Air Source Vs Ground Source

[Big Neil]

Hello All,

 

I once had an installer estimate for a Nibe GSHP, including the need for boreholes, for a circa 1800 square foot house over 2 floors with UFH throughout. Was around the 17 grand mark. Obviously only a fag packet estimate based on experience, but verified in part by an individual borehole company. This was to supply all heating and hot water for said house.

 

Firstly would anyone in the know please confirm, is this broadly the same as the cost for an equivalent ASHP? Which would theoretically cost more to run, or are they both roughly the same?

 

What other considerations apart from available land to run a loop/dig a borehole, has anyone who has installed either of these, found would be relevant as opposed to conventional wisdom?

 

Also, knowing my luck exactly the same question has been asked before and if so feel free to point me to the right post and i'll delete this one if required.

[Jeremy Harris]

I did some rough calculations for our house and found that a GSHP would cost around 3 to 4 times the initial cost of an ASHP and would save us around £20 a year in electricity cost.  This meant it would never ever recover the additional initial cost during its lifetime, so we switched to having an ASHP (which meant a minor change to our planning consent - ASHPs are a planning issue, GSHPs aren't, usually).

 

In practice I doubt that there's even £20 a year difference, as our ASHP seems to operate with a COP of well over 3, and closer to 4 a lot of the time, so pretty much the same as a GSHP, after you take into account the additional energy cost of running the collector pump.

[jack]

You also need to factor in the cost of changing out the fluid in the brine loops of the GSHP - I seem to recall that that's a couple/few hundred quid every few years.

[Big Neil]

Ahhhhhh interesting. I had no idea it would be that significant. Is noise a significant factor? - ooooo if planning is an issue with ASHP, would working in some sort of recess into lets say a garage wall, in order  to have it flush instead of blistered onto the outside of a building, potentially help that situation?. Could mount it on a slider for ease of access to Service.

 

And what about RHI? Does it still apply to ASHPs. A couple of finger in air discussions with people regarding Ground source units had them estimating the cost could largely be recouped, over the 7(??) year 

[Declan52]

Have you done any heat requirement calculations for your build yet??

Depending on what ground conditions you find when doing a borehole will determine if you can get enough heat out of the ground. Different types of rock produce different amounts of  heat. Also how many water strikes will have a major influence on how many watts of heat you can get out of the ground.

You can check geology maps to find what you're likely to find in your area and then take it from there but there's no guarantee it will work.

 

[Big Neil]

Morning Declan,

 

No calcs yet, not even got drawn up plans. Was just interested in general terms. I guess given Mr Harris comments it would be interesting to know his ground conditions??

 

Also the question of RHI still stands. 

[Jeremy Harris]

26 minutes ago, Big Neil said:

Morning Declan,

 

No calcs yet, not even got drawn up plans. Was just interested in general terms. I guess given Mr Harris comments it would be interesting to know his ground conditions??

 

Also the question of RHI still stands. 

 

We were going to use a second borehole, as we already had to have one for water, so the extra cost of drilling a second one for a heat pump collector was relatively small, as a big hunk of the borehole cost is in mobilising equipment to the site and setting up.

[Big Neil]

righteeo - and the Air Source unit was still cheaper.. And does the RHI still count. Seems if one is to own a property for some time, at least the 7 years i've got in my head, that there might be a benefit in initial cost vs planning issues, being reduced over that time. If the same rules and payments apply with Air though, strikes me as a nobrainer. Just don't know enough about the tech yet...

[joe90]

Technically I did not mention the ASHP to either the planners or the DNO although the plans fir planning had it on (very small). I had heard that DNO,s don’t like ASHP and sometimes want a bigger supply but I have read that modern ASHP,s draw less current than electric showers!!

[Jeremy Harris]

9 minutes ago, Big Neil said:

righteeo - and the Air Source unit was still cheaper.. And does the RHI still count. Seems if one is to own a property for some time, at least the 7 years i've got in my head, that there might be a benefit in initial cost vs planning issues, being reduced over that time. If the same rules and payments apply with Air though, strikes me as a nobrainer. Just don't know enough about the tech yet...

