A ground source heat pump drilling saga
The process:
1. An assessment of your project’s heat load (energy requirements) is made from your plans.
2. A desk-based assessment is made of the local geology of your site, usually followed by a full soil analysis of a sample from your site.
3. The results of these reports recommend the number and depth of bore holes required (in my case 2 x 180m deep bore holes).
4. Bore holes must be at least 4m apart, they work in pairs and contain the heat exchangers. They are then connected via a manifold to the heat pump in your plant room / utility.
5. The boreholes and manifold need to be proximate to your heat pump otherwise heat will be lost over distance.
6. The borehole, manifold and linking trench locations form part of your technical site plan alongside locations of drains etc...
7. Drilling should take 2 to 5 days for one home, depending upon size.
8. Expect 1-2m3 of “risings” (e.g. what the drill coil brings up from the ground when it drills down) for a residential project.
9. The heating engineer connects everything, leaving only the manifold cover visible.
My experience:
Call me naïve but the first surprise was 2 blokes arriving with a drill rig on a lorry which wouldn’t have looked out of place in Texas. Silly me, how else were they going to bore down 180m?
Although we had space planned for the boreholes between a newly renovated building and the footings for the new house, we hadn’t really considered the size of the kit required to drill them.
Thankfully the lorry and rig were squeezed in and the drilling began.
The geology the site in question was Chama Sands, for which read sand, sand and water, then more sand.
The drill tried over and over to get beyond 80m, but because of the wet sandy geology, the borehole kept on collapsing around itself.
The drill bit broke and had to be replaced. The riggers returned and tried over and over again. In the 3 to 5 days scheduled we had achieved precisely nothing by way of boreholes, but boy were there “risings”. If you drill into something solid like rock or chalk all should be well, but, as we and the riggers were discovering, they were just bringing up sludge, sludge and more sludge!
Sticky sandy green sludge which did not dry out was everywhere. Splattered across the newly renovated building, all over the ground. I got Covid and whilst I could not go on site the riggers decided the best place to put it all was in the trenches of the footings for the new house!!!
We eventually got hold of the riggers boss who understood the process and our horror. The plan for 2 x 180m boreholes was amended to 4 x 90m boreholes and the work was finally completed. They had been on site creating chaos for about 3 weeks. I was never so pleased to see something go!
Lessons:
1. Understand your geological report and overall plan – solid ground produces manageable risings; sandy ground creates costly sludge.
2. Have a plan to accommodate risings on site (they can be used for landscaping, or simply buried). If not removal to a specialist tip can get very expensive!
3. Large amounts of mess-creating kit needs space. Do this work on site FIRST. If retrofitting only consider if you genuinely have space for the riggers to work, which must be close to the plant / utility room which houses your heat pump.
4. The experts make the initial calculations and recommendations for a plan. This can be altered if necessary but more boreholes require more space. This might adjust the overall cost, but the main expense is getting the rig there in the first place.
5. If it does not feel right – stop the work! These chaps are used to drilling in fields and car parks, the nature of the task means that they can be no respecters of residential property – retrofit at your peril!
6. Read all the reports, asks practical questions and know how to contact the person in charge!