<blockquote><cite>Posted By: SteamyTea</cite><blockquote><cite>Posted By: JSHarris</cite>something that apparently happens from time to time during heavy rain fall periods)</blockquote>
Does down here<img title="" alt="" src="/forum114/extensions/Vanillacons/smilies/standard/sad.gif"></img>

So this treated sewage is just allowed to flow onto the lawn (or similar) and the rain takes it away or it just soaks in. Even when I did some work for Klargesters I never asked how the systems worked. Are they small enough to put on a narrowboat?</blockquote>

The rules are pretty tight on discharge now. The old days of being allowed to just have a septic tank (like an older Klargester "onion") with land drain run-off for the "liquid" element have gone. All small scale treatment plants have to be approved to EN 12566-3 2005 and typically have a settling tank, a means of aerating the liquid and often a substrate (equivalent to the coke beds used on a large scale plant) that can support a biofilm of aerobic bacteria to "digest" the waste. The final liquid that comes out is fairly clear and has a low potential for depleting the oxygen in any watercourse or drain system. Drainage of this is either via land drains, like a septic tank, or by discharge to a watercourse.

The problem with conventional septic tanks is that the treatment took place in the land drains, where soil bacteria would aerobically "digest" the waste. Unfortunately the biofilms this produces would clog the land drains after 5 to ten years, resulting in reduced aerobic activity and the discharge of raw effluent into the ground. This is the reason for making the use of proper treatment plants mandatory, although it'll be years before all the old septic tanks get converted, I expect.

There are different techniques for getting these treatment plants to work, but all rely on passing air through the system in one way or another. Some use electric air pumps, some use electrically powered rotating discs but the one I like is the Biorock, as it's passive and uses convection to get the air flow needed (see here: http://www.biorock-uk.com/biorock_how_it_works.html).

I don't think any of them would fit on a boat, unless it was a pretty large one.

<blockquote><cite>Posted By: stones</cite>Dragging the discussion back to the heating solution, our house, floor area equivalent 150sq metres has an actual heat requirement of 3.4kw at 0C to maintain our 18.5 C inside - a bit more than you but reasonably low. When considering how to provide heat, I too was fixed on the CoP of a heat pump and eventually went for an exhaust air heat pump, thinking like you, that I would always benefit from the 3 to 1 ratio. Turns oyut my unit is way undersized and has to rely on immersion back up to meet the shortfall. Working on a solution at the moment, but it has made me very wary and is the only real regret I have about my house. Without drivelling on about my woes, all I would say is do not get too fixated with heat pumps. As suggested, panel heaters / kickspace fan heaters could be a simple and cheap solution. Small gas bolier off LPG (twin 47kg cylinder arrangement like static caravans use rather than big tank) plumbed to a small radiator circuit, would be a bigger outlay to panel heaters, but fuel price is lower (at the moment) and perhaps worth considering given the flexibility it would provide for DHW.</blockquote>

Thanks for that. Practical experience is always more valuable than any amount of theorising!

I'm gradually moving away from the idea of an exhaust/in-house ASHP, for much the same reason as you've mentioned - I will need a fair bit of supplementary heat when the weather gets down below freezing.

Panel heaters were my original first choice, before wondering about a small GSHP. This process of discussion on here has been incredibly useful though, because it's led to me working through the pros and cons of a number of ideas that I probably wouldn't have otherwise considered.

I'd like to be free of burning fuel locally, so an all-electric solution is attractive. I can probably get another kWp of solar up on the retaining wall at the back of the house, giving me about the maximum the DNO will allow of 3.7 kWp. This would pretty much ensure that the house was carbon neutral, I think.

Now that the borehole option has come up I need to look at this seriously. It already makes sense just for the water supply and the additional cost of a second borehole to allow the use of a ground water source heat pump will be small - the bulk of the cost is in getting the rig on site and lining the extraction hole, putting the pump down etc. The recovery borehole would be much cheaper.

Using a small (say 4kW) heat pump with the ground water source for heating and DHW, together with solar thermal and a fairly large thermal store, seems like a good option. Even if the pump is a bit too large it can run into the thermal store as a buffer, then turn off and wait for the thermal store temp to drop before coming on again. This should give a fairly healthy degree of "spare" capacity, should it be needed, and still allow an apparently over-sized heat pump to operate efficiently.

Posted By: JSHarrisAll small scale treatment plants have to be approved to EN 12566-3 2005 and typically have a settling tank, a means of aerating the liquid and often a substrate (equivalent to the coke beds used on a large scale plant) that can support a biofilm of aerobic bacteria to "digest" the waste.

