Peter wrote:
Sure, however you are recovering low grade heat (max ~ +20C) so you need another heat pump to recover that.
Of course you can’t heat the house in that way. The point is reducing losses caused by ventilation
Graham asked:
So is there any point in putting panels on my roof? Or just a massive waste of money?
It depends on the wholesale electricity market, which is subject to government/political interference (banning onshore fracking, restricting North Sea licences, closing coal-fired and nuclear power stations, “windfall” taxes, “green” levies, and generally dancing to the tune of Russia’s useful idiots like Thunberg and her disciples). Putting all of our eggs in a basket of “low carbon” gas held by a dictator who has used chemical weapons against us – what could possibly go wrong?
But however bad the effect of government interference on any human activity, there will always be people who argue that if regulation isn’t working we obviously need more of it.
If, on the other hand, you subscribe to the view that however much of a mess successive governments have made there’s nothing that a new government can’t make worse, then there may be merit in generating, storing and using energy independently of the grid, almost regardless of capital cost.
In aviation terms, the cost is not outrageous. A hybrid energy storage system with 10-15 kVA pass-through inverter and 15 kVA diesel generator (which may need to run for less than an hour a day) costs about £15k for the average house and effectively relegates the grid AC supply to backup status.
gallois wrote:
Insulation is one of the best ways to cut your energy bill. However you should not over insulate. You may have noticed lots of adds during Covid about opening the windows to let out the bad air and let in fresh air.
Actually the german building codes assume an even higher rate, IIRC it’s five times a day. Nobody does this, of course.
I don’t understand what you mean by “overinsulation” Basically the best thing you can do is insulate your house. Unfortunately the whole thing is no longer at an engineer level of competence but full of snake oil salesmen and people with alternative facts. Opposed to conventional wisdom airing your house doesn’t lose much energy, because air doesn’t store much energy – it’s in the high mass concrete and brickwork. That’s the reason the house gets warm again, quickly, when you shut the windows. Compare this to a cold house where you turn on the heating for the first time. What seriously wastes energy is keeping your windows tilted.
Another simple example for the issues that plague thermal insulation. You have an old house and you want new windows. You ask your local window guy, he makes an offer, there you are, triple glazed windows. Suddenly you find condensation and mold on the frame.
What happened? Well, your windows used to be the worst part of the insulation and suddenly, it’s the best. What remains is the mediocre to poor brickwork right where the frame connects to the wall in the window reveal. This is now the coldest point, with the windows now being much more windtight than before also helping condensation along.
Who is at fault? At a guess 90% of the window salesmen/craftsmen have no clue. The remaining 10% could tell you that you also need to insulate the window reveal, either outside, which could lead to smaller windows and/or plastering work (you suddenly need someone else, because the glazier doesn’t do this) or from the inside (which needs some kind of painter AND opens Pandora’s box of interior insulation). If he did that, he probably won’t get the job. So he keeps his mouth shut.
I believe it’s the buyer who shouldn’t have asked a glazier for new windows but an engineer on how to improve thermal insulation. One can only hope that he has a clue.
PS: Also Air Heat Pumps don’t work at very low temperatures, at some point they activate a simple resistor, like an immersion heater. On average, days where the outside temperatures are so low that this is necessary, are quite rare.
Air Heat Pumps don’t work at very low temperatures, at some point they activate a simple resistor, like an immersion heater. On average, days where the outside temperatures are so low that this is necessary, are quite rare.
Air sourced heat pumps go into reverse cycle (usually it is not a simple heater there) whenever the evaporator ices up, which doesn’t need a low OAT. It is exactly like carb icing. If the OAT is say +20C, and the heat pump exhaust is 21C cooler than OAT, the evaporator will ice up. I see this all the time, typically at OATs like +15C or lower, and there is no solution other than a much more powerful fan which then reduces the delta T. But only a bit.
You guys have to get yourselves real heat pumps 
Seriously. It’s a bit like complaining that cars can’t be used in cold weather because the heater in the 1964 VW Beetle “doesn’t really work”
The Mitsubishi Kaiteki is one of the best selling air to air heat pumps in Norway.
