Part Thirty One – It’s Slow Going On Your Own…………

This thirty first entry was published originally by JSHarris on the 6th July 2014 and received 1,212 views on the closed forum

Sorry for the delay in updating this blog. Lots has been happening but things are really grinding along slowly now that I’m working on my own on the fitting out. I’ve spent many days trying, and failing, to sort out the ongoing water quality problems from the borehole, interspersed with re-doing work I thought I’d completed because I wasn’t 100% happy with it. In between that I’ve been trying to complete the bathrooms and utility room fit out.

The ongoing borehole saga

Much of this has been related in later entries on a long running thread that was on the old Ebuild forum, but is attached here as a pdf file:    . In essence, it seems that our borehole is partly filled with sand and whenever the pump runs for any length of time it sucks sand up into the supply. I’ve played around fitting fancy filters, with centrifugal spinners to spin the sand out, but having tried two of these devices both clogged solid within a few minutes use. The main filtration system uses a combined pressurised sand bed and manganese dioxide media system, in a thing that looks like a giant scuba tank, with an electronic timer and control valve on top. The top of it can be seen in this photo – it’s around 1500mm high and 300mm in diameter, to give a sense of scale.


The timer/control valve box on the top of this unit switches a set of internal valves over to backwash the filter to a drain line every couple of days. It does this at 2 am in the morning, when there’s not likely to be any household water demand, although there is a 100 litre pressure vessel of clean water that can continue to supply the house during the backwash cycle. The fine sand in the filter traps sediment and the manganese dioxide oxidises the fairly high dissolved (ferrous) iron content of our water. This filter also has a compressed air space at the top, through which incoming water is sprayed. This aerates the water and drives out the small (but smelly) dissolved hydrogen sulphide content (think Bath spa water smell, for those who’ve been there). Pretty clear water from this filter then goes to a 5µ filter and activated carbon filter (the filters at the top left), then flows through a UV steriliser to kill any bugs, before flowing to the house. Inside the house there is a check valve, a 100 litre pressure vessel (that runs at 5 bar pressure) and a pressure reducing valve that regulates the house hot and cold water supply to 2.8 bar. There are also full bore valves at various points so critical bits of the system can be shut off, a bit like the stop c o c k on a mains supply, but a bit more complex because of the need to separately isolate pressure vessels full of water.

The problem I was getting with sand from the borehole was mainly during the filter backwash cycle. This flushes around 250 litres of water from the pump backwards through the sand filter, to blow any accumulated sediment up and out through the drain. It does this in 8 minutes, so demands around 30 litres a minute of water from the pump, at as high a pressure as possible. This is right at the maximum capacity of our borehole pump, so it is working pretty much flat out to deliver this flush water. Unfortunately, running at this rate causes it to suck up sand from the bottom of the borehole, clogging the pre-filter I put in place. After back flushing, the timer/valve unit on the filter activates a small venturi suction pump in the valve head that flows pump water to the drain and sucks all the water out of the filter pressure tank, leaving it full of air (the air is drawn in via the small grey tube with the white check valve and filter). It takes 15 minutes to do this, and uses around another 50 to 100 litres of water to create the suction. Once full of air the filter switches back to operational mode and fills with water from the pump, which compresses the air in it (as the inlet is at the top and the outlet is at the bottom) and leaves the compressed aeration pocket at the top.

Having found that trying to filter the pump water when it was running at 30 litres per minute or so during back wash was pretty much impossible, I decided to add a storage tank and second pump, primarily to deal with the high flow rate back wash flow requirement. In normal use the borehole pump will probably never need to deliver more than around 15 to 20 litres per minute, if that, so adding a relatively cheap tank and pump just to deal with the high flow rate seemed reasonable, and also gives the option of having another store of water, should we have a problem with the borehole pump at any time in the future. So, I now have a 650 litre tank in the water plant shed, fed via a solenoid valve and an aeration venturi (it made sense to fit this, as pre-aerating the water used for back flushing stops it from reducing the oxidising capability of the filter media). This 650 litre tank has a float valve that turns the solenoid valve off when it’s full. The tank is re-filled at a relatively slow rate from the borehole pump (flow rate is limited by the 6mm orifice in the venturi aerator). A second pressure pump draws water from around 3/4s of the depth of this tank and feeds (via a check valve) the supply to the filter. This pump set is controlled by a second pressure switch and a time switch, so that it only operates when the filter is going to backwash, normally (although over-riding the time switch would allow this pump to feed the house if needed). Thankfully the extra tank and pressure pump set wasn’t that expensive. The tank was around £220 and the pump another £120.