One thing to be wary of is what can go down the drain (no fat/bleach etc). That put me off as that would inevitably happen in our house!!

Posted By: JSHarrisThe snag is that the noise comes from the fans, which are outside in free air and have to be for the thing to work (it outside noise that's the issue, there's little noise from the components inside the house). Anything that restricts airflow to make them quieter is going to reduce the effectiveness of the ASHP I think.

Yes, I know - I deal with ASHPs a reasonable amount at work for computer room cooling. Same with ductwork.

If you keep the free area of your sound proofing solution large, then the pressure drop from the acoustic baffling will be extremely minimal (far less than that across the radiator fins) - pressure drop varies with the inverse cube of the duct cross-sectional area.

So the only reason that this isn't done by the manufacturers is cost and size...

Tim.

I've been doing more research into using boreholes and have some basic figures that look encouraging.

If you don't abstract more than 20,000 litres per day then you don't need a licence or any form of paperwork from anyone, you can simply drill the hole and use it. You do have to abide by the regulations intended to protect groundwater, but these don't need any oversight from anyone.

In terms of using two boreholes, one for abstraction of drinking water and source water for a heat pump and one to return the heat pump water back to the aquifer, then this means you pump a lot of energy out of the ground without needing any licences or paperwork, plus you get as much water as you need, free from water charges, hosepipe bans, etc, and that water is considered to be sustainable, it's classified as being the same as rainwater harvesting for things like CfSH.

If I assume that the potable water abstraction rate will be 300 litres per day (just two of us, and that's probably an over-estimate), that the water coming out of the aquifer is at 8 deg C and that the return water to the other borehole from the heat pump is at 3 deg C, then I should be able to legally extract about 411 MJ (about 114 kWh) of heat energy from the water per day.

The maximum heating demand will be around 1.7 kW, so about 41 kWh per day, plus maybe another 7 or 8 kWh for DHW (assuming no sun). It looks as if I could comfortable extract all the energy I could ever possibly need from a borehole system, whilst staying well within the legally allowed licence-free limit.

It's not all positive though, as the pump needed to get the water out of the borehole will consume some power and effectively reduce the true COP of the system. However, this power should be modest, as it's likely that the water will only be a few metres down, so there won't be a need for a lot of pump power to shift it. Also, the pump power may be offset by the higher COP that the heat pump itself will have, from running with a fairly high and constant inlet temperature, that shouldn't be affected by seasonal variations.

You can do either, but the tank needs to be high if you want a decent gravity flow rate. I don't have the height available for a gravity feed tank, so will probably opt for a pressure controlled pump. From what I can gather these use a pump and pressure vessel, rather like those used on heating systems as expansion vessels, together with a pressure switch. This gives the standard 2 to 4 bar pressure at the taps instantly, as the pump only fires up when needed to re-pressurise the system. It's possible to combine a tank with a pressure pump, which allows fitting the tank underground, but as I'm likely to have a flow rate from the borehole that will be massively greater than our needs (based on the data I've seen from nearby boreholes) I think I should be able to extract directly.

It's still early days in digging out the details of all this stuff, but I'm quite surprised that boreholes seem so relatively free of regulation, and that, in my case, the cost will be less than half that of connecting to the mains water supply. I can see why many of the borehole companies give relatively short payback periods, with the price of water steadily increasing it doesn't take long to claw the cost of a borehole back.

Posted By: JSHarrisIf I assume that the potable water abstraction rate will be 300 litres per day (just two of us, and that's probably an over-estimate), that the water coming out of the aquifer is at 8 deg C and that the return water to the other borehole from the heat pump is at 3 deg C, then I should be able to legally extract about 411 MJ (about 114 kWh) of heat energy from the water per day.

You're way off base here. 300l with a delta T of 5C is only 1.75kWh (300 * 4200 * 5 / 3.6MJ).

A heatpump will need in the order of up to several litres per minute to work. The COP of open loop systems is high because the collection area is large so you don't get the input water temp declining, but you'll use a large amount of pump energy to abstract the water in the first place compared to a closed loop system. Also, unless the water has no mineralization, you'll have to descale the heat exchanger every year.

Paul in Montreal.

<blockquote><cite>Posted By: Paul in Montreal</cite>
You're way off base here. 300l with a delta T of 5C is only 1.75kWh (300 * 4200 * 5 / 3.6MJ).

A heatpump will need in the order of up to several litres per minute to work. The COP of open loop systems is high because the collection area is large so you don't get the input water temp declining, but you'll use a large amount of pump energy to abstract the water in the first place compared to a closed loop system. Also, unless the water has no mineralization, you'll have to descale the heat exchanger every year.

