ground source: soil, well or 'lake'?

[Jeremy]

Jeremy, I guess I must be way off on the borehole thing.... maybe you live in a really good spot for it ?  or maybe most of the reading I have done has been of boreholes in other, dryer countries, it sounds like you'll have plenty of water...


but... I still think you will have corrosion/sediment problems, no matter what the heat pump people say (and lets face it, they would say that)

even just tap water builds up lime scale/sediment at a pretty decent rate - if your water has filtered through chalk then it will pick some up and deposit it in your heat exchanger ? and it'll be too small to filter out :-(


I would be very tempted to run the bore hole water through a second heat exchanger a with a loop plumbed in pumping water between the heat exchanger and your heat pump - if you use a big heat exchanger (single walled water/water are pretty cheap) and add some antifreeze into the water (helps lower surface tension and increases thermal capacity) then you should easily be able to get the temperature difference down to 1'C or less!  :-)

I think there's a lot of variability between boreholes in different types of rock and different areas of the country.  We're just lucky down here that the chalk works like a massive sponge, that both holds an enormous volume of water and filters it very effectively so it comes out very clean and clear.  The chalk is like solid rock a few metres down, yet still permeable enough to flow a lot of water.  In effect we've got a massive water filter down there that has already removed all the solids and even the bacteria from the surface water that's slowly seeped down to it.  Because it's solid it doesn't result in sediment or particles coming off during use, apart from the initial few weeks when all the drilling debris and the inevitable muck from the gravel packing that's put in the bottom of the hole washes out.  The boreholes are lined and grouted around the outside at the top, so that they stay clean and muck doesn't find its way in and the hole itself is under a sealed cover to keep out rainwater etc.  Most local boreholes here give drinking quality water with no filtering or other form of treatment needed, as long as they don't feed storage tanks.

Using a second heat exchanger pipe array significantly impacts the HP efficiency, because of that heat transfer loss which lowers the HP inlet temperature.  If the HP has a corrosion resistant heat exchanger (as all the ones I've looked at do) then there's no problem.  The oxygen content of borehole water is low anyway, so it tends not to be corrosive and will happily flow through stainless, or even copper, pipework for a few decades without causing any problems.  There is a slight risk from lime scale build up after a few years, but this is not a significant problem because the temperatures are low, so the sublimation rate is also very low.  It's no worse than the build up in a normal household cold water system (I recently removed a few metres of 30 year old copper pipe from my house and the lime scale build up was non-existent on the cold pipes, although it was more noticeable as a green tinged thin layer on the hot pipes).

Finally this isn't something new or novel, it's a fairly commonplace way of providing heat input to a GSHP in many places, one that's frequently used in preference to a buried pipe closed loop system wherever it makes economic sense and there is a good groundwater supply available.

[Peasant]

FWIW Iv'e seen mention here of cleaning fluid that is left sit in the heat pump for a number of days each summer and the neighbour I spoke to last week mentioned specifying particular good quality valves in his system so he could back flush it every year. So it seems like there are particular risks but if you have a reasonable preventative maintenance regime you should minimise them.

BTW I have come across an inverter controlled GSHP from Nibe the 1250 but it is only available in one size with output from 4.5 - 16kW does anyone one know of any other inverter GSHPs in a smaller output, max. 6 or 8kW?

[Richard Owen]

What would  the effect be of duplicating or triplicating small diameter pipe laid as slinkies in the same trench ?

The extra cost would be very small as the trench work is already done. Would not he extra surface area in contact with the soil  improve heat transfer ?

Just a random thought but why do the plastic swimming pool solar heaters have micro bore pipes - and come to think of  all solar heaters I am aware of use small pipe.

Just my 2p worth

Sorry guys, just catching up after a few days of being distracted by work and other things.

More pipe would indeed improve heat transfer, but if heat transfer is already good enough, more pipe isn't go to do any better.

How you measure what is 'good enough' is difficult.

[Richard Owen]

In the interests of accuracy I have to say that when I posted this earlier

"Looking at the latest MCS 022 ground array lookup tables and using a  couple of mid range values for average  ground temperature,  9 deg C and thermal conductivity  2  w/m/K it gives the maximum extract values for 25mm and 32mm pipe ( no 40mm)  using an average value of 2400 FLEQ hours it gives maximum values  of 13W/m for 25mm pipe and 41W/m for 32mm pipe."

