Of all of the technical systems for the house, the heating and hot water solution is the one that evolved the most from my original ideas to the final selection. There’s no mains gas available at the site (the gas distribution network gets to within about half a mile but I didn’t even bother asking for a quote for a new connection over that distance) which takes away the option of a mains gas boiler and opens up a whole range of other possibilities, all of which have their own pros and cons.
There were two really big decisions to make, covered in their own separate pages:
- The fuel choice
- The system architecture (especially whether to have the main house and the outbuildings share the same heating system)
In the end I opted for a Ground Source Heat Pump.
Main Pros and Cons of GSHP Systems
- Can use electricity from any source, including renewable (wind, solar etc.)
- High levels of efficiency in cold conditions (compared to ASHP alternatives)
- Good level of payments under the Renewable Heat Incentive scheme (19.64 p / kWh as of mid 2017, compared to 7.63 p / kWh for ASHPs)
- Suitable for fully automated operation with little maintenance required
- More expensive to install than ASHP systems
- Need either a large area of ground or deep (expensive) bore holes for the collector pipes
- Some models have a high start-up current which can pose a challenge for electricity supply cabling
Heating System As-Installed
The main GSHP unit is a NIBE F1145 rated for 8 kW, supplied by Carbon Legacy and installed in the Plant Room on the Second Floor. This supplies warm water to the heating circuits via a buffer tank and a low-loss header.
Ground Source Collector Pipes
There are two 50m “slinky” coils buried under the field to the west of the house, in parallel trenches spaced 10m apart. The collector pipes are polythene, 32mm diameter with a 3mm wall thickness. These connect at an external manifold chamber made by Muovitech with 50mm polythene pipes linking back to the house and then 28mm copper pipes from the ground floor to the Plant Room.
There are three sets of heat emitters all connected to the output from the heat pump:
- Underfloor heating pipes in the polished concrete parts of the Ground Floor (about half of the ground floor area), configured as four separate zones (entrance hall, kitchen, family room x2)
- Towel radiators in every bathroom and WC (so 7 in all) over-sized by about a factor of 4 to account for the fact they’re only getting water at 30-40 degrees
- A heater on the MVHR supply duct
These operate independently using their own pump and zone valve, connecting back to the low-loss header.
The NIBE heat pump does most of the control itself, monitoring the outside temperature and adjusting the flow temperature through the heating circuit. The idea is to deliver water at as low a temperature as possible to just balance the heat loss, since any heat pump operates more efficiently at lower flow temperatures. This means that the pumps on the heating emitter circuits are normally left on all the time and they just pump slightly cooler or warmer water depending on the heating needs.
The other main control is on the underfloor heating zones since the pipes are buried about 100mm deep in the 150mm thick concrete floor slab and the different zones heat up at different rates. Some of these floors are next to big south-facing windows which benefit from solar gain in the winter and could get too warm if the slab was allowed to get to e.g. 25 degrees – and then the sun came out. In summary, the controls consist of:
- Temperature probes inserted in conduit tubes embedded in the floor slab adjacent to each of the UFH zones. The probes are from Zennio and connect back to a Zennio QUAD KNX input module which reports temperatures to the KNX bus
- An MDT KNX heating controller which generates a PWM signal for 230V zone valve heads based on the temperatures reported by the Zennio probes
- Each of the zones has its own Set Point which is adjustable via the OpenHAB home automation control panel
NIBE offer a service called NIBE Uplink whereby the GSHP unit connects to the Internet and regularly uploads operational monitoring data (and any alerts) to a server on the Internet from where it can be viewed and downloaded, including via an API. This also allows some GSHP settings to be adjusted remotely.
The temperatures from the UFH floor probes are logged and graphed via KNX and there are also portable temperature sensors from Oregon Scientific in most rooms which report data via RF to an RXFCOM receiver for logging and graphing.
In order to confirm the Coefficient of Performance of the GSHP there are Kamstrup Multical heat meters on both the Heating and Hot Water outputs, which measure the temperatures of Flow / Return and the flow volume from which they can calculate the kWh output. There is also a sub-meter on the electricity feed to the GSHP unit to measure the kWh input.
The primary role of the Renewable Heat Incentive (RHI) payments is to compensate for the up-front investment made when installing a renewable heating solution. As such, they’re probably best considered as helping to repay the mortgage (which included the cost of the heating system) rather than offsetting the running costs of the heating system.
TODO – Confirmation of GSHP CoP (likely to be different for hot water and heating, because of the different temperatures involved).
At a nominal CoP of 4.0 and 15 p / kWh for grid electricity, each kWh of heating costs 3.75 pence. That compares to mains gas which would be something like 4.5 p / kWh by the time the efficiency of the boiler is taken into account.
The prospect of smarter electricity tariffs makes this more interesting since – as long as the building has a high level of thermal mass – it would be possible to avoid running the heat pump at the most expensive times (typically early evening) and benefit from the much lower cost of electricity at other times (typically overnight).
Initially there were lots of concerns about the noise generated by a heat pump installed on a suspended timber floor above a bedroom and the NIBE model was selected largely on the basis of it being particularly quiet. In addition, the unit is installed on thick rubber pads in an attempt to reduce the vibration transmitted to the floor – plus there is mineral wool insulation in the ceiling void. These measures have been quite successful and while the GSHP is just audible in the bedroom below it the noise isn’t intrusive; it’s no worse than a fridge in the same room.
- An over-temperature alarm on the GSHP unit caused by one of the plumbers forgetting to re-pressurise the heating circuit after some other work. This was successfully alerted via NIBE Uplink.
- The Honeywell zone valves are not really intended to left on (i.e. open) 24×7 for weeks on end and sometimes these can stick open. It’s best to open and close them periodically.
Hints and Tips
- It’s better to site GSHP units on the Ground Floor than the Second Floor.