1. Most people design for worst case

Typically heating is done for the ‘design day’ – this results in systems to cope with the peak demand on the coldest days of the year, but most of the time your building will need much less capacity. 

This is fine for extreme conditions, but most of the time your system will be working under ‘partial load’. You still need to cope with the 1-2% of the year that that extreme level of heating is needed, however, with a few simple design tweaks (such as variable flow temperatures or ‘peak shifting’ with local storage) it is possible to run your system year round much more efficiently. It also saves you money in not buying so much kit in the first place!

2. Increasingly complex

Systems are becoming more and more complex. Introducing new concepts and technology can sound a good idea, however, it is good to have the discipline of looking into the whole system ‘due diligence’ and asking questions such as: “has this been done before?”; “can we see working projects?”; and “what happens in the future in terms of servicing and longevity”. There isn’t really a Which? guide so having an independent engineer who has seen these technologies being deployed in reality can save you learning out the hard way. 

3. Increasingly expensive

Recent analysis has shown what we suspected: that 50% of all systems are oversized. This is due to lack of maths as trades are using simple ‘rule of thumb’ design approaches to sizing systems. This is costing ~23% more for your up front cost for heating , ventilation systems. The difference in a fossil fuel system probably wasn’t that much (a 20kW gas boiler is basically the same cost as 25kW- so they just stuck the 25kW one in), however, in a heat pump project that choice could be thousands if not tens of thousands of pounds difference.  Basically paying to size the system properly is much cheaper for you in a renewables scenario than just randomly selecting the heat pump based on a rule of thumb.  It also helps keep the grid connection size down and thus easier to permit.

4. Rely on suppliers to select the system

As design can cost upfront many are relying on suppliers/ and or installers to provide you the system . Increasingly we see a particular make or size promoted above say a smaller cheaper one- they are in the business of selling the kit, and more kit equals more margin. 

5. Lack of comfort

Overheating, under-heating and poor ventilation are common problems in buildings.

The Heat Balance Method states that the “sum of all space instantaneous cooling/heat gains at any given time does not necessarily (or even frequently) equal the cooling/heat load for the space at that same time”

CIBSE guides and methodologies have really helped (such as TM59), however, to answer the question you really need to model all the spaces in the building and in a dynamic way in at least hourly intervals. For example each wall in the room will have a different radiation effect on the space heating for that space.

Otherwise you are relying on individual, unconnected static methodologies or the ‘rule of thumb’ again. Obviously it takes more time (and money) to do the proper calculations before you build, but this is less emotional than sitting in an under-performing building at the end of the project. 

6. Vague specifications

Concepts are good, but if you leave the detailed design to the installer then you run the risk of them selecting components without knowledge of the overall design or in worst case just the cheapest that are on offer at the suppliers merchant. 

7. Commissioning problems

Any professional will tell you that it’s a fact that often the set up, balancing and commissioning is typically time pressured as its the last phase in the construction.

Verifying say a flow rate from a ventilation duct, or the set up of your heat pump, is nearly always left to the installer.

We see housing occupiers are left with the systems on say constant (higher) flow temperatures. The last thing that installer needs is a call back and he/she will want the customer to report back that the heating is hot and working really well. The issue wont turn up for months or years in terms of impact on bills. 

You are best to use local trades in your community or independent service engineer verification.