no

Would someone care to say a bit more about this?

I've just been given a surprisingly large estimated bill (proposal) for topping up biocide in one GSHP. I would have thought this was a fairly trivial thing to do so I was going to ask in here if anyone did this themselves to save money and what kit I would need. But I searched first and found this and now I am curious to know more.

The GSHP manufacturer recommends that biocide be used and kept topped up.
And I notice that several installers also mention it on their web sites as being necessary.

The main product used for this seems to be Sentinel R700. BUT I don't see a whole raft of alternative products from other manufacturers as you might expect with such things. Sentinel also produce a test kit that includes agar slides for culturing the bugs to diagnose how bad they are.

And since the ground loop is full of glycol+water I can imagine that there would be bugs that would happily live on the glycol even without air and that this could change the consistency of the ground loop brine.

The original post here is pretty old now. Have views about this changed in the last five years?

I used to use this in POS units that had fake beverages in them, even after nearly 20 years they still look good.

http://www.aako.nl/product-groups/personal-hair-care/products/preservative-biocide/nipacide-cfx4

Interestingly the fluid in my ground loop is a mix of water and methanol - we're allowed to do that here and it has better performance compared to the glycol-based mixes. Propylene glycol is relatively viscous and this affects performance a little bit.

Paul in Montreal.

> why would you risk bacterial contamination of the heat transfer fluid for the sake of £40

Thanks. I certainly would not.
But the estimated bill I have is for £500.

FYI, this GSHP has 1800m of 42mm PEXA pipe in 3 boreholes.
I make that total volume about 2.5m3, so requiring a few litres top-up.

Installer also estimates a whole day labour to install. I am surprised. I thought/guessed maybe 1 hour?
Hence I was wondering if it was worth saving by DIY.
But I was not sure what equipment cost this would require.

Then I was surprised to see question of whether biocide was necessary.

All very interesting.
More info/opinions please.

Posted By: SprocketFYI, this GSHP has 1800m of 42mm PEXA pipe in 3 boreholes.

Should that read 180 metres total of borehole each of 60m drill depth; if so, that will give you a total pipe length of 360 metres. 42 mm OD pipe of 2.4mm wall thickness will contain 1litre per metre of fluid therefore 360 litres of fluid. Glycol needs to be mixed for -15'C freeze protection and this is usually around 33% concentration. Based on 360 m of pipe you are likely to require 120 litres of monopropylene glycol.

Posted By: SprocketNo, it really is 1800m. Three 150m deep boreholes. Two loops (ie. duplex) in each.

That's a lot of pipe. What's the rated output of your heatpump?

That's a 16kW heat pump (two 8kW compressors).

Spec selected by initial GSHP installer based on BGS data was four 110m boreholes with single loop of PE100 in each. But we got a good deal on 6 lots of duplex pexa pipe surplus to another job and I couldn't bring myself not to make the most of it.

However, I think if I had known how much trouble the borehole drilling was going to be we might not have tried so hard for the extra depth.

It's that time of year again. Estimated bill was looking even larger this time so I decided to look at this more closely. My conclusion is that this annual biocide addition is completely unnecessary and probably quite a bad idea anyway. Here is why...

Normal procedure (as recommended by biocide manufacturer) to be done at the annual service is:-

1) remove a small amount of fluid from the primary/ground-loop
2) rinse a dipslide (basically an agar plate) in the sample.
3) take the dipslide away, keep it in a warm place for 24 hr and see if anything grows.
4) use a test strip to measure biocide level in the sample.
5) order required amount of biocide.
6) return to customer to top up biocide.

Sample test kit is about £140. This has 5 dipslides. So I need one of these every year (3x GSHP).
The biocide is £65 a litre. My setup requires about 7 litres.
So that is £600 materials cost even before factoring in two site visits by the maintainance company.

Bear in mind that the ground-loop is sealed, so if it was sterile at install and has not been topped up then it should remain sterile. Where are the bugs that require fixing supposed to be coming from?
Conditions for 1, 2 & 3 are hardly laboratory standard. I would suggest this test is pretty pointless conducted this way, especially since it plays no part in determining amount of biocide to add; it is just meant to be an early warning that something nasty is growing in your ground-loop. But it's far more likely the fluid (which must surely already be sterile) will be contaminated at sampling enough to show some bacterial growth.

A little bit of investigation reveals that the biocide is just 3% Hydrogen Peroxide. That explains the test strips too as peroxide test strips are readily available from any lab supplier (or ebay/amazon).

The half-life of that dilution of H2O2 in water circulating in those pipes at this temperature and pressure is likely to be a matter of days or weeks, not years. So at the annual service it will certainly be all gone and require a full top-up again so measuring it with test-strips after a year goes by is pretty pointless.

3% H2O2 is the old 10-vol peroxide. ie. it contains/releases 10x it's own volume of oxygen.
So 7 litres of this peroxide will dump something like 70 litres of oxygen (at 1atm) into my ground-loop. That strikes me as a pretty bad thing to do as it's going to interfere with the ground loop pressure and risk creating airlocks and require yet another maintenance visit to bleed the system and check the pressure.

The bacteria that can grow on glycol include several aerobic and anaerobic types. The conditions in the groundloop would normally be anaerobic so the aerobic ones (ie. that need oxygen) can't survive there. But of course adding all that oxygen creates conditions that can support the aerobic ones too. So if you were to top up the ground loop with water a week or more after the biocide addition you probably risk growing more bugs than if you had not added biocide in the first place.

The dipslides are pointless (but easily bought from lab suppliers). The test strips are available at 50 for a few pounds. The biocide is readily available as food-grade 3% peroxide at about £10 for 5 litres (less if you buy more or more concentrated). So £600 is a huge mark up on these materials.

But after talking to the heat-pump manufacturer about the advisability of adding all that oxygen to the primary we concluded that if the ground loops are sealed and pressure is holding up OK then they are best left well alone. The only test recommended by the manufacturer is to check the glycol concentration occasionally to make sure you still have adequate freeze protection. This is easily done with a cheap refractometer.

Result! Big savings and probably a more reliable GSHP setup too.