Air Source Heat Pumps

thechippy

richardc1983 wrote: »

Yeh but steves system is lower rated in cooling but provides more heat than my unit that is rated at 3.5kw in heating!!!

Its almost twice as efficient as mine!

That's simply a difference in the overall design and quality of the systems.
The mitsi's have probably got a much more efficient condenser and evaporator.

thechippy

mech wrote: »

I think 2.3 is the carbon emission break-even point, not the running cost break-even point. Not sure where -3 degrees comes into it unless you get a COP of 2.3 at -3 degrees.

My electricity costs just over 3 times as much per unit as my gas. 3 times 90% (minimum A rated boiler efficiency) gives a COP of 2.7 (ignoring DHW provision).

I'm fairly certain it's running cost (but could be wrong!) When the outside ambient is less than -3c, the cop of the ashp is reduced to the point when the condensing boiler becomes the cheaper to run. However, this is largely irrelevant in the UK climate, so the ashp is always on average going to be the cheaper form of heating.

Cardew

thechippy wrote: »

I'm fairly certain it's running cost (but could be wrong!) When the outside ambient is less than -3c, the cop of the ashp is reduced to the point when the condensing boiler becomes the cheaper to run. However, this is largely irrelevant in the UK climate, so the ashp is always on average going to be the cheaper form of heating.

That might well be true - but it might not.

You have to know the price of gas, price of electricity, efficiency of the gas boiler..

Then of course you have to factor in the cost of heating hot water 365 days a year.

However as I understand it, the quoted COP at various temperatures does not take into account the electricity required for defrosting.

The amount of ice formed is surely dependant on the humidity of the air. It would not be possible to calculate this and factor it in to the figures.

I repeat, that is as I understand it, and can find nothing on-line to confirm this latter point; although I have read a couple of times that the energy taken by defrosting at low temperatures "kills the COP figures"

Anyone any definitive information on this issue? - not opinions!

thechippy

@ Cardew,

As you say, the amount of ice formed and frequency of defrosts is in relation to humidity and outside ambient.

I don't see how the defrost cycles will kill the cop figures. The system will reverse cycle for quite a short time to melt any ice build up. This is controlled by sensors and only happens when necessary. When the outside ambient reaches a certain point (can't remember what) the defrost cycles become non existant.

Hermann

Cardew wrote: »

You have to know the price of gas, price of electricity, efficiency of the gas boiler..

You also have to factor in the cost of maintenance and servicing, likely to be much higher with Gas than heat pump, factor in the high failure rates of gas boilers (anecdotally at least, difficult to find reliable figures) which means expensive monthly contracts or pay for a new boiler long before its 'payback' time.

You may also want to take into account the Governments love affair with gas boilers and the tax revenue possibilities they provide.

richardc1983

thechippy wrote: »

That's simply a difference in the overall design and quality of the systems.
The mitsi's have probably got a much more efficient condenser and evaporator.

Yeh thats what you get for top market systems more efficiency so its worth it.

@ Cardew... defrost cycle uses less electricity than the actual heating cycle, it just runs in reverse, mine uses about 300watts for a 5minute defrost cycle.

thechippy

Ok,

I've got my gas and elec prices from my latest bills.
Elec 12.51p pkwh
Gas 3.51p pkwh

Assume a 90% efficiency of an A rated condensing gas boiler.

0.12
0.035
x 0.9
= 3.09
Therefore, the break even point for running costs of an A rated combi to an ASHP is a cop of 3.09. On an average UK day, most ASHP's will have a cop of 3.5+ , going up to 5.5. Depending on the make of the ASHP, the break even outside temp will be 0c or less. So, under general UK conditions an ASHP will always be cheaper overall to heat your home than gch.

Cardew

Hermann wrote: »

You also have to factor in the cost of maintenance and servicing, likely to be much higher with Gas than heat pump, factor in the high failure rates of gas boilers (anecdotally at least, difficult to find reliable figures) which means expensive monthly contracts or pay for a new boiler long before its 'payback' time.

You may also want to take into account the Governments love affair with gas boilers and the tax revenue possibilities they provide.

I assume you have not read all the posts on ASHPs?

I think most of us are fans of heat pumps and the points you make in your first paragraph are valid and have been covered in previous posts.

What some of us are trying to determine is realistic overall efficiencies of ASHP systems and installation costs.

Additionally for some of us air to air ASHPs would not be acceptable - I want a discrete heat source in all my rooms - not have doors to bedrooms and bathrooms left open.

This means an air to water system and existing radiators are not suitable as the water does not get hot enough.

The Mitsubishi Eco dan, which I understand is one of the leading systems, claims a COP of "up to" 3.5. Normally if a manufacturer includes the phrase 'up to' it is a fair bet that it will be considerably less in the real world.

The problem is that there is no reliable data available.

I don't understand your point on tax.Why is there more scope for taxing gas rather than electricity?

richardc1983

Can you guys stop talking for tonight now am trying to watch tv and relax lol and keep getting emails popping up about new replies lol! And out of curiosity have to check back to see what the reply is!

Its saturday night lol!

thechippy

Cardew wrote: »

I assume you have not read all the posts on ASHPs?

I think most of us are fans of heat pumps and the points you make in your first paragraph are valid and have been covered in previous posts.

What some of us are trying to determine is realistic overall efficiencies of ASHP systems and installation costs.

Additionally for some of us air to air ASHPs would not be acceptable - I want a discrete heat source in all my rooms - not have doors to bedrooms and bathrooms left open.

This means an air to water system and existing radiators are not suitable as the water does not get hot enough.

The Mitsubishi Eco dan, which I understand is one of the leading systems, claims a COP of "up to" 3.5. Normally if a manufacturer includes the phrase 'up to' it is a fair bet that it will be considerably less in the real world.

The problem is that there is no reliable data available.

I don't understand your point on tax.Why is there more scope for taxing gas rather than electricity?

I agree with most of this. I install these on a daily basis, but decided against in my own home when my boiler recently failed. My concern was hot water. You can install a gas powered on demand dhw, but our home did not lend itself to one.
All homes / layouts are different and ashp's are not suited to all.

I also agree with not having to leave doors open etc for heat distribution. However, this can be overcome by using multi splits and having an ahu in each room.

I'm not keen on air to water systems. Rads need to be changed, as the existing will be undersized and often the dhw temp is boosted by a 3kw inline heater. It's an expensive system and you're better off using an A rated condensing boiler.

Re; servicing.
The ahu filters can be user cleaned, BUT contrary to popular belief, just brushing the condensers is not good enough. They have a very close fin spacing and often have secondary fins. These need to be foam cleaned by a suitably qualified engineer twice a year. Cost about £40 per visit.
Reliabilty is certainly better than most combi's and service life is the same if not better.