 

You need to work out the heating and hot water requirement to see whether it's worth going for an approved installation so that you can claim RHI.  When I did this, the cost of an ASHP that was smaller than the one we have now, supplied and fitted by an approved installer, was around £4,500, IIRC.  The RHI payments would have been £84 a year for 7 years, so a total of £588.

 

I paid £1,700 for our ASHP, installed it myself (very easy) and so paid probably under £2,000 for the whole installation.  I don't care about the RHI, because even allowing for that with an approved installation I still saved close to £2,000.  It made it daft to think of using an approved installation, as I'd never have got the additional installation cost back.

[Jeremy Harris]

28 minutes ago, joe90 said:

Technically I did not mention the ASHP to either the planners or the DNO although the plans fir planning had it on (very small). I had heard that DNO,s don’t like ASHP and sometimes want a bigger supply but I have read that modern ASHP,s draw less current than electric showers!!

 

The DNO issue goes back a decade or so to the time before we had inverter controlled heat pumps, when they were unmodulated, "on-off" machines, with a conventional direct online starter.  The starting current could be two or three times the running current, and it was this that caused the DNOs concern, especially as most early heat pumps were quite high output units, so tended to have a pretty high input current to start with.

 

The 7 kW unit we have draws a maximum current at full output of 10 A, and most of the time ours runs at around 2 A.  There's no startup surge, as the inverter starts the pump and fan slowly, so the current remains under 10 A at all times. 

 

An electric shower rated at 10 kW at 230 VAC draws a bit over 43 A.

[ProDave]

22 minutes ago, joe90 said:

Technically I did not mention the ASHP to either the planners or the DNO although the plans fir planning had it on (very small). I had heard that DNO,s don’t like ASHP and sometimes want a bigger supply but I have read that modern ASHP,s draw less current than electric showers!!

For anyone reading, in Scotland you need PP for an ASHP unless it is >100M from your boundary.

 

Luckily I discovered this during my planning application and was able to update the plans to include the ASHP. 

 

I was previously thinking of a GSHP (which are permitted development in Scotland) but discounted that idea when I found the ground collector pipes and brine to fill them with would cost more than the heat pump itself.

 

I am willing to bet my Fridge has a higher starting current than my inverter driven ASHP but the DNO's do ask on the new supply form if you are having a heat pump.

[Big Neil]

Silly question then. If they're so efficient, and can be in the broader scheme of things relatively cheap to buy. Why doesn't everyone have them. Should I get one in my current house and throw the boiler out maybe??

 

And I'm not missing something am I. Just as with GSHP units, they can be sized to supply all hot water and heating yes?

[Jeremy Harris]

30 minutes ago, Big Neil said:

Silly question then. If they're so efficient, and can be in the broader scheme of things relatively cheap to buy. Why doesn't everyone have them. Should I get one in my current house and throw the boiler out maybe??

 

And I'm not missing something am I. Just as with GSHP units, they can be sized to supply all hot water and heating yes?

 

Heat pumps are best suited to a relatively low heat loss house, for a couple of reasons. 

 

Firstly, the size of the heat pump needed is set by the worst case demand, often the heating demand in very cold weather for a house built to a relatively poor standard in terms of heat loss rate.  That then means the heat pump will probably be quite large and so more costly, usually.

 

Heat pumps don't like delivering water at more than about 50 deg C, even a GSHP will lose efficiency when the flow temperature gets this high.  Heat pumps generally work more efficiently if the flow temperature is reduced to around 40 deg C or less.  An older house may well need water fed to conventional radiators at a higher temperature than this in cold weather, so would need either retrofitting with UFH (if practicable, as there's a limit on how much heat UFH can deliver) or have the existing radiators replaced with larger ones.