Paul in Montreal.</blockquote>

I'm pretty sure my calcs are spot on.

The maximum legal flow rate is 20,000 litres per day. Take away the 300 litres used as potable water for domestic use and I have an allowance of 19,700 litres per day left for energy extraction before I need a licence.

19,700 litres with a 5 degree delta T gives me:

4.18 x 1000 x 19,700 x 5 = 4117300000 J or 411.73 MJ or 114.369 kWh

I think you've got the sums wrong Paul, not me................


EDITED TO ADD:

Just realised you misread my post, Paul. I have 20,000 litres a day as the maximum for the heat pump, less the 300 litres per day drawn off as potable water for domestic use, leaving me with 19,700 litres per day for use by the heat pump.

The pump and pressure vessel setup I've been looking at uses about 400W to deliver around 1000 to 1500 litres per hour at the sort of head I anticipate, so not too high a premium on top of the heat pump consumption. My guess is that the higher COP would probable give me that sort of gain on a 4 kW rated HP.

Good point about scaling, yes, it probably will need descaling as the water around here is very hard,

As I understand it the danger involved with using an open loop is if your incoming water temperature falls below 7C or so you are in danger of freezing your heat exchanger and trashing your HP but by the sounds of it if you are moving enough water across your heat exchanger you won't be stripping enough heat out of it to risk freezing, but it's something to be aware of and account for.

<blockquote><cite>Posted By: Chris P Bacon</cite>As I understand it the danger involved with using an open loop is if your incoming water temperature falls below 7C or so you are in danger of freezing your heat exchanger and trashing your HP but by the sounds of it if you are moving enough water across your heat exchanger you won't be stripping enough heat out of it to risk freezing, but it's something to be aware of and account for.</blockquote>

I'll admit to still being in the research phase at the moment, and hadn't picked up on that risk, so thanks. I'm digging around to try and find HPs that accept direct water input. It seems one or two of those marketed as Dimplex do, as do the Viessmann HPs. Presumably these must have some form of protection against freezing, perhaps by shutting down if the outlet temperature gets close to freezing.

Posted By: JSHarrisJust realised you misread my post, Paul. I have 20,000 litres a day as the maximum for the heat pump, less the 300 litres

Ah right, I did misread. 400W to run the water pump is at least 10X the power consumption required to run a closed loop system, though all the waste heat of the pump will end up in the water, just with a COP of 1.0. The higher entering water temperature, though, compared to a closed loop system will mean that you will probably come out even. So long as the flow rate is high enough, you won't have more than about a 4 or 5C delta T on the water side so there should be no danger of freezing the heat exchanger. This won't damage anything, it will just knock the capacity for six as ice isn't very conductive. Most decent heatpumps have sensors on the water loop anyway and will shutdown if freezing point is approached. Mine has a jumper that selects for 1C or -10C depending whether the loop has antifreeze or not. When it was first installed they forgot to select the -10C jumper and it shut down as the entering water temperature is about 4.5C where I am.

Paul in Montreal.

Posted By: JSHarrisI'm digging around to try and find HPs that accept direct water input.

Erm, they all do in effect I'd say. It's just water in a pipe whether it's from an open or closed loop. Of course, you have to be sure the heat exchanger is corrosion resistant, but I would think most are. Certainly my GSHP's instruction manual gives performance data for both open and closed-loop modes of operation and the heat exchanger is appropriately designed for either.

Paul in Montreal.

JSH, I have no experience of running a GSHP from a borehole but I do have experience of borehole use as I have installed and use 2 on the farm. You can not suck water up more than 9m and 6m is the practical limit. If the water is deeper than that you will need a submersible pump designed for boreholes (vibrating membrane submersible pumps must NEVER be used in boreholes). Borehole pumps are about 75mm - 100mm dia. and your typical borehole is about 120mm or 150mm dia., its not reckoned to be a good idea to put 2 pumps down a borehole as the pipes and cables interfere with each other. As boreholes and wells work and water is drawn the passages in the water bearing layer open up and more water becomes available i.e. boreholes produce more water the more they are used. I would have a concern that if the 2 boreholes were too close the waterways between them would open up and short circuit the flow, this may not be a problem for the GSHP but could provide a contamination possibility for your drinking water. For this reason I would prefer to see a plan that discharged the GSHP water to the stream rather than put it back into the ground. (Also might be cheaper than 2 boreholes)

Water in the ground is in layers separated by impervious layers, the first layer is considered this years rain water and is fairly local and subject to annual variation. Lower layers come from further distances and are usually more secure in the supply. From what you say you will be tapping into a lower layer. It is very important that who ever drills the borehole properly seals the pipe between the layers to prevent cross contamination between tha layers. First layer water is subject to contamination from leaking cess pits, agricultural run-off ect. so this is very important.

