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.