Ground source or air source heat pumps?

[Karl Hofmann]

I have a customer who has recently renovated a large house. He has under floor heating, solar panels for his domestic hot water and a Worcester 40kW condensing oil boiler. Having considered his oil prices, he has now decided to supplement his heating system with some form of heat pump, either air source or ground source. My dilemma is do we fit air source which is cheaper to install but stands to lose efficiency to a higher degree when he needs it most, or go for the more expensive ground source which should lose less efficiency in the Winter especially in his very large field (About 6 times the footprint of his house) with a high water table.
He would like to use the heat pump to heat as much of his home as possible and use the oil boiler as a back up.

Can anyone advise on a good quality GSHP system, what kind of COP I could expect to see in the real world (I have seen 5 and 6 bandied about but find this hard to believe). I have always skilfully avoided Ground source as I hate mud but feel that this could well be a worthwhile project..

Advise and suggestions are welcome

[nike123]

Check this Viessmann brochure.

[Karl Hofmann]

Thanks for that Nike... I really never gave Veissmann a thought but their stuff should be really good if their Boilers are anything to go by. I also had a look at NIBE of Sweden, they have been recommended to me also. The guy has a lot of land and for the more stable ground temperatures and higher COP I'm inclined to steer him in the direction of groundsource unless anyone knows of any really efficient air to water gear just about to come on to the market.. I do seem to remember a nifty little animation of Daikins Ecocute system a little while ago but that seems to still be on test in Japan

[Peter_1]

.... what kind of COP I could expect to see in the real world (I have seen 5 and 6 bandied about but find this hard to believe).

5 is possible but 6???
You already thought on a DX heatpump like the system of Nordic, Waterfurnace?
This will give you the highest COP.

[Abby Normal]

40 kW input 32 kW output is a fair sized residential heating system and would easily handle a 600 square meter home in Canada up against -30C outdoor temperatures.

You need to make sure on the size of the load. Quite often, money is best spent insulating and sealing up a home.

The heat output of air source equipment falls off as the ambinet temperatures fall below freezing, while the ground source unit has temperatures 'more constant' to extract heat from.

COP is going to depend on the temperature of the heat source. Maybe if the temp source in near 0C you will get COP of 3, at 5C maybe COP of 4, 10C maybe more.

[chemi-cool]

ground temp below 1 meter do not change.
It does get drier though when you run the HP in cooling mode.

[Karl Hofmann]

Abby, The house is big.. real big. It is a Victorian farm house that has been incorporated in to the existing barns and had another wing built on to it for his offices and workshops, most of which have 3 floors.. I've not even had chance to measure it out yet, but just by looking at it and from its construction would estimate its heating requirements to be well over 30kW (Not forgetting the 6kW hot water requirement for adequate DHW recovery time). Insulation was brought up to spec last year whilst under reconstruction and it's heating requirements were calculated by Nu Heat when they designed the underfloor heating so at the moment I have no reason to feel that they are anything but correct.

I hate working on this house because there is just so much damned walking.

Chemi there will be no cooling as the heating pipes are already set in to the floors and clad in marble, the prospect of condensation on polished marble is a scary one but the ground is nice a moist as the water table is quite high.

Peter, From what I can make out, most GSHPs seem to have a COP of around 4.3 but the Waterfurnace unit bucks the trend for some reason but we shall have a closer look at these, prices from the US should be favourable at the moment, thanks for pointing these out to me

[Abby Normal]

ground temp below 1 meter do not change.
It does get drier though when you run the HP in cooling mode.

I would say they are constant when you get down around 15 to 16M deep, down at one meter if you plotted the yearly temp it would look like a big sine wave lagging behind how the average air temps fluctuate

Not uncommon in cold places to have frost down 1m with snow cover, yet it thaws out each year

[majo]

from a depth of +/- 2m ground temp over here (so app. the same in UK) stays 12°C all year round. At chemicool might be slightly different since he's in israel.

[Abby Normal]

sine - cosine close enough

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd180_e.html

you guys in the UK would probably be similar to Vancouver, milder climate so less fluctuations

[Abby Normal]

a lot of north american ground source research in the 80s use to refer to a 'far field temperature' which was basically the temp 50 ft down (15 to 16 m) which was taken as constant

[Karl Hofmann]

Ha! No way am I digging a hole 50 feet deep...

