Hot water with a heat pump

A combination boiler makes hot water the instant a tap is opened, by firing at 25 or 30 kW into a plate exchanger. A heat pump has perhaps 8 kW to give and cannot produce water at 60 °C efficiently anyway. So it heats a cylinder in advance, and the house draws from that.

For a house that already has a cylinder this is no change. For a house with a combination boiler it means finding somewhere to put one, and that, more than anything to do with the heat pump itself, is often the practical obstacle in a retrofit.

How much hot water a house needs

MIS 3005-D gives a formula for the daily demand in a domestic building:

V_d,average = 45 × N

where N is the greater of the number of bedrooms plus one, and the number of people actually living there. For a new build, where the occupancy is not yet known, the bedroom count is used.

So a three bedroom house is designed for four people and 180 litres a day. If six people live in that three bedroom house, the known occupancy wins and the figure is 270 litres.

Forty-five litres a person a day is a design allowance, not a measurement, and a generous one. Metered studies put real consumption lower for most households. The generosity is deliberate: a cylinder that runs out produces a complaint, and one that is slightly large produces a standing loss of a few pounds a year.

Sizing the cylinder

The cylinder does not need to hold a whole day's demand, because it refills as it empties. What it must cover is the largest draw-off before the heat pump can catch up, and that depends on how fast the heat pump reheats.

MGD 007 reproduces the BS 6700 guidance, with a bath taken as 100 litres at 40 °C, or 60 litres at 60 °C blended with 40 of cold. The minimum volumes come out like this, in litres:

The pattern is worth noticing: a bigger heat pump needs a smaller cylinder. The reheat formula behind the table is

minutes = (volume × temperature rise) ÷ (14.3 × kW)

Heating 60 litres from 10 to 60 °C takes a 3 kW heat pump 70 minutes, which is far too slow for a second bath 25 minutes after the first, so all of it must be in store. The same job takes a 10 kW machine 21 minutes, so the second bath is largely reheated in time and the store can be much smaller.

MGD 007's own worked table then adds capacity for real life on top of these minimums, and rounds to sizes cylinders are actually made in: 150 to 210 litres for most three and four bedroom houses on a 6 kW heat pump, 250 to 300 where there are several bathrooms.

The coil is the part that matters

The single most common mistake in a heat pump retrofit is reusing an existing cylinder.

Heat crosses from the coil into the water in proportion to the coil's surface area and the temperature difference across it. A boiler puts 75 °C water through the coil of a cylinder holding water at 20, so the difference is large and a small coil suffices. A heat pump puts 50 °C water through the same coil, and the difference is half as large, so the coil must be much bigger to move the same heat.

An old cylinder with a 1 m² coil connected to a heat pump will reheat slowly whatever its volume, and it will drag the heat pump's efficiency down while it does, because the machine has to run hotter to force heat across an inadequate surface. A cylinder made for a heat pump has 3 m² or more, often as two coils or a finned one.

MIS 3005-D allows an existing cylinder to be reused, but only under conditions, and the coil is the first of them. The specification of the heat exchanger has to follow the manufacturer's recommendations, and the honest answer for most pre-2010 cylinders is that they do not meet them.

Storage temperature and legionella

There is a real tension here. Efficiency wants the cylinder as cool as possible: a heat pump producing 45 °C water runs far better than one producing 60. Safety wants it hot, because legionella bacteria multiply between about 20 and 45 °C and are killed reliably above 60.

The usual resolution is to store at 50 to 55 °C, which is warm enough to suppress growth and cool enough to be reasonably efficient, and to run a periodic cycle above 60 to sterilise the whole cylinder. That cycle is normally weekly, and is often done with the immersion heater, which runs at a coefficient of performance of 1 and is a genuine if small cost.

MIS 3005-D requires domestic hot water systems to incorporate a means of preventing bacterial growth. It does not prescribe one method, and the arrangement should be visible in the design, not left as an assumption.

A separate point: water stored at 55 °C or more will scald, so the outlets need thermostatic blending. That is a building regulations matter as well as a sensible one.

Practical consequences

A 200 litre cylinder occupies a footprint of roughly 600 mm square and stands about 1.5 m tall. Full of water it weighs a fifth of a tonne, and nearer a quarter once the cylinder itself is counted. It needs a floor that will carry that. Airing cupboards, the traditional home, are frequently too small for a modern heat pump cylinder, which is taller and fatter than the vented cylinder they were built around.

An unvented cylinder, which is what most installations now use, needs a discharge route for its safety valves and annual servicing by someone qualified for unvented systems. A vented cylinder needs a tank in the loft. Both are workable, and the quotation should say which is proposed.

Finally, the heating and hot water compete. Most systems give hot water priority and pause the heating while the cylinder reheats, for perhaps 40 minutes. In a well insulated house nobody notices. In a cold house at the design temperature, a long hot water cycle in the morning is felt. Scheduling the reheat for the small hours, on a cheap tariff if there is one, deals with both the comfort and the cost.