I have the borehole drilling company coming out to air lift the accumulated sand in the borehole, too, and am going to replace the borehole pump higher than it was, in the hope that it will stay clear of any sand that runs back into the hole via the 0.5mm wide slots in the bottom 12m section of the borehole liner. At the moment, the whole of this slotted section of liner is packed hard with sand, so all the water the pump is drawing is being sucked up through the sand, which is one reason why, at high flow rates, the sand gets disturbed and pulled upwards into the pump. Fingers crossed that I may get a reliable water supply within the next couple of weeks. Thankfully there is no shortage of water, the borehole can easily sustain pumping at 2000 litres per hour for days on end, non-stop, and I doubt we’ll ever use more than around 500 to 600 litres in a whole day, and then only when the filtration system backwashes.

Fixing things that should have been done properly the first time

Like most blokes, I suspect, I’m not good at reading instructions before I launch in to doing things. Although the ASHP install went well (around 1/2 day) and it worked as expected (once I’d fathomed out all the missing information from the manufacturer on what the controls really did) it did create some noise inside the house when it was running. This was really odd, as it was almost completely silent outside, yet inside the house there was a distinct hum in the utility room whenever it was running. Not really loud, but enough to be annoying. After reading through the installation instructions (properly….) I realised that my attempt at reducing heat losses might be to blame. The manual makes it clear that the flow and return pipes should be connected to the rigid pipes coming from the house using 1m long full bore flexible hoses. I had thought this was OTT, as it would have meant having long loops of hose outside, so I hooked it up using 300mm full bore flexible hoses, and wrapped these with insulation. It turns out that the long flexible hoses are needed to isolate the compressor vibration from the house structure. Here’s a photo of the short hose I originally used (after I’d removed it) and another photo showing the new long hoses (before I fitted armorflex insulation over them:


The difference doing this has made is staggering. The ASHP is now inaudible inside the house, the only way to see whether or not it’s running is to check the display. Clearly the short pipes I’d used were conducting vibration into the rigid pipework and hence into the house, whereas the longer pipes stop this completely. Had a read the instructions properly I’d have realised this, and could have saved a couple of hours work in draining (and saving) the antifreeze, replacing the pipes and refilling the system.

The second re-work was very similar, and relates to noise and me not following the manufacturers instructions for the MVHR. Although the MVHR is virtually silent most of the time, whenever it goes to active cooling mode there is some noise, mainly from the extract ducts. To be fair, the installation manual makes it clear that duct silencers should be fitted to the big ducts close to the MVHR unit, but because I changed my mind and only decided to fit this big Genvex unit after the house was designed, I ended up with insufficient space to fit silencers. I also had to re-jig the service area layout to fit the big Genvex unit, and this also reduced the space available for silencers. After a bit of head scratching, and researching how duct silencers work, it seemed easy enough to make some custom shaped ones. Essentially they are just absorption silencers, that are lined with a layer of sound absorbing material. They aren’t very high tech, all that’s needed is a big enough volume to allow the air through without restriction, plus some sound absorbing foam.

The fresh air supply silencer was easy, as it could just be a big rectangular box that fitted on top of the supply manifold chamber, with a central baffle and a couple of duct fittings. Unfortunately it’s impossible to photograph, as it’s sat down in the narrow eaves space behind the MVHR unit. The extract silencer was more of a challenge, as the only space I had to fit it was up against the underside of the roof service space battens.Here are some photos, first the front view:

The extract manifold is the thing with all the blue ducts to the left, the bit of left over green vapourblock board forms the front of the silencer. The just visible short sliver flexible duct goes to the MVHR extract intake.

Here’s a side view of the silencer, which gives an idea of the shape and size.

Finally, this is view of the acoustic foam lining:

These silencers have made a very big difference to the sound level indeed. Even on full boost cooling the MVHR is virtually inaudible in the bedrooms and living room, and can only just be heard in the utility room (which is right underneath the service area). If I’d had the space I’d have perhaps made them a bit bigger, as they are so effective at reducing noise that I think it should be possible to make the MVHR virtually inaudible everywhere in the house, even at full boost when cooling or heating the house.

