A Hybrid Heat Pump System

The more I look into the idea of an air-sourced heat pump heating system the more it appeals. In particular, the idea of heating my home without burning fossil fuel to the greatest extent possible is within grasp and there are even solutions out there that could renewably source the gas I burn. More on that later.

A chap from the prospective installation company came to measure up the house in order to calculate its space heating requirements. He went through each room with the tape, drawing a little plan on his clipboard and adding numbers as he went. He then discovered what a sad git I am! (Sad being a modern term for a geeky person who is deeply happy but few understand her or him, and their compulsive ways.) I introduced him to the spreadsheets with which I have documented our daily energy usage and generation over nearly ten years of patient data gathering. He seemed impressed and I glowed with pride. All we had to do was add up a complete year of gas consumption in kWh. The answer was 23,650kWh of energy. With an estimate of 20% being used for water heating, this led us to a figure 19,000kWh as representing our space heating requirements for a year.

He suggested that the way forward is to replace our existing combination boiler with a Daikin Altherma hybrid heat pump system. This consists of two units that work in series to supply the heating and water needs of the house. Most of the space heating comes from an outdoor heat exchanger, like an air-con unit in reverse. This can extract up to 8kW of heating power from the outside air and it will do this right down to about 3°C. This covers a lot of the typical winter weather in the West of Scotland which is characterised by relatively mild, moist air coming off the Atlantic Ocean on the prevailing south-west wind. The outdoor unit measures 735mm x 832mm x 307mm and is sited at least 150mm away from the house wall just outside where the boiler is.

A graphic from Daikin's brochure to illustrate how the heat load required by a typical European house can be met by an air-sourced heat pump. For the milder Scottish climate, I suspect this curve would move to the right somewhat.

When the temperature goes below 3°C, the indoor gas condensing boiler comes into play to augment the output from the heat exchanger. The two sources of heat are blended under computer control which also takes into account the tariffs for gas and electricity, on the assumption that the user has entered these values. At around -4°C or less, the 33kW gas boiler takes over completely. Although slightly larger than our existing unit, this boiler will fit into the available space. The system so far described is entirely capable of working through our existing system of 13 radiators. But that's not the plan.

Our home was built in 1970 and a builder extended it in 1990. We believe this was done purely with the intention of making money out of it and thus, shortcuts were taken. For example, the back room extension has a floor suspended on old scaffolding battens instead of conventional joists. One imagines a pile of these were lying around and so they were pressed into service instead of having to buy new wooden joists. They do the job perfectly well but they illustrate the shortcuts taken.

The 13 radiators I have are a mixture of very old single panel units and relatively new double panel units. I'm guessing some are original with the house, nearing a half century old, and some had to be bought in when the extension was done and thus are over a quarter of a century old. There is one further small one that we added about 15 years ago.

All these radiators are fed via a mixture of 8mm microbore copper piping (popular in the 1970s) and 15mm small bore pipes with a little bit of plastic piping for our additional radiator. To the guy from the installation company, this was less than ideal. It is apparently difficult to properly balance the flow rates through the system when there are pipes of different sizes all over the place. Moreover, the heat pump system can work more efficiently at a lower temperature than conventional systems and therefore benefits from having larger radiators in order to transfer sufficient heat to the rooms. Given the advanced age of the system's components, we decided that the job should include the complete replacement of all radiators and their associated pipework. That way, we start with a clean sheet and a single point of contact should there be a problem with the heating system as a whole.

Finance

Our initial quote for this work comes to a shade under £11,000. Three quarters of this can be covered with an interest free loan from the Home Energy Scotland renewables loan scheme. This is paid back over a term up to 12 years. Consequently, the up-front cost to us would be £2,745, an amount that compares favourably with the cost of simply replacing our boiler with a reputable condensing unit.

To further help with the overall cost of the system, I ought to be able to take advantage of the Energy Saving Trust, Scotland's Renewable Heat Incentive (RHI) scheme. I'm told that the way this will work is that for a period of seven years, quarterly tax-free payments will be made to me based on readings from meters that monitor the gas and electricity used to run the system. The installation company has given me an estimate of how much this would be, and their figure is £6,860. This is £1,373 less than the loan so that, over the piece, the cost to me would be £4,118.

