Electric wet central heating options?

dekh

I can't fathom how these air heat exchangers work.

If it's minus 2 outside at the back of the house (which is south facing) how does it raise the front room up to 18 degrees?

paceinternet

How does your refridgerator stay at 5 degC inside when it is 20 degC in the room - think of the heat pump reversing this process.
http://www.heatpumpcentre.org/About_heat_pumps/HP_technology.asp

By the way, most heat pumps, and particularly air source, perform differently depending on outside air temperature and the required water temperature for the heating system.
At + 20 degC outside and a water temperature setting of 35 degC, they might have a COP of 4+.
At -10 degC air temperature and water temperature setting of 50 or 60 degC the COP will fall to maybe 2.
Change either air or water temperature, and the COP will be somewhere in between. So it isn't good for people to post numbers without some parameters for those numbers. Also beware, some figures show water temperature into the heat pump, and others show it out from the heat pump.
However, over an annual range of air temperatures, some quote a "seasonal COP rating of 3.6".
Mitsubishi Ecodan shows a COP of about 2 at A-12/Wout45. COP 3 at A1/Wout 45, COP 4 at A8/Wout 45, and COP 5 at A 17/Wout 45.

Just take care with anyone who doesn't take all this into account when specifying a solution. The right specification will provide a very efficient and eco friendly answer for most people.

dekh

paceinternet wrote: »

How does your refridgerator stay at 5 degC inside when it is 20 degC in the room - think of the heat pump reversing this process.
http://www.heatpumpcentre.org/About_heat_pumps/HP_technology.asp

A motor and compressor squishes HFC (assuming it's a new fridge) until it turns into a liquid. The liguid then hits a tiny hole that it has to squeeze through. The other side of the hole is low pressure, the liquid turns into gas and gets very cold in the process of vaporising. It then takes heat energy from the surrounding area (the fridge) to warm back up. Electricity is used to drive the liquid round the system.

Something like that anyway.

Still don't understand how these heat exchangers work. Talk of COP and all that is just nonsense to me.

If the ambient temperature is -2oC how much electricity is required to raise the temperature in a 27m2 room from -2 to 18?

Cardew

dekh wrote: »

Talk of COP and all that is just nonsense to me.

If the ambient temperature is -2oC how much electricity is required to raise the temperature in a 27m2 room from -2 to 18?

COP(coefficient of performance) is simply a multiplication factor.

If you have, say, a COP = 3, then for any given consumption of electricity you get three times the output in heat.

e.g. Your heat pump uses 5kWh of electricity and you get an amount of heat that would take 15kWh to produce using normal electrical heating.

Given the question asked about a 27m2 room it is impossible to give a specific answer as it would depend on the insulation and losses of that room.

However the simple answer is that however much electricity using normal electrical heaters it took to raise the temperature of that room from -2C to 18C, a Heat pump system with a COP of 3 would use one third of that amount of electricity.

dekh

OK, I understand the number COP now seems quite straight forward. I take it the higher the COP the better?

What is the typical COP for systems operating at -2C?

How much to buy and install such a system?

Cardew

dekh wrote: »

OK, I understand the number COP now seems quite straight forward. I take it the higher the COP the better?

What is the typical COP for systems operating at -2C?

How much to buy and install such a system?

Yes the higher the COP the better.

There are 2 types of heat pump. Ground Source Heat Pump(GSHP) and Air Source(ASHP) The GSHP is very expensive to install.

There are 2 types of ASHP, warm air and wet(with radiators.)

Costs depend on far too many factors to give a figure. Could be between £3k and £20k

You should read this thread for more information. Samtheman has a wet system and in particular posts objectively on the pro's and cons.

http://forums.moneysavingexpert.com/showthread.html?t=1464827

paceinternet

As has been said, you would need to do a proper assessment of the heat losses in the room, but as guide, a room of 27m2 with average losses might need 100 watts per m2. So, 2700 watts, so size at 3kw.
But depends what radiators or underfloor, or fan convector you have to put the heat into the room. Because, the lower the temperature of the water, the bigger these parts will need to be. And the lower the temperature of the water, the more efficient the heat pump will be.
As an example, at -2 degC:
water return of 35 and out of 40 could have a COP of 2.9
water return of 40 and out of 45 could have a COP of 2.7
water return of 45 and out of 50 could have a COP of 2.1

Remember that a conventional fuel fired boiler might be running at 70+ degC water temperature out, so radiators often need to be bigger for a heat pump system.
If your room target temperature is 20 degC and heating water temperature is 70 degC, this is a temperature difference of 50. Most radiator outputs are quoted at this.
If you ran the heat pump at water out of 50 degC, the radiator size would need to be based on a difference of (50-20) 30 degC, which means a factor of 0.515 needs applying to the radiator output. So 2.7kw for your 27m2 room would need radiators of about 5.4kw std output.
Running the heat pump at water out of 40 degC ........... radiator factor of 0.304 is needed, so a radiator size of about 9kw standard output.

dekh

More numbers than I can comfortably take in, but you are helping me get there.

One of the posts in the air source heat pump thread gave figures.

£1.50 to £2 to run the system 24/7. £730 per annum.

Compared to a quarterly gas bill of £200. £800 per annum.

Assuming there are no other gas appliances like cooker.

Then I don't get it.

Ok add on annual safety check and service to gas CH.

I still don't get it.

What's the typical cost of one of these systems? Dirt cheap? Simple DIY install?

10 year warranty?

I'm just very dubious about being an early adopter. By comparison...

Amazon have a De'Longhi Dragon 3 TRD0820ER Oil Filled Radiator, 2kW.

It costs £113.99 to buy, nothing to install, nothing to maintain, has a 10 year guarentee and the customer reviews state that it heats up large rooms easily.

Reviews are also contradictory about noise, some say very noisy some very quiet - hey ho.

So if we go for a COP of 4, (so I like easy maths), and both systems are heating the same room (not at the same time!) how many months to hit break even point with the Air source heat exchange system over the cheap rad?

Saving 1.5Kw/Hour?

Running it 24/7 for 6 months of the year would cost £227 for the ASHE and £597 for the radiator. (Based on 1000KwH at 22.7p/KwH and remainder at 10.8p/KwH.)

Saving £365 a year for heavy usage.

With a COP of 2 obviously you get much less saving.

I just don't see it. Then again my math is probably terrible

Thank you for helping me to understand this at the moment I see it as little better than the solar water systems which are just a big con.

paceinternet

Yes, your maths is terrible (or mine is).
24/7 * 26 weeks = 4368 hrs
2 kw for those hrs = 8736 kwh
Not sure about your tariff, but to use your numbers:
1000 * 0.227 = £227
7736 * 0.108 = £835
Total = £1062
As said above, average COP will depend on temperature, which is unlikely to average -2 in the UK, but
At COP 2 save £531
At COP 3 save £708

If you are really only interested in heating one room, then an air to air system may be best if you can accept a fan type heater in the room. Cost for a 3kw unit installed may be less than £1000, so payback could be not much more than a year.
You would give more consideration to the air to water heat pump if you were heating more rooms and your hot water.
If you move forward on this, do talk to at least 3 suppliers to get their different solutions.

Cardew

As paceinternet indicates, most people will look at an ASHP to provide a complete CH/HW system for a property.

Whilst ASHP running costs are comparable with Gas CH, IMO it would not make sense to replace an existing gas CH system with an ASHP - particularly if you have a gas combi.

As well as the cost of the heat pump, for a wet system you would need to have a HW tank fitted and upgrade your radiators. So you are talking £thousands.