 

Hot water is another potential problem area, especially if an existing house only has a small hot water tank, run at a high temperature.  With the upper limit for most heat pumps being around 50 deg C, the house may need a larger hot water tank in order to be able to deliver the same quantity of usable hot water at the taps.  You only need water at around 40 to 45 deg C at the hot water tap, but often hot water tanks will be run at around 60 to 70 deg C and the water at the tap will be mixed down with cold to give a comfortable temperature, which then allows a smaller hot water tank to be fitted, as less hot water is drawn from it due to the much higher temperature.

 

If there is a mains gas supply available, then generally it's cheaper, both in terms of initial cost and running cost, to fit a decent condensing gas boiler than it is to fit a heat pump.  If there isn't mains gas available then the chances are that a heat pump, with modifications to the radiators and perhaps the hot water tank, may prove cost effective, because of the reduced running cost over its lifetime.  This may be marginal though, for a house with a high heating demand; you probably need to work through the actual data for a specific house in detail to see if it would make sense.

 

 

[ProDave]

^^^ Following on from the above, you seem to hear some people saying how useless heat pumps are., but that appears to be people that have simply replaced a boiler with a heat pump with no proper regards to any design, and then complain it does not work well.

 

Out house has a maximum heat demand (-10 outside +20 inside) of a little over 2KW which should comfortably be met by our 5Kw heat pump feeding under floor heating ans a large hot water tank ensuring hot water at 47 degrees is okay.

[Stones]

@Big Neil I looked into retrofitting an ASHP or a GSHP at our last house.  The cost of the GSHP was significantly inflated, as the salesmen put it, 'look what you get back in RHI'. 

 

When I actually drilled down into the figures, I found that a GSHP was going to cost MORE to run with a heating demand of 5000kWh/yr compared to an ASHP (due to a combination of the energy used to pump around the ground loop, and the cost of changing the fluid in the ground loop).  The higher the heating demand, the more the balance tipped to GSHP.  DHW was also an issue for the GSHP requiring immersion top up, although the overall delivered cost of DHW was the same as an ASHP.

 

ASHP installations are eligible for RHI (the rate recently increased but maximum annual payment now capped)

 

I keep linking to it but my recent blog entry gives some performance data for our ASHP

 

 

[Ballynoes]

Hi All,

 

An interesting thread indeed.

 

I am under the impression that an ASHP is not that efficient in the winter when needed the most, just out of ignorance really, as I don’t see how it can get much heat from a temp outside of say -10.

 

Please feel free to correct me as I am new to all this.

 

We are planning a new build in 14 months’ time, so I am looking at GSHP or ASHP.

 

I have 10 acres, so land for the GSHP loops is not a real problem, but reading some of these posts it looks like there is not much difference in them.

 

I will be installing underfloor heating, so what’s the secret to an ASHP, as the initial cost will be significantly lower, but I want the best option for our build, by the way it will be well above code as far as insulation is concerned.

 

Also if I may, how are ASHP at heating water?

 

We also wish to install a log burner for those days during the winter we are sat home.

[joe90]

Hi @Ballynoes, I have installed an ASHP and will be using it for UFH via a buffer tank and also heating DHW. I believe the trick is to limit the water temps so the ASHP does not go into defrost mode, some here say 40’ will do that but others say they are getting hogher DHW temps. Mine is only just working so I cannot give you actual figures but other here will. I do believe it also depends on your location and weather conditions.ASHP,s are used in countries with very low temps but also low humidity. 

[Ballynoes]

On 02/10/2018 at 18:00, ProDave said:

^^^ Following on from the above, you seem to hear some people saying how useless heat pumps are., but that appears to be people that have simply replaced a boiler with a heat pump with no proper regards to any design, and then complain it does not work well.

 

Out house has a maximum heat demand (-10 outside +20 inside) of a little over 2KW which should comfortably be met by our 5Kw heat pump feeding under floor heating ans a large hot water tank ensuring hot water at 47 degrees is okay.

 

Hopefully ProDave will comment here, I also read his Blog about his house build, very interesting it was too.

 

We are going to build a modest house of around 120M square with a U value of 0.1, as we have lived in a 200 year old drafty house for long enough.