But that's not the whole story. The calculations by the installation company suggest that my running costs for the system will reduce by £341 per year. If we add that over the seven years of the RHI scheme, then we would have saved £2,387. Moreover, this assumes that all the electricity used to power the system has to be bought. It does not take into account the fact that for substantial periods of the year during daylight hours, there can be a surplus of electricity coming from my solar panels that can be used to help run the heating, something that does not happen at the moment. Typically, we run our heating from late October to springtime. In the depths of winter, it is very rare that the panels produce spare power but from mid-February to late October, it is much more common.

Living with the system

Installation of such an air-source heat pump system might change our usage habits. With our gas-only current system, we tend to bring it on only when we perceive it is really needed in an effort to save gas. This means that throughout the heating season, it is on for 3 hours in the morning, 3 hours in the late afternoon and three hours in the evening. In midwinter, the house is relatively quick to cool and there comes a point when we just force it on full time.

I suspect that controlling a heat pump system would be quite different. Our current system has few controls and what there is depends only on each room's thermostatic radiator valve (TRV). In the new system, there would be a central thermostat/control panel, probably located in the centre of the house. Additionally, the (new) TRVs would allow control of an individual rooms but the system would work to maintain a temperature at the control panel. The heating characteristic of an air-source system is different because it is less responsive. From all accounts it is better to leave it on most of the time and instead, programme a desired temperature profile for the day/night cycle. Moreover, it might be easier just to leave it on through most of the year. As the temperature warms through May and June, the demands on the heat pump would lessen while there would be far more available solar energy to power it. I can imagine only turning the heating off for two or three months in summer, and if its control system is good enough, maybe never.

Insulation

As an additional but separate job, I'm going to get the installation company to insulate the floor of the back room extension. This room is my office and has three walls to the outside. When it was built, barely 2cm of insulation was added to its roof space and none beneath its floor. Though it has two radiators, it always has a cold feel in winter. Last summer, I cut a hatch in its ceiling and laid 20 or more centimetres of insulation into its narrow roof space. This has helped a little but since the two radiators are to be changed anyway, along with all their pipework, I thought this to be a good opportunity to insulate the floor. Unfortunately, there is inadequate space available below the floor to carry out this work from beneath so I've asked for a quote to lift the room's entire floor in order to add an appropriate layer of insulation between the joists.

The next task is for the installation company to come out and finalise the plans. Additionally, I need to fill out the loan application form that arrived from Home Energy Scotland.

Gas from grass

It can be pointed out that while a hybrid heat pump system ought to hugely reduce my dependence on fossil fuel, it doesn't exactly cut it out completely. This is perfectly true but I see a possibility here. I recently watched a video on YouTube produced by the excellent Robert Llewellyn for his 'fullychargedshow' channel (though it is not released yet). In this, he interviewed Dale Vince, the boss of the environmentally aware energy company Ecotricity. Like other power companies, they allow you to buy both gas and electricity from them but most (95%) of that gas is sourced from fossil reserves.

What Vince wants to do is arrange a series of 'Green Gasmills'. A 5MW example would be supplied with grass grown in 1,200 hectares of surrounding land, much of it from marginal land. Farmers are already well practiced in grass growing for silage production as animal feed. Anaerobic digestion would be used to break down the grass and release methane. The gas is harvested, scrubbed to make it compatible with the national gas grid, and the remains would be returned to the fields to act as natural fertilisers for the next growing cycle. Such a facility would supply enough gas for about 3,500 average homes; more if hybrid heat-pump technology achieves more widespread adoption. Ecotricity hope to install these across the country. And, unlike a fracking site, with its limited lifetime, a Green Gasmill will keep producing gas indefinitely.

