How to turn a lovley old house with solid walls into a warm efficient home

AIR SOURCE HEAT PUMP - MECHANICAL VENTILATION & HEAT RECOVERY

SPACETHERM INTERNAL WALL INSULATION - UNDERFLOOR HEATING

Choosing a heat pump

 

 

In my 'Initial Research' article I concluded that Air Source Heat Pumps do work and that they can run efficiently if used appropriately, but if they are installed in such a way that the pump has to work its socks off then it would most likely not be an efficient heating system. In short, it would be folly to simply rip out a conventional boiler and replace it with a heat pump without ensuring its suitability. As I had undertaken major works to my home anyway I took all measures I considered feasible to insulate, draught proof and ventilate my home as well as install UFH in most of the building.

Given all the work I had done to build my extension and renovate my home I really felt it would have been a retrograde step to install an oil boiler and tank, so I really was keen to find an alternative. I was skeptical about claims that an ASHP would save me 70% on my heating bill over an oil boiler, but ultimately my reasoning was this: I had to install a whole new heating system of some sort anyway so provided the initial cost of installing an ASHP wasn't excessively more than an oil boiler and tank then an ASHP actually only needed to match the efficiency level of a new oil boiler to make it my preferred option, anything else was a bonus.

I did my heat loss calculations for the house, room by room and concluded that it would be possible to heat my home from a heating system with the low temperature water a heat pump produces. I already had a wood burning stove in one room and planned to install a wood burner in my new lounge. Given that these could be used during really cold periods to assist when an ASHP may not be at its most efficient I felt I could have a fairly flexible system.

I was also aware that there was lots of talk about a new Government scheme offering ongoing payments to ASHP installations (a bit like solar PV). There was no guarantee it would happen at the time I did my installation (it is now in place), but if this scheme came to fruition then I would be much better off with a pump. I decided to consider a heat pump despite my initial reservations and started to look into whether one pump was any better than any other.

 

I invited six companies to come and quote me for a pump. Prices varied widely for pretty much the same setup, and for many I certainly got the impression they were charging whatever they thought they could get away with. The Mitsubishi Ecodan was the most popular recommendation, but like so many electronic good these days when you look into it you find that many of the components are made by a third party and a good deal of the difference is the badge on the front. All of the pumps that were recommended were 'modulating' heat pumps, but when I quizzed the sales guys about this I have to say most seemed clueless – they simply assured me that they were best but could not really tell me why. I understood that a modulating heat pump was one that could modulate its output dependent upon demand (ie) It is not simply on or off but could run at say, 70% capacity. This sounded a great idea – if my house needed just a top up of heat but not lots it made sense that the heat pump should run at half throttle, so to speak. However, I was concerned about how this worked in practice and the complexity involved if every room of my house had to communicate with the pump about its own demands. It turns out that this is not quite what happens, so I delved more.

In principle, a heat pump is a simple replacement for a boiler in that water from your heating system is circulated round and round, gaining heat as it passes through the boiler/heat pump. There are plenty of articles on the web that detail exactly how a heat pump works, so I won’t deal with that here. As with any heating system it is necessary to size the boiler/heat pump to suit the house. I was surprised that of the six companies that visited my property I had recommendations of pumps that ranged from a 7Kw output to a 16Kw output. Straight away this rang alarm bells – some of them must have given me bad advice! Part of the problem arises from the fact that when the mercury drops and we have a really cold spell the house needs more heat than in autumn when it’s just a bit chilly. An oil boiler’s output does not appreciably change with weather conditions – it will still burn the same oil and generate the same amount of heat from it. The boiler has to work harder only because the house is losing its heat more quickly. A heat pump, however, uses the air as its fuel. As the air gets colder it has less available energy for extraction – a heat pump with a rated output of 16Kw, for example, may only be able to produce 12Kw in cold weather. With a heat pump, therefore, we have the double whammy in very cold weather where the heat pump’s output is reduced as well as the house losing heat more quickly. Sizing a heat pump, therefore, is somewhat trickier than sizing a conventional boiler, and this, at least in part, accounts for why I received such varying recommendations.

