Air Source Heat Pumps

Cardew

giscool wrote: »

Thanks Cardew I have taken off my green tinted spectacles and green blinkers now.

If you think that COP of 1.8 is normal for these systems and not just suppliers learning to cover themselves then ASHP must be a dead duck for retrofit. But I cant bring myself to even contemplate a new oil CH system.

You raise the issue of return on investment but dont you have to factor in that interest rates are 3-4%, inflation about the same but electricity price increases are 15%.

A few years ago it was put out that electric prices were set to double in the next five years. Doubting this claim I looked at my electricity bills for the previous five years and to my surprise they had doubled in that time = 15% pa. There is no reason to suppose they will not continue to rise if not even faster with someone having to pay for the 43p/ kw FIT. So isnt any investment in energy saving better than sticking it in a bank - by about 10% pa?

The overall COP of a system should be better than 1.8, but it depends on the house. You need to read the EST year long trial report on 29 ASHP retrofit systems - link in the 'Green' forum - to see that a COP 1.8 is not out of the ordinary; and indeed several got less.

You say you couldn't bring yourself to contemplate Oil fired CH, but give no reasons??

I agree with you about investment in energy saving but would ASHPs be a great improvement on your present Storage heating set-up with your hot water also heated by E7?

Cardew

albyota wrote: »

Urmm, if using UFH at lower temps means a heat pump is at its best efficiencies, it is because the low temperature water flowing through the pipes 70mm below the screed is a very very large radiator....placed where your feet and body are closest to....on the floor, unlike a standard radiator which due to heat rising....heats from the ceiling down, hot head......cold feet, fan convector type radiators blow the low grade heat across the floor to where you are..... before rising to the ceiling.....so using the correct type of fan convectors means.....you can run the heat pump at lower temperatures, thus increasing the COP.

Albyota, It is not in dispute that UFH is best suited for ASHPs.

However with fan assisted radiators you might have the better initial distribution of heat when switching on, but natural convection will mean that the heat distribution in the room will quickly equalize.

Or a £10 fan blowing across a normal radiator will achieve the same effect

albyota

But the OP has no radiators yet for a £10 fan to blow across.....

giscool

Cardew wrote: »

You say you couldn't bring yourself to contemplate Oil fired CH, but give no reasons??

Firstly if oil is 6p/kW I am paying 4.5p for my stored electricity and at worst allowing for peak rate top ups I am paying 6p there is no advantage and saving to pay for the invesment.

Also in the 21st century I beleive it is unacceptable to increase the demand fo oil. Yes part of my heating electic is oil based but getting less.

If you believe as I do that technology is advancing at an exponential rate (check out Ray Kurzweil and the Singularity) the magntitude of change that has taken place in the past 100 years could happenin he next ten and so on, the price/performance of PV panels (and other renewable technology) will mean that soon it will make sense to put them on my east/west facing roof or something else.

So its a NO to oil for me.

zeupater

albyota wrote: »

.... 14kW Ecodan with 250L DHW cylinder average cost £8,660 + 5% vat fully fitted, 15 radiators c/w TRV's between £2,500 - £3,000 depending on quantity and size.

Hi

Considering that a number of ASHP manufacturers claim that their units are easier to install than a gas boiler, why is it that installing a gas boiler costs less than £1000 (excl boiler cost) and installing ASHPs costs over £4000 ? ..... :think::whistle:

Z

Cardew

jeepjunkie wrote: »

Hi Cardew,

Fan convectors typically have a about a litre of water in them whereas a convential rad will have considerably more especially if oversized. As there is much less water mass to heat with the lower grade heat this allows them to heat up very quickly with the advantage of being able to heat large rooms effectively. Some of our rooms have two purely for btu room sizing. This is achieved at similar temps to UFH. It's my understanding that rads cannot work at temps as low as 25-30c. The higher the temp you ask from the ashp the lower the cop.

The fans don't run all the time as once rooms are up to temp the heat pump shuts down only to come on periodicly as there is much smaller mass of water to heat.

Best regards

Hi,

Well extending your argument, if the Fan Convector only contained, say 2cc(0.002 of a litre) of water. Then there would be even 'less water mass to heat up' so it would be even more efficient??;)

Can you not see the flaw in your reasoning?

paceinternet

Beware of not specifying this correctly. It needs to recognise the flow water temperature you expect to use.
ASHP efficiency will be lower if you expect to use higher temperatures.
The radiator suppliers quote their outputs at T50, meaning a 50 degC difference between room temperature and the flow water temperature. If room was expected to be at 20 degC, then flow water would need to be 50 degC higher, meaning 70 degC. (To be strictly correct they talk about "mean water temperature").
www.stelrad.com show delta T50 degC as having radiator output factor of 1.0.

Reduce the mean water temperature to 50 degC and you have a delta T of 50-20 which is 30, so the output factor is 0.515. So you would need a radiator double the size.

Run the ASHP at 35 degC output and the delta T is only 35-20 = 15, which has a radiator factor of 0.209. So you would need a radiator about 5 times the normal rated out put for your room.