The downside for me? Gas and electricity from Ecotricity is more expensive than my current supplier, EDF. This is especially true since I make sure to renew my tariff offer from EDF. With Ecotricity, I would gain a £40 discount for having an electric car and some free access to their motorway-sited electric car chargers; both of which are small beer. Nevertheless, I feel minded to help Ecotricity along by signing up to them.

Renewable heating

So far, in my bid to have a modern twenty-first century lifestyle that's leans towards being sustainable, I've acquired solar panels to supplement my electricity consumption, and I have an electric car in order not to burn any more fossil fuel than I have to while getting about.  Now I want to take the next step because I want to deal with our house's heating.

Here's the picture thus far. We live in an average house built around 1970 when insulation was barely thought about. It was extended in 1990. We have fully insulated the loft and the roof space above an extension room. Our floors are not insulated and, because access to the underfloor space is extremely difficult, the only way this is going to happen is to empty all the downstairs rooms, lift their carpets and then their floorboards. Where we can, we have had the cavity walls insulated.

16.5 years ago, we installed a 24kW Vokera Linea combination boiler and water heater. This burns gas to drive a central heating system of 13 radiators on a hot water circuit; pretty standard stuff in the UK. One of those radiators is permanently switched off. The water heater comes on when it senses demand, burning gas to heat water as required. Because of this, we dispensed with the hot water storage tank that the house had at that time.

The boiler has been annually serviced by British Gas on a Homecare agreement since it was new and it is reasonably reliable. However, I'm not prepared to wait until it fails on me before getting it replaced because that will force me to make that replacement rashly. In the knowledge that the boiler is approaching the wrong end of the 'Bathtub Curve', we have decided to start planning for the changeover now.

For those unaware of the bathtub curve, this is a theoretical curve that plots the reliability of complex mechanical systems as time goes on. At first, there is likely to be a rash of failures. These will mostly be due to manufacturing or installation defects and it is these that the one-year guarantee is meant to catch. It is the steep end of the curve but then we enter a long period of reliable operation. The systems are doing what they were designed to do, within the timeframe they were expected to do it. But as they age, the probability of failure gently begins to rise as parts wear out or extreme circumstances exceed the ability of the system to cope. The shape of the graph is like the cross-section of a bathtub.

In the case of the boiler, heat exchangers are likely to corrode, gas burners may clog up, sensors might fail. Moreover, an old boiler is never as efficient as a new one, given the improvements in boiler design over the years. The Vokera Linea was 77.5% efficient when new. A new Worcester boiler is over 90% efficient.

The big question then is how to upgrade the system. There are lots of options but I'm tending to focus in on two.

Boiler changeover

The easiest solution is simply to swap my old combi boiler for a new condensing one. The Worcester brand seems to have gained a high reputation in this regard. If I do follow that path, then I'll likely follow the advice I've been given to get one that has a 30kW rating instead of my current 24kW. The feeling seems to be that the Vokera is struggling, given the size of the house.
Although a new condensing boiler will reduce the gas we consume for heating, there are solutions that can take this reduction much further. Most seem to be impractical for our house but there is one that might work.

Solar thermal

This consists of an array of evacuated glass tubes mounted on a roof. The tubes house collectors for the Sun's infra-red output and the resulting hot fluid usually heats a storage tank of water. That's as fine as far as it goes but it would mean installing a new storage tank. Further, it would not be capable of heating radiators and, worse still, it works well at the wrong time of year. From May to October, when the Sun is most capable of heating water, I've got no need for it because my central heating system is shut down.

Environmentally sourced heat

The environment around us is brimming with heat energy. If it wasn't, we'd all be frozen blocks at zero Kelvin (-273°C). Even on what we call cold days, there is plenty of heat around us. It is just that it is low-grade warmth. If we could gather this energy and concentrate it, it would provide an inexhaustible source of heat for our homes. Amazingly, we can achieve this by using the same principle of heat energy transport that is used in refrigerators and air conditioning units. These work by having a compressible fluid and a compressor. The fluid flows around a circuit absorbing heat in one place and giving it up in another.

