Part 25 - Heating and DHW system performance
Entry posted by Stones
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In Part 22, I detailed my decision making process in relation to my choice of a pre-plumb Mitsubishi Ecodan 8.5kW ASHP based DHW and heating system.
I now have a full set of data covering 12 months so can provide figures in respect of how the system, and our house has performed.
My baseline requirement was to maintain 21.5C in the house 24/7 throughout the heating season (October to April), and a supply of DHW water that would allow multiple showers to be drawn off without a drop in the temperature of water delivered at the tap.
The Mitsubishi FTC5 master controller / thermostat is set to 21C, and is located in the hall next to the vestibule. DHW is set to and stored at 50C.
Over the 12 months March 2017 – March 2018, heating COP ranged between a February low of 3.3 to an October high of 4.6 over the course of the heating season, with an overall SPF of 3.7
DHW COP ranged between a February low of 2 to a summer high of 2.5, with an overall SPF of 2.3
Based on a kWh electricity unit price (inc standing charge) of 12.3p, I paid 3.32p per kWh of delivered heat, and 5.34p per kWh of DHW (inc losses).
It should be noted that DHW cylinder losses do slightly reduce my heating demand, albeit at a higher cost than if delivered via UFH.
For a reminder of our layout:
In winter, with a set temperature of 21C, the house sits at a comfortable even temperature, the main living section of the house tends to sit at 21.5C, the 2nd and 3rd bedrooms at 21C and the master bedroom at 20.5C. I suspect that the slightly lower temperature in our bedroom is due to the fact I set the MVHR vent at a higher supply rate than the other bedrooms. This would tally with my experience of doing the same in our last house.
The two biggest factors that impact on our heating demand are wind speed and solar gain. In modelling our heating requirement, I took both into account, along with incidental and household gains. The weather data set was based on a combination of met office and local home weather station information.
Our average wind speeds are significantly higher than elsewhere in the country, and combined with the effect of storm force wind speeds (which we get a fair bit of) we do have a higher heat demand when compared to the same house being located in a sheltered inland area. The impact of wind speed, and the differential in pressure it causes is illustrated here:
http://www.wanz.co.nz/ConversionChart
A doubling of wind speed sees the pressure increase by a factor of four.
Average winter wind speeds of 15-20mph (which equates to the standard air pressure test) are common if not the norm here. Average storm wind speeds of 40-50mph gusting to 70-80mph are also common. The impact of the pressure differential that such wind speeds cause was illustrated to me during the build whilst I was decorating. Having masked off the windows with polythene it was noticeable that when wind speed exceeded 40mph, the polythene would inflate on the windward side of the house, and be sucked onto the glass on the leeward side. Whilst we’re not aware of any drafts and the house isn’t any way uncomfortable, looking at the daily heating requirement when wind speeds are high, you can see an increase in the amount of energy used. Part of that will be air leakage (as evidenced by the effect of pressure differential on the windows) part is the unbalancing of the MVHR (gusting wind from a particular direction can cause the fans to struggle), and part is the lack of solar gain on such stormy days.
In terms of solar gain, the vast majority of any gain manifests in the public areas.
In winter this provides a useful uplift in internal temperatures. Depending on how clear it is, and how long the sun is out, the uplift sometimes compares to having a WBS stove on and really is quite pleasant. More generally, with mixed winter weather, the gain is less noticeable in terms of a temperature spike, but does have the benefit of reducing our heating energy use.
In summer, the gain can be significant and does require a cooling strategy.
Without any active cooling, the house has at times risen to 25C in the public areas and 24C in the bedrooms.
Alongside the MVHR summer bypass (set to activate when extract air is 22C or more) we cool the house down to a more comfortable 22C using cross ventilation, opening windows / taking account of the prevailing breeze. We also have a velux window upstairs, which when opened in combination with a downstairs window, creates a chimney effect that is very effective in exhausting hot air.
The biggest downside in using cross ventilation is that it doesn’t work when the ambient temperature is high (not a very common), nor when there isn’t a breeze (again, not very common). You also have to factor in the unexpected as we had to recently as our neighbour undertook ground works, which created vast clouds of dust in the dry weather. Opening windows simply wasn’t possible on those days.
Overall the predicted impact of solar gain is as I modelled it using data from the following two sites:
http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php
PVGIS provided daily average data, and from susdesign I was able to work out a peak solar gain multiplier to determine what the maximum likely amount of solar gain would be on a clear, cloudless day.
Modelling solar gain for both heating and cooling requirement was a very worthwhile exercise as I was able to determine what our worst case requirements were for both, and what strategies would work.
I’m fortunate in that the prevailing weather conditions here mean cross ventilation is a viable and workable strategy to deal with overheating. I am however in no doubt that had we built our house in a sheltered location in a warmer part of the country, that we would have a very real overheating problem and would have to use a very different strategy, most likely combining solar films on windows and active cooling.
I do have the option of actively cooling my house using our ASHP, via the UFH and if I wanted by retrofitting a duct cooler into the MVHR system, although haven’t felt the need to do so yet. One plus point of the Mitsubishi Ecodan ASHP is that activating cooling is simple (changing a dip switch setting to enable the master controller).
All in all, I’m very happy with the way the house is performing in terms of retaining heat and providing a comfortable environment in both winter and summer.
The performance and running costs to date are certainly more than satisfactory.
Of particular value to us is having sufficient heating capacity to deal with spikes in heating demand (resulting from especially stormy weather) as and when needed, without having to resort to auxiliary heaters or peak rate top up, and the simplicity of use of the master control system. Whilst I could if I so wished set flow temperatures and heating curves, the onboard auto / adaptive program requires one user input – internal set temperature, and the controller works out the lowest temperature way of delivering it. Whilst I had a very good idea of what our heating curve should look like, using the auto / adaptive mode saved a lot of trial and error, and having monitored flow temperatures, have not seen them exceed 32C. For those not comfortable with developing their own programming or control systems, this is a very big plus.
Having looked at a variety of options, I concluded that an ASHP would be the most cost effective solution (even after taking into account the cost of replacing the outdoor unit after 10 years) to meeting our requirements, and 12 months on, I have absolutely no doubt that I selected the right system for our requirements.
Whilst I have no hesitation in recommending the ASHP system I have, it is important to recognise that low energy or passive type builds really do need to be modelled and individual requirements identified to determine what type of heating, cooling and DHW provision is required.
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