Cardew

jeepjunkie wrote: »

Now your being silly

But your right it is a fine balance to get the most out of an ASHP.

Cheers

Actually not being silly, but serious - your explanation is flawed!

I am not disputing the merits of fan convectors, but as I said above:

If your room needs xxxx BTu, you size radiators to achieve that output. That can be with big radiators, UFH or Fan assisted radiators.

To achieve that heat output, at any specific water temperature, your ASHP will be producing exactly the same xxkW output.

All they do is, in effect, increase heat dissipation such that a smaller radiator will produce the same output(heat/BTu) as a larger radiator.

That indeed is exactly the principle of Under Floor Heating(UFH) - it is in effect a very large radiator holding a lot more water than your fan heater's 1 litre; and I think we are all in agreement that UFH is ideally suited to work with a heat pump.

If you argue that Fan convectors increase efficiency because they only hold little water, where does that leave you with UFH that holds lots of water?

albyota

Cardew wrote: »

That indeed is exactly the principle of Under Floor Heating(UFH) - it is in effect a very large radiator holding a lot more water than your fan heater's 1 litre; and I think we are all in agreement that UFH is ideally suited to work with a heat pump.

Although UFH is designed to be at a low temperature and has lots of water, the fact is, that once that heat has heated you, about 1.5 metres off the floor, its job is done, it does not need to be heating the ceiling, we do not live up there.

Cardew wrote: »

If you argue that Fan convectors increase efficiency because they only hold little water, where does that leave you with UFH that holds lots of water

A fan convector can still heat a room sufficiently using lower temperature water 35-40 degrees, due to the air being heated and blown directly towards you, where you are sitting or certainly low level across the floor, same principle as air conditioning systems, (air to air heat pumps in heating mode, air on = 25 air off = 40), or a cars heater matrix / climate control / A/C.
Whereas, a standard radiator, (usually under a window), and due to its size has to be much hotter to radiate the heat into the room unaided, a lot of this heat will rise from about a metre away and heat the ceiling first before dropping down towards your head, hence a typical 60-70 degree boiler.

Cardew

albyota wrote: »

Although UFH is designed to be at a low temperature and has lots of water, the fact is, that once that heat has heated you, about 1.5 metres off the floor, its job is done, it does not need to be heating the ceiling, we do not live up there.




A fan convector can still heat a room sufficiently using lower temperature water 35-40 degrees, due to the air being heated and blown directly towards you, where you are sitting or certainly low level across the floor, same principle as air conditioning systems, (air to air heat pumps in heating mode, air on = 25 air off = 40), or a cars heater matrix / climate control / A/C.
Whereas, a standard radiator, (usually under a window), and due to its size has to be much hotter to radiate the heat into the room unaided, a lot of this heat will rise from about a metre away and heat the ceiling first before dropping down towards your head, hence a typical 60-70 degree boiler.

Whoaa - come on albyota read the posts!

Who is talking about 'standard radiators' - certainly not myself or jeepjunkie. This was his statement I was disputing.

For even massively oversized rads to work you are going to drive your ashp very hard so your cop will be rubbish therefore expensive to run and in winter as your ashp is going to have quite a job to lift the temp to something useable if at all which could cause the unit to ice up frequently.

Also I am disputing the explanation that the small quantity of water in the Fan convector is the key to higher efficiency.

It is blindingly obvious that a standard radiator cannot dissipate enough heat at water temperatures where an ASHP is most efficient.

That is why I specifically stated:

If your room needs xxxx BTu, you size radiators to achieve that output. That can be with big radiators, UFH or Fan assisted radiators.

Clearly that will mean something like a big triple radiator(or UFH;)) to dissipate the same amount of heat(BTu) as a much smaller Fan convector radiator.

As you concede, there is lots of water in UFH, as there is in a 'Massively Oversized' radiator. So the amount of water is not a factor in lowering the efficiency of the heat pump.

Put simply, if a fan convector radiator, UFH or a 'massively oversized radiator' are all dissipating xxxxBTu(at the same water temperature) then their demand on the ASHP will be the same.

While we are at it, there is nothing any of us can do to stop warm air rising, regardless of that heat coming from any form of heater -including UFH.

All warm air in a room is subject to convection currents and much as we might not want the heat to rise to the ceiling(and stay 1.5 metres off the floor) the laws of Physics still stand.

http://paul-a-heckert.suite101.com/understanding-convection-currents-a63549

<H3 class=dynamic>What Causes Convection Currents

Imagine a cool room with a radiator at one end and no fans or any other forced air systems to blow the warm air to the other side of the room. How does the radiator heat the entire room? The key is convection currents. The hot radiator sets up convection currents that transfer thermal energy to the rest of the room and eventually heat the entire room.

How do convection currents work?
The hot radiator warms the air that is closest to the radiator. The warm air expands, becomes less dense and rises to the top of the room. When the air reaches the top of the room it is pushed sideways towards the far wall by the more recently warmed air rising from the radiator below. In this way warm air moves to the other side of the room. Once on the other side of the room the air drops down both because it has cooled a little and because the air behind it continues to push on it. The air then continues to circulate back to the radiator and repeat the process.

</H3>