The foam I used is fire-resistant acoustic foam used for insulating studios and the like. It wasn’t that expensive, as I bought some big sheets to line the walls of the service room, and just used offcuts for the silencers.

Finally, for anyone looking to use the same Genvex MVHR unit, I can offer another tip to same money. This unit has the option to add humidity sensing to the extract duct, so that if it senses increased humidity (such as when a shower is running, or perhaps from cooking) then the MVHR will automatically go into boost mode, to increase the extract and fresh air supply rate. There are no-volt contacts on the MVHR to switch the unit to boost, so what’s needed is a humidistat, placed in the duct so that it closes a pair of contacts when the humidity exceeds a set point. Standard duct mechanical humidistats are both expensive (well over £100) and fairly crude, with poor adjustment of the true relative humidity switching point (because they are also very temperature sensitive) and no control at all over the hysteresis (the relative humidity difference between the switch on and switch off points).

After a bit of digging around I found a Polish manufacturer that makes a very neat little unit (…latory/dhc-100/ ) that can be programmed both for set point and hysteresis, plus has some added features like time delay switching (so you can set an over run time if you wish). Best of all is that this unit is 100% compatible with the Genvex (it has no-volt relay contacts, and will run from the 24V supplementary valve power supply that the Genvex has) and only cost £43 delivered. It also has a nice RH display, and is accurate to 0.5%, so should be a handy way of monitoring the extract humidity level to get the right set point for switching the MVHR to boost.

I’m cracking on with finishing the second bathroom this week, then will start the main bathroom, after which I can have a good clean up and start fitting the bamboo flooring on the first floor. I will be glad to finish with doing plumbing, especially all the stuff to do with the bloody borehole………………


joiner 06 Jul 2014 03:04 PM:

 With so many female contributors on here now, this is not the time to go reinforcing the “men don’t read instructions” illusion that women are so fond of insisting has more than just a grain of truth in it!


 jsharris 06 Jul 2014 03:25 PM:

 The problem is that for both of the above cases it is, unfortunately, true! In the case of the ASHP install I thought the requirement for 1m long flexible hoses in the instructions was daft, as it would cause additional energy loss from the hot/cold pipes to the outside air in winter/summer. I didn’t realise until reading the instructions more carefully that the real reason for needing long flexible pipes was to get rid of the noise transmission path.

 A quick wander around our build will find quite a few unopened plastic bags with instructions inside, I’m afraid……………


 ProDave 06 Jul 2014 03:38 PM:

 the trouble with men is they don’t just need to be told HOW to do something, but we need to know WHY. If we are not told why, then we do it our own way.


 joiner 06 Jul 2014 04:22 PM:

 I know, but no need to go advertising it!

 Thank god the wife doesn’t read this stuff or my life would be misery.


 jsharris 06 Jul 2014 04:50 PM:

 Spot on Dave (ProDave). The mistake with the ASHP pipes was me thinking I knew better than the manufacturers, and that shorter pipes would always be a good thing, not realising that they knew that noise transmission was a problem unless you use longer pipes.


 SteamyTea 07 Jul 2014 08:36 AM :

 What are these instructions in plastic bags you talk of. I have never seen them.

 I made a Temp/RH logger with some DHT11 sensors, these are cheap, made for A/C units, but do seem to require airflow to get an accurate reading.


 Calvinmiddle 08 Jul 2014 04:19 PM:


 I know you used the HB+ ducting but looking at the manifolds that you used it looks like you used the flat 400 x 400.

 I’ve been looking ta the CVC website and there is another manifold that looks like it also might act as like a silencer as there is a much bigger air volume in them.

 Did you notice these? Or have I got it wrong and they aren’t mean to act as a silencer.



jsharris 08 Jul 2014 05:06 PM:


Yes, I did look at these, but they were too big to fit in the limited space I had, which is why I went for the smaller manifolds. I think that these bigger manifolds might silence a bit, but they don’t have any acoustic lining to absorb sound, so could only really work as expansion box silencers. My feeling is that this would be fine for airflow noise, but probably not for the transmitted noise from the motors in the MVHR unit we have. The big advantage of those bigger manifolds is that it is a lot easier to gain access internally, which would be a big help when balancing the system. Balancing is done by inserting annular rings of differing diameters into the manifold duct fittings, and needs access to the inside of the manifold to undo the quarter turn locking ring on each incoming duct, insert the ring, replace the locking ring, refit the 150mm duct, then go back around the house checking all the terminal fittings to see how the change has affected the flow rates. This is a bit of a tedious and iterative process, and gaining access into the smaller manifold is tight and has to be done by feel through the 150mm duct hole, making the process even harder than it need be.