I have just noticed that while I had been  looking at the pipe size for horizontal arrays in the tables I  had failed to notice the heading had changes to slinkey, so there is no data for single pipe runs other than 25mm that I can see.

That may well be true and it may well be very interesting but I can't see why it has any relevance at all.

The pipe in the ground does two things: it collects heat and it transports heat.

We've already seen that smaller pipe (unless the ground is very dry) does a better job or collecting heat than does bigger pipe.

That leaves us with transporting heat. Look at it this way; you have a house heated by a gas boiler and radiators. The radiators are connected by 10mm pipes to a pair of manifolds which are connected by 22mm pipe to the boiler.

The numbers above say that larger pipes holds more heat (because they hold more water) than do smaller pipes.

So, would your heating system work any better if you replaced the 10mm pipe with 22mm pipe?

Of course not. In terms of transporting heat, the 10mm pipe moves enough water for the radiators to work efficiently so increasing the heat carrying capacity of the pipe doesn't effect the efficiency of the system at all.

Am I missing something obvious here?

[Richard Owen]

Here in Finland, no company offers ground loop for realistic price. That is because everyone can calculate price tag for 8-12h excavator work + 400m ground loop.

Much easier is offer bore hole and after company is made "all inclusive" offer and you have shaken hands, the company takes drilling company to drill and install collector pipes. Rest of the job is pure plumbing job. Installing GSHP is no rocket science.

Sami

Heat pumps are a bit of a cottage industry over here in the UK and the economics work differently. It's more expensive to bore a hole than dig a trench.

[sam123]

So, would your heating system work any better if you replaced the 10mm pipe with 22mm pipe?

Of course not. In terms of transporting heat, the 10mm pipe moves enough water for the radiators to work efficiently so increasing the heat carrying capacity of the pipe doesn't effect the efficiency of the system at all.

Am I missing something obvious here?

In GSHP system larger pipe is better. Always aim to as low radiator/UFH water as possible. COP is always better when you can use better flow and lower temp radiator water.

Circulation pump to maximum speed in winter time, and heating water temperature as low as possible.

In gas heating, then only gain from larger pipes is that you can keep your boiler little a bit cooler. Which means little less heat loss from boilers and pipes. If boiler and pipes are in living area then it is all same. Sometime you can keep your circulation pump in lower speed with larger pipes and if your boiler is downstairs you can get "free flow" with larger pipes, which helps during power failure.

@ Peasant

I wouldn't go with inverter GSHP. It just useless with UFH. It might be useful in radiator house without buffer tank. In ASHP you need inverter, because there is nothing where you charge your heat. In UFH situation, there is tons of concrete where you can load.

IVT has also inverter pump, but it is same power range. Just go for Nibe 6-8kW...no reason to pay more. You get easily COP 4.5-5 in +5/30 situation.

I have UFH water temperature is +31 degree in outside temperature -20. My house is from early 2002-2003. You do probably same with +27...more COP. Inside temperature +22.

[Richard Owen]

In GSHP system larger pipe is better. 

So where's the data that supports that claim?

I can buy your argument that in Finland, where boreholes are the common method, that a single 40mm pipe connected to the GSHP is the way to go.

In the UK, where ground loops are the norm, everything I look at shows that multiple, thinner pipes are better.

I'd love to be wrong. Show me.

[Peasant]

@ Peasant

I wouldn't go with inverter GSHP. It just useless with UFH. It might be useful in radiator house without buffer tank. In ASHP you need inverter, because there is nothing where you charge your heat. In UFH situation, there is tons of concrete where you can load.

IVT has also inverter pump, but it is same power range. Just go for Nibe 6-8kW...no reason to pay more. You get easily COP 4.5-5 in +5/30 situation.

I have UFH water temperature is +31 degree in outside temperature -20. My house is from early 2002-2003. You do probably same with +27...more COP. Inside temperature +22.

I'm concerned that I will have problems with short cycling, given that my heat demand calculations are 2.6kW based on the coldest day in the last 6 years, if I choose a heat pump which is delivering a fixed 6kW surely I will have a lot of compressor starts in a day? When I spoke to the neighbour with the bore holes last week he mentioned that he felt his 11kW heat pump was oversized as when fitted first he was averaging 35 compressor starts a day!  By tricking around with settings he is down to 20 a day now and admittedly he has big old radiators rather than UFH but I still find that figure crazy.

