Decisions, decisions: HVAC System Selection
Bill Jolly explains his company's HVAC system selection process
In the first instalment of MEP Middle East’s new technical review section, Bill Jolly, Ramboll’s director of MEP and sustainability, explains his company’s HVAC system selection process
There are many challenges facing MEP design engineers in the construction industry today. The built environment is changing along with clients aspirations which require MEP services to be cost effective, sustainable, low energy/carbon, low maintenance, and have low running costs.
MEP engineering solutions and selections need to be very carefully considered where many different aspirations play a vital role in the selection process. Gone are the days when engineers simply rely on previous designs.
MEP design engineers today sit at the centre of this selection predicament and are pivotal in the selection process for ensuring the right solutions are offered.
Utilisation of a system selection matrix is a comprehensive way to demonstrate the most cost effective sustainable solution which holistically satisfies client's needs and aspirations for any given project/condition. This paper offers a detailed description of how to prepare and document such a process.
System Section Process
Everybody goes through a decision-making or selection process every single day. A typical example would be simply getting to work in the morning where most people have decisions to make.
Would you travel by car, walk or use public transport? Automatically, people go into system selection mode where the criteria in this instance would be quickly determined. Time, comfort and cost are perhaps the selection criteria for this typical example.
Walking would be free but it would take time and perhaps would be uncomfortable. Driving would be much quicker and comfortable, however, driving is expensive.
Using public transport is perhaps a relative content mix between the two options where cost would be reasonable, time would also be balanced and the trip would be relatively comfortable.
Once the selection criteria has been determined and scored then the important factor in any selection process is the weighting of the selection criteria.
The question here is: "What's most important to the individual?" If time is of the essence in this particular example then I suppose driving the car would perhaps be the best way.
However, if time is not critical yet cost is, then walking may be the most preferred option. The weighting of the system selection criteria would determine the preferred option when deciding how to get to work.
The selection of MEP services should follow a similar comprehensive, well thought out process. ASHRAE standards also offer guidance where selection criteria is defined and evaluated along with weightings of that criteria before an overall selection is defined [Further information can be found in ASHRAE Handbook Section 1.3 – HVAC Systems and Equipment]
In order to provide a comprehensive system selection process, we at Ramboll adopt and prepare a complete system selection matrix. This matrix thoroughly considers various feasible systems (options) which are reviewed against different selection criteria.
The selection criteria is scored and weighted for each option/system. The overall selection matrix then offers the most cost effective and sustainable solution for the project satisfying client's aspirations.
Firstly design engineers should establish the various options which are to be reviewed. This is normally decided in conjunction with the clients and normally with the clients’ FM teams or technical engineering staff. The typical sample example shown in this review considers Water Cooled Chillers, Air Cooled Chillers and a District Cooling solution.
2. Selection Criteria
Engineers should determine the selection criteria. This is normally undertaken in an interactive manner with the various clients, where it is often the engineer’s responsibility to extract from the selection criteria which is important to review.
Once the selection criteria has been determined, design engineers should undertake a review of this criteria against each of the various options under consideration i.e. a capital cost review shall take place for Options 1, 2 & 3.
Likewise a carbon emission analysis would take place for options 1, 2 & 3 etc. Here are a few typical selection criteria which are normally reviewed as part of the selection process at Ramboll.
Capital Cost: The capital cost of any solution is always a major consideration for any client. In order to accurately review the capital costs, engineers should work closely with quantity surveyors and suppliers.
The capital cost can then be evaluated for each option. It is important to recognise each option should have a holistic MEP capital cost review. Air cooled chillers might have addition capital cost due to increased transformer requirements whilst water cooled chillers have increased water tanks and cooling towers.
The whole MEP capital costs need to be carefully considered for each option and fully documented in any proposals offered to the client.
The solution with the least capital cost would normally attain the highest score whilst the most expensive solution would receive the lowest score. If there are five options then there would be five points available.
In the typical example there are three options and three available points. District cooling has the least capital cost and attains all three points whilst the water cooled option is the most expensive and therefore receives one point.
Carbon Emission: Engineers should work in conjunction with a building physics team to establish the carbon emissions of the various solutions before the scoring system takes place.