 

Underfloor heating is the preferred option, but I was looking at a GSHP, as I assumed they were better, however reading on here it looks like a properly specked ASHP will do just as well, as the heating demand for the house, I hope will be very limited, compared to our old drafty oil fed house at present.

 

A log burner will also be installed, for those really cold evenings when we want to toast ourselves.

 

Like Dave we live in the highlands, in the countryside where temperatures can get down to -15 in bad winters.

 

The hope is that the house will be warm for getting up in the mornings, then off to work, and warm for coming home, as our old oil boiler does at present

 

Am I therefore correct in that an ASHP will be effective at keeping the new house warm, electricity costs don't matter too much, as we have more flood lights round the outdoor riding school, than I care to mention.

 

 

[joe90]

We also have a woodburner for chilly evenings and despite being told we would overheat if we used it we have not (so far).

[jack]

4 minutes ago, Ballynoes said:

Like Dave we live in the highlands, in the countryside where temperatures can get down to -15 in bad winters.

 

Those sorts of temps aren't necessarily a problem, because by then there's very little water in the air. For example, have a look at the numbers for these Panasonic models.

 

COP will be lower than at high temps, of course, but if sized properly there shouldn't be an issue.

 

As has been discussed elsewhere, the real problem is when temps are low single figure and the air is moist. This causes a lot of condensation on the heat exchanger, which in turn freezes. It's the defrosting of that ice that causes the big hit on COP and average amount of heat that can be delivered. 

 

[Ballynoes]

 

14 minutes ago, jack said:

 

As has been discussed elsewhere, the real problem is when temps are low single figure and the air is moist. This causes a lot of condensation on the heat exchanger, which in turn freezes. It's the defrosting of that ice that causes the big hit on COP and average amount of heat that can be delivered. 

 

 

This is slightly concerning as I said we live in Highland Scotland... renowned for its, cold wet winters, this I assume as you say can freeze on the condenser.

 

Maybe I need to read more about ASHP's before making my decision.

 

Any recommended sites to visit...

Edited November 23, 2018 by Ballynoes

[ProDave]

Hi @Ballynoes Where in the Highlands are you?  I am about 20 miles north of Inverness.

 

My heat pump is working fine. The lowest I have seen so far this winter is -6 and it continues providing 37 degree water for the UFH without issue.  It takes heat out of the air by cooling it, so if the air is already -10, it will probably exit the heat pump at -13 or even colder.

 

The only "problem" with an ASHP in cold weather is when you ask a lot of it, e.g for heating hot water.  At least 3 times now I have seen mine need to defrost when heating the hot water to 48 degrees, but I have never yet seen it defrost when just keeping the UFH going.

 

If you are building to the insulation level you say, you will find the house only cools down very slowly when the heating goes off, so forget the concept of the house being "cold" in the morning before the heating comes on, it will just be a little less warm.

 

If you are passing by this way you are welcome to call in for a look.

[Ballynoes]

Hi Dave,

 

thanks for the reply, we are nearer Dunkeld, so compared to to you not "really" in the Highlands, but since we are on our own in the country, the weather can get pretty chilly in the winter, and when it snows it snows, so we can get temperatures down as far as -15 because of our location.

 

My thoughts were to install a GSHP as we have 10 acres, so ground loops are not an issue, I just thought naively an ASHP was less able to heat the house in winter, but since joining here, I am having second thoughts.

 

The house will be built to 0.1 U value, that's a given, as our old stone cottage is 200 years old and costs a fortune to heat, and when the wind blows it just strips the heat from it.

 

Could someone explain what you mean by "defrost" is this because the ASHP is working overtime and causes itself to ice up... I assume then it takes extra energy to defrost itself before producing heat ??

 

As you say with a well insulated house it will probably stay "relatively warm" overnight so in the mornings require less heat to bring it up to a comfortable temperature. Is it best to go for an ASHP which is Over specked, just in case or not necessary.

 

Thanks Again.

[PeterW]

I’d be looking at an ASHP that can provide 100% of the heating input at 50% capacity. This means it will have a larger condenser so should defrost less often.