TLA’s – three letter acronyms. People love them… and hate them… and get them mixed up… The ones we’ll talk about today are COP – Coefficient Of Performance, SHX – Subsoil Heat Exchanger, and DHW – Domestic Hot Water.
Compact Heat Pumps for heating water and for space heating are very efficient units for transferring heat. Using the same unit for multiple purposes makes sense if it saves you energy. Taking advantage of all the free ways to boost your heating potential like employing a subsoil heat exchanger could make sense too. Obviously placing a subsoil heat exchanger comes with some expense but if it pays for itself and can realize a way to supply the natural heat of the earth to pre-warm incoming air or a liquid without additional expense after it has paid for itself through energy savings then you are getting free heat.
Let’s look at those TLA’s. The first one we come to when filling in the Compact worksheet is SHX. This stands for Subsoil Heat Exchanger. This is a tube that is buried in the ground below the frost line and air or a liquid is drawn through it transferring the heat from the ground to the air or liquid which then interacts with another system in the building to transfer the heat it has picked up from the ground to an air or liquid system.
I say air or liquid because some subsoil systems will have a closed loop system that circulates a liquid that never leaves the system, but at some point is able to transfer its heat by coiling around another pipe or container that has cooler temperatures. Other systems use air as the medium for heat transfer. A good resource for more information on ventilation systems that use subsoil heat exchangers is the PDF by Prof. Dr. Harald Krause entitled “Technical Installations in Passive Houses, Part 1. Ventilation Systems”
The next TLA we get to is COP. This is NOT the Heat Police, but rather the Coefficient Of Performance. It tells how efficient a system is expected to operate at. For a heating unit it is the amount of heat supplied to the heat reservoir divided by the heat consumed by the heat pump to produce that heat. More information can be found on the COP Wiki page: http://en.wikipedia.org/wiki/Coefficient_of_performance PHI certifies Compact Heat Pumps and those units’ COP. If you are using a system that has not been certified I recommend speaking with PHI early in the design process to see what is needed to record the COP of the unit you want to use.
Our final TLA is DHW and that stands for Domestic Hot Water. The heat energy from the SHX (if one is used) is transferred by our Compact Heat Pump (with the super great COP that we picked out) and passed on to our DHW. In the course of all that we can also tap into the system to use some of that heat for additional space heating if necessary as long as we size the unit accordingly. You will need to state if the unit is primarily for DHW or space heating on the Compact worksheet. PHI has a good discussion on Compact Heat Pumps on their website.
To complete the Compact Worksheet you will need to identify your system and the specifications of its operation. If you do not see your unit listed as an option on the pulldown menu of the worksheet you can add a new unit. The PHPP manual states that the test point data for heating may include 4 test points, but only 2 are actually required with one of those being under -3deg C. A similar arrangement is made for the DHW, but the final test point will need to be at 20deg C so the PHPP can consider hot water production in the summer months.
A discussion in the PHPP details the calculation method for determining the COP of the standby mode if it changes from the operational mode. If your system does not operate differently you will not need to enter the information in for the standby mode, the PHPP will obtain it from the “DHW storage reload section”.
Refer to section 30.2.6 of the PHPP manual if you will be using a SHX with your system. There are entries that are required on the Ventilation, Aux Electricity worksheet as well as the compact worksheet when this type of system is used.
Finally, the information on the Compact worksheet depends on cells in other worksheets to be filled in appropriately. The Ventilation sheet needs to be told that you are selecting a Compact unit on the Compact worksheet. The Heating Load worksheet sets a default of 52deg C for a max supply air temperature but the PHPP manual advises that most compact units do not achieve this temperature and the actual measured temperature should be entered into the PHPP.
Should your compact unit use an internal circulation pump this does not need to be included on the Aux Electricity worksheet, but if it uses an external pump you will need to supply that information to the Aux Electricity worksheet. Be sure to let the Aux Electricity sheet know if you will be using it for DHW in the summer and if you will not be using a SHX you will need to record the max temp for frost protection in the Aux Electricity worksheet as well.
You don’t need to adjust the DHW + Distribution worksheet for the losses of the storage tank because the compact worksheet covers this. You do need to adjust the SolarDHW worksheet as per the PHPP manual under section 30.2.7 if you are using one of these.
Now that we have figured out the TLA’s of the Compact Worksheet we will be moving onto something else that might be just a bit of a mystery to those of us in the US who do not know what District Heat is!
Up next – District Heat