If a gas is compressed, it will heat up. A good example is a bicycle pump with the outlet blocked. As a volume of air, maybe 25cm long, is compressed into a much smaller volume, the end of the pump can become seriously hot. This is because the quantity of heat that allowed that air to be at, say, 15°C ambient temperature has been forced into a much smaller volume. The energy is concentrated and the temperature will rise, maybe enough to scald.

If this hot, compressed gas is allowed to cool to ambient while being held compressed, then what happens when it is allowed to expand out to its original volume? The small quantity of heat energy remaining is spread through the larger volume and as a result, the air's temperature will fall precipitously.

Refrigeration works in the same way. A compressor squeezes a fluid which warms up as the heat within becomes concentrated. That warmth can be given up to the room via a radiator. The fluid is then allowed to expand within the fridge or freezer's interior. It cools markedly and begins to absorb the heat within before being recompressed and giving up that heat to the room. By the same principle, we can capture heat from the inexhaustible supply in our environment, and transport it into our houses. There are three common ways to achieve this.

1. If a body of water, preferably flowing, is to hand, then huge quantities of heat can be extracted from it. Water has an astonishing capacity to store heat energy, even when it is cold. Unfortunately, no such river runs near to our house.

2. Were the land around the house to be blessed with great green lawns, we could install a network of pipes beneath the surface that would be 'warmed' by the low grade heat within the ground. An appropriate fluid can be used to transport or pump that warmth into the home. Unfortunately, we do not possess a suitable amount of land, and anyway, such 'ground source heat pump' systems can be expensive to install.

3. Like the water and the land, the air around us can be used to supply sufficient heat to warm a house. Furthermore, it is easily accessible to an 'air source heat pump'. I'm going to investigate this source of heat as a possibility for our house.

Air source heat pump

On my Facebook feed, I've been seeing adverts from a Glasgow heating company pushing air source heat pumps so I got in contact with them looking for information on boiler replacement. The first guy that answered immediately launched into a sales spiel and I ended up with a quote for a replacement Worcester combi boiler for £3,384, including the replacement of two radiators in the coldish extension room. This seems a little high for me, but I pressed on.

As I began to query him about heat pumps, I learned some interesting stuff: A system would cost in the region of £8,000-plus. However, the UK and Scottish governments are keen to promote this technology to reduce carbon emissions and have a system of 'Renewable Heating Incentives' (RHI) which are paid out quarterly to offset the overall cost. Further, they offer interest-free loans to help with the initial outlay. It was suggested that the two schemes roughly cancel out as the RHI can cover the loan repayments and so an air source heat pump system can be installed for about £2,000. This is promising.

I was then passed to a gentleman who had a much more practical tone instead of the sales hyperbole and he explained the system to me a second time. As best as I can make out, the Daikin system they sell consists of two units. An external unit looks like an air-con right down to the large fan that dominates its volume. The fan draws air through a heat exchanger, cooling the air and warming the refrigerant which is then compressed to concentrate the heat. The extracted heat is then pumped into the house. There, it can be used to warm the central heating circuit. The system also has an internal unit that essentially consists of a compatible combination boiler so that if there isn't enough heat coming from the heat pump, it can be topped up from the burning of gas.

We now have an appointment in a week's time to assess the house and give us an accurate quote for the installation of an air source heat pump system.

Since then, I've gone searching for more information on YouTube and managed to learn a few things that may or may not be relevant. Apparently, because the warmth from the heat pump is at a lower temperature, the house needs larger radiators to get this heat into the rooms. I'll need to see if this applies to the Daikin system or whether the gas can be used to supplement the temperature.

Some of the videos I've watched show an external unit that is rather large – about the height of a man. As we would want to position ours over a narrow path, we will need to see if the Daikin unit is as large and if it can be mounted 2.5 metres above the path to maintain access.

Finally, I'm not sure this is going to be cheaper to run in terms of my fuel bill. While the gas bill drops, the electricity bill must rise. It takes 1kW of electricity to produce 2.5 to 3kW of heat. But electricity is 3 to 4 times more expensive than gas per kilowatt. At least some of the electricity would come from the solar panels.