either central VRF or multi splits. i have the running currents etc, but he wants to know the "average" time the compressor will be running.

As the title says, how long is a piece of string, any ideas very roughly.


Daddy Cool
22-12-2004, 06:42 PM
I look forward to it. i did start with all the formula and trying to take the average ambients for the months and then draw a line across the graph, working out TD's etc etc, but it was taking ages and will only serve to raise more questions in my direction. i was looking to say to them that an educated guess would mean the compressor/s were running approx 25% of the time the fan coils were on or something of that ilk, but wanted some input from others and maybe take an average of the guesses.

The software would be really useful, as i had to do an Excel sheet showing the CO2 emissions caused by running different heat pump a/c units as opposed to a condensing boiler. That was fun, but never really got looked at.

Any input would be greatly appreciated.

22-12-2004, 10:28 PM
sounds like you have the same problem as i do with "tyre kickers"
although it is proberbly bad for business,with these sort of questions i now tell them to go and find out for themselves, and if they really are interested and want a realistic quote, give me a call.
they know what "air conditioning" or comfort cooling to me and you does, they also know that in a residential development it is a luxury not an essential. therefore the running cost are irrelivant.
if he still insist on a figure, tell him for heating it is cheaper than electricity and more expensive than gas, and for cooling there is no alternative, so the cost doesn't matter
these are the type of people you spend a lot of time trying to please, yet are the most ungratfull of you efforts.
as you pull away in your van upon completion of the job, he will be sat there, in his BMW, thinking of a way to withold payment.

merry chistmas


22-12-2004, 11:13 PM
Hey, Daddy Cool, are you involved with the design or just the pricing of the equipment ?

If you are only pricing then refer the questioner back to the building designer. Beware getting involved in 'design responsiblilty' unless you have professional insurance.

Do you have all of the design criterea for the building to estimate heat gains etc?; if you have then you have worked out the amount of cooling required at the maximum heat gain period of the day. Give a rough guide of 100% at maximum design temperature dropping to say 50% at other times - be honest and say that without knowing exactly how warm/cool every resident wants their premises how can you accurately estimate running time.

Daddy Cool
23-12-2004, 10:09 AM

As far as "design" goes, i have to employ the classic cop out that "final design responsibility lies with others" when asked to elaborate, i tell them that i can only offer advice as i do not have design liability insurance. Too many times in the past i have been asked to select a unit, to take it to the extreme, say a 2.5kW wall mount, the mechanical consultant does his extensive calcs and tells us the duty is 2430watts, so can i re-select! yes of course i say it's a 2.5kW wall mount like i told you before you spent a fortune on a consultant.

To fill in a few blanks with this project, they are running two very old chillers (circa 1961) that are dead on their feet. They have been ripped off for maintenance for years to keep them limping along. They run all the time and never stop as they never reach setpoint. The residents are upset that they pay a fortune each month for service charges and don't get what they pay for. The general idea was to replace with new VRF equipment, but being joe public they want to know how much it will cost to run on top of the capital expenditure. they won't accept it is substantially less than the old museum piece they currently have without some figures. i have done one sheet showing all rooms in all apartments running at 100%, but this is obviously miles over what they will ever use. i did point out that this is worst case situation and doubt it will ever happen, hence they asked for a realistic figure, back to how long is a piece of string.

23-12-2004, 12:28 PM
You can't say this, Brian, because this is usually what the client assumes when she asks the question in the first place.Fair comment.

23-12-2004, 03:24 PM
Hi Daddy cool,

The way I do it, is ask a Design engineering office to go with me on the project, they give me all the details so I can give the quote and if I get the job, they design and I pay them.
If I don't get the job, they do not get payed.

It works fine for many years.

Chemi :)

23-12-2004, 03:26 PM
If you install Inverter multi splits then the compressor will be running from the minuite its switched on in the morning untill its switched off. VRF's are a bit harder, multiple compressors turning on-off at will!?! if it was me i would say they would be running at full tilt 20% of the time, 3 quarters full load 25%, half load 40% and 25% load 15%.
ofcourse, this doesnt include for defrost cyclces, and it all depends on what sort of diversity the system has been designed on.