SCOP: 5.2
COP at -25 deg C : 2.0
Energy “marks?” : A+++
I got a much older model, but it for sure works just fine at -20 deg C and anything above. COP of 1.5 at -25
Is that a warm-air blowing heater? Does it sit on the inside of an external wall, exchanging with the air outside and heating a single room? What do they cost?
When we refer to air-source heat pumps, at least in the UK, we mean units that sit on an outside wall (or the patio) and heat a buffer tank which then pushes warm water around a traditional central heating system of radiators and pipework. The one unit does the whole house. Reports of really poor COP, nothing like what the manufacturers/installers claim, are common. Also common is the house never gets really warm because the central heating system was sized for radiators at 80 degrees C. The installers usually persuade customers to add a few radiators or change to larger ones, but this is not always practical and sometimes it’s the pipework and/or pumping that makes a lower temperature system impractical.
My CH system was sized for 80 degrees C flow. When we bought the house some of the radiators got hot but the house didn’t and the boiler kept short cycling. The boiler never condensed either, so consumption was excessive. It took me a bit of research and experimentation to work out that the boiler (LPG, 30kW) couldn’t actually modulate its output down low enough (minimum output approx 12kW) to prevent the CH flow temperature exceeding the max safe temp (90 or so) so that was why it kept cutting out. It was such a big boiler heating and pumping (relatively slowly) such a small volume of water, so it’s obvious when you think about it. I replaced most of the old single panel radiators with double panel ones, probably almost doubling the capacity of the system. Then I turned up the pump setting on the boiler so it pushed the hot water around more quickly and got the whole system warm rather than just the closest parts of it. Result is the boiler brings the system up to temperature rapidly then hums away on minimum output for as long as the room stat demands it, condensing all the time and with no short-cycling. Gas consumption is down, rads are evenly warm and house is much warmer. The idiots that chose the boiler (Worcester Bosch combi) had doubtless followed the new housebuilders rule of thumb, basing it on perceived hot water demand for a large-ish house running two pumped showers and the kitchen hot tap all at once. Problem is, the boiler is only for heating and doesn’t provide hot water – the hot water circuit of the boiler isn’t connected to anything, so we don’t need anything like that power. 20kW would have been more than enough.
Radiators sized for 80C water is a fairly aggressive cost reduction tactic You also need a big boiler to achieve that. Very wasteful.
A heating boiler should do HW as well. That provides you with hw at a small cost, and negligible when the heating is used.
Can anyone work out how the above Mitsubishi heat pump avoids evaporator icing?
Peter wrote:
Can anyone work out how the above Mitsubishi heat pump avoids evaporator icing?
They all have automatic deicing function. Short bursts of running in reverse I would guess, combined with the fan. Around here it is important to mount them high up (over the snow) and have a roof over it so it won’t snow down. That’s the usual problem for icing. If it is snowed down, it wont get rid of the ice easily, or at all. If it still ices down, the only solution is to run it it cooling mode until it’s free of ice. Icing has never happened to me (yet)
OK; that is what the ones I know do. They can hardly do anything else. It goes into reverse cycle for a minute or two. But the manufacturers quote the COP without reverse cycling, so the effect of this depends on how dry the air is.
Also at temps below about -15C you don’t get much icing.
Same as aircraft really. It is basic physics; not a lot one can do about it.
Deicing the heat exchanger is a normal feature. Yes, air sourced heat pumps don’t work at an optimum when it’s cold outside. And even the calculations the engineer does do not result in realistic power consumption values as he uses average climate, which in turn – at least in Germany – is influenced by politics. Because, as the politicians see it, it would be unfair that someone in a part of Germany where it is, on average, warmer than in another part should be required to have less insulation. I kid you not. Then add mulitpliers for the source of energy – use district heating and suddenly your old factory is highly recommended energy efficient.
The issue is, all things being dumbed down to a single number that is influenced by vested interest of multiple partys won’t result in expected performance.
So, if you purchase an air sourced heat pump and run it with 80°C flow temperature, you’ve already done the worst thing. That’s what I mean by talking to an engineer instead of talking to the guy who wants to sell you the device. Just touching one part of the system won’t work, you need the low flow temperature, the corresponding radiators and a decent insulation, too. Just looking at the COP and then calculating the expected power consumption from your former oil consumption is bound to fail and will lead to frustration.