Any tanks should be through flow. The standard pressure tank for CH should not be used as they create a stagnant volume of water that can allow bacteria build-up. (German regs. prohibit single pipe pressure tanks in drinking water systems for this reason). When looking at pressure vessels you should use a large vessel 100lts or more to prevent short cycling of the pump.

My deep borehole (120m) has a submersible pump to a holding tank then a surface pump to a pressure vessel (on to another surface pump and pressure vessel to increase available pressure (height /distance issues)). The shallow borehole (30m) has a submersible pump directly feeding a pressure vessel. The differing designs are driven by depth of the borehole and the height required to pump. The holding tank has 1 winter days supply as its volume and there is an iron filter in this system due to the high iron content of the water.
I have never had contamination problems, but care has to be taken with maintenance ect.

I hope you find this useful.

Peter

I briefly considered open-loop when we started here. Eventually we decided against it.
I mean I wussed out worrying about unforeseen complications;
Worry about grit in the water and abrasion, filters, limescale, freezing etc
So instead we currently have a closed loop system with 6x 150m deep boreholes with two loops of 42mm PEXA in each hole. I have a feeling we might have overdone that a bit but that's just in my nature :)

So I can't comment on open-loop beyond my paranoia. I would be really interested to hear from other folks practical experiences with open loop.

Some warning I do feel a bit more qualified on is ... be wary of any assumptions about local geology.
We had a lot of drilling complications that pushed our installation price up quite a lot and added months to our schedule. I wish I had gone for a fixed price deal on the drilling.

Thanks for the tips, Peter. The pumps I'm familiar with (we had a borehole on the farm) were submersible, around 100mm diameter and would pump up a head of a few tens of metres easily enough. They were simply dropped down the borehole on a rope, with a length of plastic pipe and waterproof cable to take the water up and electricity down respectively.

Discharge into a surface watercourse is forbidden - rules and regulations! Discharge back into the aquifer via another borehole is, however, fine, as long as the water hasn't been polluted. As it will simply be flowing through plastic pipes and the heat exchanger in the pump it will be just as clean when it goes back to the aquifer as when it comes out, provided the pipe work all stays in good order. The tops of the boreholes need to be protected against run-off water getting in, but that should be fairly straightforward.

The geology here is pretty simple - chalk downland with guaranteed clean water in the chalk aquifer. Being chalk, percolation is fairly slow and the water at depth is always clean and from rainfall some years before. All of the mains water comes from the same source, although it's pumped into storage reservoirs and treated with chlorine.

From what I've read so far there doesn't need to be much separation between the outflow and inflow boreholes - a few metres is usually OK. I can probably get 20 metres separation without too much trouble. As the water going back will be as clean as when it came out, just a bit colder, I can't see why contamination should be a problem. The Environment Agency are pretty hot on anything likely to contaminate water sources and they would rather that heat pump outflow go back down a borehole than be discharged into a watercourse (based on there view that the contamination risk is very low and exceeded by the increased flood risk of surface discharge).

I won't go for a storage tank, just a pressure vessel and pressure switch to control the pump on demand. This avoids the need for any bacteriological treatment, as there won't be any standing water for bugs to breed in.

Sprocket, thanks for the advice. I intend forking out for a geological survey, but based on the sampling data from surrounding boreholes (all of which show very similar layers of rock strata and aquifer depth relative to Ordnance Datum) I'm reasonably sure we'll hit clean water at a relatively shallow depth. The shallow depth is one of the attractions, as it means the boreholes won't cost a fortune to drill. I'd need to drill down to around 60 to 80 metres to use a borehole for an indirect (piped) heat exchange system, which would cost a fair bit more, plus I'd still need the shallower borehole for the domestic supply, as the grouted in pipes in the GSHP borehole would preclude using it for water abstraction.

I think I'll get the drillers to drill the abstraction borehole first and see what depth we hit water. If it is around where I think it will be, then I'll proceed with the plan to drill a second hole for the heat pump return. If we don't hit water at a reasonable depth and need to drill deep, then I'll consider abandoning the second borehole and look for an alternative for the heat pump source. One alternative option might be to see if I can get a licence to abstract water from the stream, filter it, feed it through the heat pump and return it to the stream. As this wouldn't pose a risk of contamination or increase the flood risk, it may be that the Environment Agency would look kindly on it..................