 

Some of the companies that quoted me decided that the best solution was to consider worst case scenario when it’s freezing outside and ensure they quote me for a heat pump man enough to still heat my entire house under such conditions. Other companies think it is most suitable to quote for a heat pump that will heat the house in all but extreme circumstances and suggest that for really cold spells you have some form of top up heating such as a wood burner. The oversizing route seems to make most sense at first glance. The heat pump will do the job when it’s really cold and because it’s a modulating heat pump it can work at reduced capacity when it’s only chilly out. Unfortunately, it is not necessarily that simple. It’s a mistake to assume that because a pump is ‘modulating’ that means it can run at any capacity from zero to full throttle. Certainly at the time of my research the pumps could only modulate down to about 70% of maximum (things may have changed with new designs). The potential drawback to the oversizing option (as well as larger pumps being more expensive to buy) is that a large pump, even modulated, might be producing more heat than is needed at a particular time. Generally, and with all the quotes I received, the heat pump feeds the central heating directly. Such a system can’t circulate a very small quantity of water, so the heating system design must be such that certain UFH loops or radiators are permanently open. This ensures there is always sufficient water to circulate. Exactly how much has to remain permanently open will be a question of design, but I have encountered situations where 25% of all rads/UFH loops are permanently open. This may or may not be an issue depending on the particular house and how well designed the system is. I have spoken to those, however, where this has become an issue. For example, assume that you have a hallway and that the UFH loops or rads in this area are designated ‘permanently open’. The hallway is perfectly warm, but a bathroom calls for heat from the heat pump. The heat pump fires up, heats the water and pumps it around the system. Inevitably, this heat will be fed to the hallway as well as the bathroom even though the hall is already warm. In severe, cases I have heard of poor designs where certain rooms overheat and the home owners end up opening windows to let heat out of one room whilst the system is still trying to heat up another. This may be an extreme example but it worth understanding how the system works and also illustrates the need for well thought through design. As I have an old house with a new extension I am aware that insulation levels in different parts of the house will vary quite a bit despite all my efforts to improve the whole building. Anyone used to an old house may be familiar with the notion that some rooms get warm quickly and others are a real bugger to heat. Designing which areas of the house should have the permanently open loops could be a bit of nightmare, I reasoned, and would be a pain if I got it wrong. In a modern house where the whole building is of similar insulation value or where a house is very open plan this may be less of an issue, but worth consideration.

Defrost is another issue with heat pumps. As the pump draws warmth from the air passing through it, the evaporator can ice up. Periodically, in cold weather the heat pump needs to stop heating your house and take action to thaw out the evaporator. It does this by back cycling the warm water from your home into the heat pump to de-ice itself. In essence, during the defrost cycle it is pumping cold water into your heating system.

I have no doubt that a system as described can be designed appropriately and work very well. I think this is easier to achieve in newer homes and the client must understand if the pump will potentially require a bit of help in very very cold weather – possibly by using a wood burner in the house.

I believe there are pros and cons, as with most things in life, but I chose a different route. I installed a heat pump that is not modulating, but to overcome the drawbacks of not modulating I have installed a buffer tank. The idea behind the buffer tank that the heat pump no longer cares a jot about the heating system. The heat pump’s job is merely to heat up a large tank of water to a predetermined temperature. When the temperature in this tank falls below another set point the heat pump kicks in and raises the tank temperature once more – and that is all the pump knows about or cares about.
The buffer tank of water is used to circulate around the heating system. No loops or radiators have to be kept open. If a particular room calls for heat then water is pumped from the buffer tank to that room and nowhere else. For me and my house I believe this is an appropriate and simple system.
The less complex non-modulating pumps are cheaper, and potentially there is less to go wrong. Within limits, the concerns of accurate sizing of the heat pump are also far less critical because it does not feed directly to the heating system of the house.

A non-modulating heat pump needs to defrost just the same as a modulating pump. However, it will draw some water from the buffer tank to defrost but will not be drawing water from out of your heating system. In essence, the heating system can still be doing its job whilst the defrost is in operation.

 

No system is without its drawbacks, however. Most obviously you will need to find somewhere to site a dirty great big buffer tank in addition to your domestic hot water tank (more on this later). Mine is 200 Litres and is built into the same unit as the DHW tank, one sitting above the other, so the footprint it takes up is only slightly larger than a standard DHW tank – it’s just taller. In modern, smaller homes this is often an issue that makes the idea of a modulating heat pump without a buffer tank a very attractive idea. Fortunately, I did not have much of an issue siting the tank.

Short cycling is a concern with heat pumps. This is when the pump switches on and off too frequently. If you can imagine a non-modulating pump feeding directly into a heating system that is not needing much heat the pump would reach temp and cut out very quickly, the room would cool again and soon the heat pump would be firing back up. This short cycling is inefficient and reduces the life of the heat pump’s compressor. An appropriately sized buffer tank should overcome this, but of course you need to find room to stick a tank of water.

I the end, having chosen to buy a heat pump with buffer tank I did my research and following a visit to Husky Heat Pumps in Southport, I decided to buy their 16kw pump and tank. I did a deal to fit the system myself, leaving them to simply come to commission the system once installed and check my installation was ok. This saved me a good deal of money, and I beleive I installed the system for little more than it would have cost me to install a modern oil boiler.

 

 

© Christopher Thompson