24-12-2004, 12:19 AM
i have a question that only requires an approximate answer.

i have to price a large residential block in London, the man making the decision is a real pain, and wants to estimate the running costs. the system will be inverter driven a/c, either central VRF or multi splits. i have the running currents etc, but he wants to know the "average" time the compressor will be running.

As the title says, how long is a piece of string, any ideas very roughly.


Degree days in the UK
If you take 15.5C as the the most appropriate base temperature and knowing the cooling degree days, it seems to me that you should be able to get an approxmation of the hours of energy use per month
Example: The formula for degree days is 15.5C (60*f) - [hi temp + low temp]devided by 2. So if we have a hi temp of 80 *f and a low of 60*f /2 = 70-60 = 10 degree days. If this condition went on for 10 days then you would have 100 degree days. This site gives you a look at the cooling degree days in the UK..http://vesma.com/ddd/std-year.htm

Daddy Cool
24-12-2004, 09:47 AM
Hi Chemi,

All the design engineers i know will attend and do the calcs for a fee, if you win the job, then fine, if not you still have to pay them. At best i could quote with the design element as an extra, if they want design calcs then they pay, but everyone wants something for nothing.

24-12-2004, 09:52 AM
just give him your calcs.

you will have to make a 100% load situation and calculate the temps.
when outside very hot they will not have all very cold houses so assume this temp alsop going up a little and recalc.
make a normal calc and see how much cap is needed so with normal in and outside temps.
make a calc where no energy is needed.
if he has these questions somebody told him to ask this or he even knows about this, so you will have to convince him you know what you are doing. As you told him the old system is **** ( and he managed it for 40 years, he will be mad at you and will test you) as i am now thinking make a calc on the old **** and just show him a new installatioon will be paid back in 6 years.

Daddy Cool
24-12-2004, 09:52 AM
Thanks Shogun, looks like i will be spending Christmas Day doing calcs. still better than being stuck around a table with relatives you don't see for years on end as you can't stand each other!

Are up early or late in Cali?

Daddy Cool
24-12-2004, 10:00 AM
Hi Shooter,

i have been trying to get some info on the old chillers, but over the years they have had endless people "tending" to them, numerous apprentices told to give them a lick of paint (including the plates!) york replaced the compressors 14 years ago, but i'm having a problem getting the 6/12 duty from york for the compressors. Carrier done the original install, but i can't find out the duty of the chillers. i am rapidly getting to the stage where i just say, thats what you need take it or leave it!

24-12-2004, 11:22 AM
just take the temps of what you have now and the amps of the compressor and with a little luck some pressures.
then just calc what you would think it can be and present it.
it will always be a lot less then origin probably 50%.
and tell him also the change of breaking down will cost him money even to get a new one in a hurry.

24-12-2004, 11:59 AM
have you got any name data from the carrier? or pictures even...

at age the plates were usually stamped so should be available -unless its french -as they sometimes used printed laminates.

I assume that this refers to the Y53 thread a while ago..sadly this is just a model number you need the rest to get the capacity..



Daddy Cool
28-12-2004, 08:23 PM
Hi Richard,

Yes, you are right, with the Y53 thread. i think a return trip to site in the new year is called for. if i'm honest i have always avoided anything to do with chillers as i just don't have the experience.i'm always keen to learn, but every opportunity has been with monsters (upwards of 1 mW) i took the model number at face value as i had loads of other things to look at, so i only have myself to blame for another journey into town. i also have a bad habit of not being keen on things that are years old and have been played with by loads of people over the years and never recording what they have done, its like looking for a needle in a hay stack, but i guess you already know that!

i will try to get some more info and maybe some pic's (if only for the sake of interest now)


30-12-2004, 08:50 PM
Why not have a look at this old thread? http://www.refrigeration-engineer.com/forums/showpost.php?p=9261&postcount=1