The solution with the lowest emission should receive the highest number of available points whilst the solution with the highest emission receives the lowest number of available points.
It is prudent to note here that normally each solution would not have the same level of points and this statement stands true for the review of each selection criteria review. Generally, solutions would not have similar emissions and likewise for the cost review etc., therefore solutions should not receive the same score.
Constructability: Reviewing the constructability and the implications of the options put forward are important. It might be necessary to utilise large cranes for heavy equipment or to reinforce structural solutions for certain applications.
This review can also be undertaken in conjunction with a constructability QA/QC process. The constructability review would occur for each individual project where unique constructability issues might occur. This needs to be considered on a project-by-project basis.
Life Cycle Cost Analysis: This is a critical component of the selection process which needs to be reviewed for all projects. The life cycle cost should be prepared such that the design engineer has a complete holistic view of costs for each option.
Maintenance: All services installations within buildings require maintenance. Some solutions require more maintenance than others. The engineer needs to review the maintenance requirements/implications for each system and then score the systems accordingly.
Space Requirements: This review should be undertaken in conjunction with the project architects and with the client. The location of equipment is essential and where possible should be located out of sight and in positions which do not compromise the architectural aesthetics of the building design.
Once the review of each system has been undertaken the engineer can score each system accordingly and insert the scores within the selection matrix.
Running Cost: The running cost review should take place by the design engineer in conjunction with the building physics team. It is important for the design engineer to understand the anticipated running cost of the various options.
Air-cooled solutions only have electrical energy costs whilst water-cooled solutions have electricity and water consumption costs. Once the running costs of the different options have been established then the scores can be applied.
The solution with the highest running cost would receive the lowest score whilst the solution with the lowest running cost would receive the highest score.
Availability: The local availability of equipment within the region can be a major element of any system selection process. If the client is looking to complete the project in a timeline where the programme is critical, then perhaps “long-lead” items would have some major implications.
It is important for the design engineer to understand the client's aspirations and then establish anticipated timeline deliveries from suppliers before the scores can be applied to each option.
The weighting is a critical element of the selection process and dramatically affects the outcome of the selection. It is vitally important to get this element right and ideally these weightings should be provided by the client.
If the client is unable to offer this information then the design engineer should work through the implications of the weightings in an interactive and informative manner to ascertain what is and what is not important for the selection criteria.
To keep the weightings and the selection process complete the collective sum of the weightings should equate to 100%.
The number and scores for the various selection criteria should be available for each option considering the review has already taken place. The design engineer should now insert the scores into the matrix.
The important thing to note is the way the matrix is prepared. The option scoring the highest number of points would receive the full percentage weighting of that particular selection criterion.
In the example, district cooling has the lowest capital cost and therefore receives the highest score.
The weighting of the capital cost is 25% therefore the district cooling option receives 25%. The water-cooled chiller option has the highest capital cost and therefore only receives one point.
This one point equates to one third of the weighting percentage at 8%. If there were five options available then this would equate to a fifth of the available 25% i.e. 5% would be carried forward.
It is important to note that options should not receive the same scores via the review e.g. when reviewing the capital cost, one solution will always be more expensive than the other.
Only on extremely rare occasions would two different solutions have the same cost. The options should therefore all have different scores.
The result is a summation of the various percentages from each of the different options. The one with the highest score is the recommended option to put forward to the client which, after due consideration of all the system selection criteria, should be the most cost effective and suitable solution which satisfies the clients requirements for the project.
In the sample example offered as part of this paper the solution with the highest score is water-cooled chillers. It is important to note that this system selection process has been prepared for demonstration purposes only and is not a generic solution for all situations.
The system selection criteria along with the weightings would significantly vary for different clients and projects ultimately offering different solutions.
The paper offers a description of a comprehensive system selection process which considers different options, selection criteria, weightings, scores and finally offers a recommended solution.
The process ensures design engineers consider each and every aspect to attain the right solution. This selection process can be provided for the benefit of both the client, who will receive "peace of mind" with the knowledge they are receiving the most cost effective sustainable solution, and the design engineer, who will have the knowledge to provide a solution which satisfies the client's aspirations for the project.
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