[dhaslam]

It wouldn't be all that difficult to  keep  the compressor running.  You just need to limit the running hours  in line with the  daily heat requirement  and set room thermostats a bit higher than you actually need.     This way the  heat pump can cut out on the timer before  the  temperature is reached.   

[Peasant]

Cheers dhaslam

I just ran the figures on that to see what it would mean in reality if I were to use a 6kW heat pump for my year round space heating and DHW (The reality is I am unlikely to have the budget left for solar thermal if I have to drill the boreholes.)

Oct. 3 hours/day
Nov. 5 hours/day
Dec. 6 hours 30 mins/day
Jan. 7 hours/day
Feb. 6 hours/day
April 3 hours 30mins/day
During the remainder of the year 1 hour 45mins running per day should fulfil my estimated DHW demand.

To limit run times in such a fashion with a fixed output oversized heat source will seriously limit flexibility in terms of heating and DHW availability, it seemed to me that a inverter solution which could adapt output to the heating and DHW requirements would be a better solution.

Makes me think that a GSHP is not the way for me to go at all.

[knighty]

if you're looking at a max of 7hours running in winder, why not go for a smaller pump?



as long as it will reheat your hot water as fast as you need it ?

[Jeremy]

I'm concerned that I will have problems with short cycling, given that my heat demand calculations are 2.6kW based on the coldest day in the last 6 years, if I choose a heat pump which is delivering a fixed 6kW surely I will have a lot of compressor starts in a day? When I spoke to the neighbour with the bore holes last week he mentioned that he felt his 11kW heat pump was oversized as when fitted first he was averaging 35 compressor starts a day!  By tricking around with settings he is down to 20 a day now and admittedly he has big old radiators rather than UFH but I still find that figure crazy.

I share the same potential problem.  My space heating demand at -5 deg C is 1.7 kW.  Ideally I'd like a smaller HP, but it seems hard to find GSHPs of less than 4 kW.  As more and more well-insulated homes get built, that only have a small space heating requirement, there would seem to be a growing demand for low output GSHPs, but so far it seems that the manufacturers haven't caught up.

[Peasant]

That's the height of it, 3kW heat pumps are very thin on the ground. Of the few I have found I have reservations about all of them for one reason or another. In one case it's mainly down to price, it's dearer than a 6kW pump from any of the big name manufacturers, so I'll keep working on them but there's a pretty big bridge to cross!

Give it a few years and I am sure there will be a far better choice of solutions but that's not much comfort now.

[A.L.]

To limit run times in such a fashion with a fixed output oversized heat source will seriously limit flexibility in terms of heating and DHW availability]To limit run times in such a fashion with a fixed output oversized heat source will seriously limit flexibility in terms of heating and DHW availability

a 200mm thick 100m²floor slap will absorb 12kWh/°C. so could heat it by one degree four times per day to cover your space heating requirement with one start for DHW

[sam123]

I'm concerned that I will have problems with short cycling, given that my heat demand calculations are 2.6kW based on the coldest day in the last 6 years, if I choose a heat pump which is delivering a fixed 6kW surely I will have a lot of compressor starts in a day? When I spoke to the neighbour with the bore holes last week he mentioned that he felt his 11kW heat pump was oversized as when fitted first he was averaging 35 compressor starts a day!   By tricking around with settings he is down to 20 a day now and admittedly he has big old radiators rather than UFH but I still find that figure crazy.

In Nibe and similar pumps it is pretty easy to reduce starts by control panel settings. As GSHP compressor take easily 150 000-200 000 starts...it is truly irrelevant issue. You can add buffer tank, but as it cost extra £1000-1500 which is pretty same as price for new compressor  

REMEMBER: You make also your domestic hot water with your pump. It takes ages to make hot water even with 6kW Nibe. It gives only 5kW for 50 degree DHW .

Half of the year it is only 2-5 starts/day for hot water only.

@ Richard

I wasn´t wrecking your theory from small pipes on ground loop, even it´s completely useless except installers point of view (who gets lot of money for connecting pipes and by selling installation accessories).

I was talking about connecting radiators with larger pipes (10mm vs. 22mm)