In passive houses the production and storage of Hot Water is usually more of a problem than space heating. There are various reasons for this. First the current PHPP modelling of our renovation suggests hot water will use about 2900 kWh a year compared to the 1750kWh required for space heating. This is common across most passive houses. Secondly space heating only needs to heat to about 20 degrees compared to the 40 degrees required for a shower or bath. Furthermore the 40 degrees at the tap will normally be supplied by a store maintained at a much higher temperature in order to minimise the volume required - large tanks of water are not a desirable feature in space constrained houses.

In practice it is easiest to separate space heating from hot water rather than adapt one solution for both. In our case space heating will be a simple 1.2kW electric heater in the air supply supplemented by two 600 watt far infrared heaters and a few hundred watts of electric underfloor heating in the en-suite. We have traded off ‘higher’ running costs for low capital expenditure. We will also have less equipment to find a home for.

The hot water solution is simplified by the decision not to have a gas supply. The original specification was to use a simple thermal store with water as the heat storage medium with energy provided via two immersion heaters run via Economy 7 or a Solar PV diverter. The 220 litre volume being sufficient for a standard bath. I hadn’t done my energy calculations but assumed a 220litre tank at around 75 degrees would be sufficient to fill a bath with about 120 litres of water at 40 degrees.

We forgot about this when searching for a bath - on the original plans we had a standard 1700mm by 700mm bath specified for the Bathroom. We then spent a very interesting afternoon looking at baths - which revealed very little. The assistant then told us you can’t choose a bath without trying them out. Baths are like beds then. This proved to be a revelation and we found ourselves liking none of the many baths we tried - they just felt awkward or uncomfortable. After more searching and trying them out we finally found a comfortable bath - the Pleasance Plus by Cabuchon that is made only a few miles from the house.

Only when Glenys told me that the originally specified thermal store was no longer made did I undertake the hot water energy calculations. This revealed a problem! Divorce was likely if Glenys couldn’t have a full bath. The capacity of the chosen bath was much larger at 300 litres. Subtract the 60litres displaced by the typical person means 240 litres of water at 40 degrees are required.

Glenys found a replacement for the originally specified Thermal Store - the Gledhill Pulsacoil Eco which stores 220 litres at 75 degrees. Assuming useful heat at the tank ends when the supplied water reaches 42 degrees (it will lose temperature traveling to the bath tap), then when fully charged the store will hold a total useful energy of:

33 x 220000 x 4.2 = 30.49MJ

Since 1 kWh = 3.6MJ, This gives 8.47kWh available at the end of the Economy 7 heating period in winter (Summer is less challenging since the solar PV should keep the tank at this temperature during daylight hours). Since neither of us likes having a bath in the morning this heated water will then sit for around 12 hours losing energy. Sadly the heat loss data provided by the manufacturers to the standard testing procedure doesn’t match normal household practice. (There is much discussion on this point on the various forums.) My best estimate is that the loss would be 150 watts so after standing for 12 hours the remaining useful energy will be around 6.67kWh. This would provide 190 litres at 40 degrees and I would have an unhappy wife!

There are two other problems with the 220 litre hot water store solution. The obvious one is the 150 watt heat loss, problematic in winter since we can’t control this source of heating but simply a waste in summer when there is no requirement for space heating. This is compounded by having the Solaredge 5000 inverter in the same space. The will generate an additional 130 watts of heat loss when generating 5 kW (i wish!). The inverter should last longer if we can keep its components cool. In other passive houses with more sensible orientations heat losses have produced overheating (not a problem in our house with very little glazing with a southerly aspect). The less obvious problem is the volume occupied by this type of thermal store - excluding the header tank it occupies 0.78 cubic metre and its location means that is essentially ‘blocks’ the accessible wall of the en-suite which needs to have access to install the plumbing for the shower mixer valve.

The ebuild web forum identified a possible solution: the SunampPV unit that essentially replaces water as the thermal storage medium with a phase change material. The main advantages are that a lower volume is required for the units (the two needed in our case) - 0.22 cubic metres and the height of 700mm means the wall of the en-suite is accessible. (we might even be able to place these under the MVHR unit thus freeing up an entire wall surface). Another advantage is the low standing loss of 24 watts per unit (placing them together suggests a loss of 38 watts for the two units). The numbers work out (just). Each unit has 4.4kWh of useable energy down to 42 degrees. So initially that’s 8.8kWh. After 12 hours with a 38 watt loss we are down to 8.34kWh which should be able to supply 238 litres at 40 degrees. Divorce is unlikely from a 2 litre shortfall! It is fortunate that Sunamp started commercial production of these units this week (w/c 11th October 2015).

For households with a standard sized bath a single Sunamp PV would suffice as long as other hot water demands are small. If showers replace a bath then the single Sunamp PV should easily be sufficient (a caveat being that my showers rarely consume more than two minutes actual use of the shower head!)