 PeterStarck 10 Dec 2015 07:07 PM:

 As a new member I’ve been working my way through this excellent blog. I can fully relate to the various problems of all types you have encountered during your build. For instance my 47m deep borehole started off as a deepbore soakaway, but that’s another story. I will be installing a Genvex Combi 185 and I was told by the suppliers that controlling it with humidistats wasn’t a good idea because of their unreliability. I’m therefore very interested in the Polish humidity controller you mentioned. I found a similar looking one on Ebay but from the price suspect that the sensors are not as good as your one. Have you looked at the one linked to below, my Polish is nonexistant and comparing them is difficult. The dimensions are not quite the same even though they look similar.…sd=191324730505


 jsharris 10 Dec 2015 09:06 PM:

 Hi Peter,

 Glad you’ve enjoyed the story so far.

 I’m not sure about the Chinese units; I went for the Polish unit for two reasons. The first is that I’ve not been wholly impressed with some Chinese made equipment, the second is that I worked for a while in Southern Poland and was very impressed with their work ethic and attention to detail.

 I have a copy of the English instructions for the Polish humidistat, and have to say that so far it has worked flawlessly. The only slight problem is that I need to change the the threshold twice a year, as the warmer months tend to be dryer, so it’s better to have boost come on at around 60 %, in the late autumn and winter the threshold needs to be increased to about 65% to allow for normal seasonal changes.

 I suspect that much of the unreliability that is reported is down to poor quality sensors plus a failure to recognise the seasonal humidity changes.

 I’m impressed with the Genvex Premium 1L we fitted. It works very well, has a LOT of programming options via the Optima controller (the same one used of the Genvex 185 I think). It takes a bit of wading through the manual to get to grips with all the settings, though.

 BTW, buying a Genvex from Denmark is massively cheaper than buying from the only UK distributor, Total Home Environment. I bought our unit from, saved around £3000 and can get back the 25% Danish VAT. Their service was good and they knew more about the product than the UK dealer.


 PeterStarck 11 Dec 2015 08:04 AM :

 Hi Jeremy, thanks for your help.

 Based on what you’ve said I’ll be buying the Polish humidistat.

 I went to Total Home Enviroment to look at the Combi 185 a couple of years ago and they were very helpful. The only problem is they ring me every few months now asking me when I will be buying the unit. I searched for Danish suppliers but never saw so thanks for that. I shall look into it.


 jsharris 11 Dec 2015 09:19 AM :

 We also went to Total Home Environment and had a quote from them for both supply only and supply and install. The supply only quote from THE for just the Genvex Premium 1L active MVHR was around £8,200 inc VAT, plus around £2,200 for ducting (inc VAT) and another £3,000 (again inc VAT) for installation labour, making the total around £13,400.

The price from Sundthus for supply and delivery of exactly the same unit was £5,200, including Danish VAT at 25% (which can be reclaimed from HMRC I found).

So, comparing just the unit price, after recovering the VAT, THE wanted about £6,830 to supply and deliver the same unit (with the same warranty) that Sundthus would supply and deliver for £4,160. Sundthus were great to deal with, spoke and wrote excellent English and now even list a UK delivery price on their web site (I was, I think, one of their first UK customers and they had to get a price specially for me).

The unit they supplied came with the English manual and was pre-programmed in English, so was no different to the one that would have been supplied by THE. I’ve just checked, and currently Sundthus have the Combi 185 L-S for 42,772 Danish Kronor (including Danish VAT at 25%), which at today’s exchange rate is about £4,134 (inc 25% Danish VAT) and UK freight charges of 1550 Danish Kronor which is around £150 (again inc Danish VAT at 25%).

Worth comparing those prices with those you get from THE – I’ll lay money that they are a lot keener!


PeterStarck 11 Dec 2015 12:34 PM:

I looked at the Sundthus site this morning and the prices are certainly much lower. I put the site through Google Translate and I think the DKK42772 price is without the VAT and with VAT it is DKK53465. It is still a large saving on the UK price though. It’s also interesting about the delivery.

What is the situation regarding the guarantee when buying from Denmark?



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