The one good thing about the British climate is that we don't get the extremes of temperature that many other places get... 30c is regarded as very hot and minus 5c is very cold and rarely lasts, certainly it never freezes the ground down to the depth of one meter so in theory heat pumps should be an ideal source of heat in the UK.

Just checked my electricity and gas prices and I have found that electricity is 3.2 times the price of Natural gas, so again I think that heat pumps are pretty close to heating with gas. My only reservation with air source is the way that the efficiency falls off as it gets colder so it is still a tough call there

[nike123]

Just checked my electricity and gas prices and I have found that electricity is 3.2 times the price of Natural gas, so again I think that heat pumps are pretty close to heating with gas. My only reservation with air source is the way that the efficiency falls off as it gets colder so it is still a tough call there

If you have that relation between gas and electricity, then consider this air/water gas powered heat pumps. You save some money on excavation and on energy here.
They are powered with gas and works with any loss of capacity until -20°C. And, they ARE made in Japan.

[Peter_1]

Digging a hole 50 ft deep isn't that difficult. It's sometime the only way to have enough place to install all the loops you need. Most land surfaces becomes really small, at least in Belgium.

You can count at +/- 2 kW/hole of 60 ft.

You have a point about the extremes in our countries: we will see the biggest benefits with a GSHP when ambient is cold but if you look how many hours/year the ambient drops below 0°C (32°F). Very rare.

We can agree that an airsource HP (AHP) will give a good COP till 5°C (41°F) outside. So for the few days we have less then this - mostly at night when you need much less heating - most AHP manufacturers say that the then lower COP and the higher running costs will never match the higher price of cost of the loops you have to install for a GSHP.
In the Alterma's of Daikin, an electrical heater is additionally installed which can help the times the COP drops to far. And if you heat during night rates, then it only cost half the price.

So the AHP has in my opinion good arguments

Karl, you compared gas prices with kW prices but have you taken the energy stored in the gas (€c/kWh or £/kWh)?
In Belgium, we can say that in €c/kWh gas costs double as electricity where 80% of he electricity is taken during night and 20% during day.

[Peter_1]

But Chemi can use the colder ground temperatures in my opinion very good when he wants to reduce running costs of the refrigeration systems he's installing in his country.
Or some sort of backup condenser in the ground for the times it becomes really hot outside.
I see big benefits in hot places all over the world.

But also in our countries where you can condens year around at +/- 20°C to 25°C (68°F/77°F)
Counting that every 1°C drop in condensing temperature gives a saving of +/-2%, dropping to 25°C will increase seriously your overall COP.

I posted some time ago a loading dock wherein we installed a condenser in the concrete around 1990:1993.

We there use the concrete as a condenser and to melt also the snow (for the few times we see snow here )

We had to install a timer on the air cooled condenser fan because it became stuck after many years the fan never had to work. Now we run it some times/day so that it rotates freely when we need it.

And you also reduce the noise for the environment.

[Abby Normal]

Ha! No way am I digging a hole 50 feet deep...

The one good thing about the British climate is that we don't get the extremes of temperature that many other places get... 30c is regarded as very hot and minus 5c is very cold and rarely lasts, certainly it never freezes the ground down to the depth of one meter so in theory heat pumps should be an ideal source of heat in the UK.

Just checked my electricity and gas prices and I have found that electricity is 3.2 times the price of Natural gas, so again I think that heat pumps are pretty close to heating with gas. My only reservation with air source is the way that the efficiency falls off as it gets colder so it is still a tough call there

Vertical loops are common, you drill wells, sometimes 60 to 75M deep.You could be a dozen such boreholes on a project like you are describing

In Sweden they found if they buried their horizontal loops down around 4 meters they initially had great performance but they eventually created permafrost

[Peter_1]

Just a small add: there were prices compared between gas and electricity.
Translating this tot a HP is in my opinion not really correct:
you get 3.5 tot 4 kW - 5 kW for a DX HP - for every kW you take out of the grid and if you compare these to night rates (here in Belgium also the whole weekend and the whole day on holidays) then you almost double this.

So, there remains a big advantage for a heatpump.