To cut a long story short, we had a GSHP installed in 2020, a 13kW popular fixed compressor speed model. This was a tad oversized even from the start with a heat loss calc at -3 of 10.5kW (I think even this was high due to renovations). Since then, major unplanned building work has happened removing almost all of the old leaky stone into modern well insulated cavity walls. Heat loss now about 6.5kW. Problem is, as it is fixed speed, the output depends on what the temperature is that comes out of the ground. In warmer conditions you see well over 20kW out of it when the house might only need 3kW. This results in major short cycling and an issue where as the pump also cannot modulate, all zones need to be left open which overheats the house even run on weather comp as you can’t shut zones or the DT widens and errors out.
My solution to it is either to replace with a 7kW model (at significant cost) or go air source. I love the ground source as it’s been very reliable and if correctly sized I think it’s a significant step up over air source particularly on those colder days and would rather stick with it if the sums can work. Does anyone have any experience at all on finding out what I would get for a used GSHP in perfect working condition. They’re £10k odd new and if I got a third of that I’d be pretty happy. None have been sold anywhere from what I can see to base it on but if anyone has any ideas that would be great!
Hello @Cjl24 that’s frustrating, how about switching to a ground source with a modulating compressor e.g Nibe S1156? Here are the ground source heat pumps on HeatpumpMonitor: HeatpumpMonitor.org some appear to modulate very well.
I’m wondering if you will actually see that much of an improvement with the 7kW unit over the 13 kW if it doesn’t modulate as even the 7 will cycle all the time anyway.
That’s a good idea. For the life of me I cannot fathom why anyone thinks a fixed speed heat pump will work well outside of a lab unless they have a constant source temperature (none do) as the variation in output will inevitably mean, as you say, the only way of regulating the output is to cycle. It’s a very odd design choice.
@johncantor of http://heatpumps.co.uk/ has a lot of experience with fixed speed compressors. I wonder if there’s anything you can do with it to get a bit more out of it? It might make more sense financially to keep it and only replace it with something else at the end of life?
What’s the emitter system that it’s connected to ? buffer etc?
Not an ideal option and it’s SO oversized that even a 150l buffer would only add a few extra minutes each hour of run time… 2 minutes 35 seconds to be exact.
Yes that’s a good shout and kind of the side I’m erring on if I can make any changes that don’t affect efficiency.
It’s radiators upstairs, UFH downstairs all sized for 35 degrees at -2… or less. There used to be a buffer but it was removed when the heat pump was moved from an outbuilding into the house as the monster pump they installed initially was no longer required due to the big underground pipe run as the electricity consumption of the pumps was hideous. Whole system runs completely open loop and no zoning. I still have the 150l buffer sat in the garage so potentially that could be installed as a giant volumiser but as I said above, that would only add about 3 ish minutes of run time onto each cycle, but thinking about it, maybe that’s enough just to take the bite out of the short cycling.
What make is it?
One slightly radical idea would be to fit a smaller compressor, but this needs a bit of care and thought, But, given heatexchangers are rarely generously sized, you could end up with a very efficent heat pump. Its something i used to do many years ago. Happy to discuss further. Anyhow, it might end up expensive.
I think there are people about who would rather go GS than AS. Given that you have a ground source, as Trystan says, could you go with smaller inverter GS?
Does the heatpump have a “degree minute” type configuration option?
How much space do you have for buffer tanks?
Using a 4 port buffer with mixer would allow longer cycles as the buffer temperature could be allowed to go significant above required flow temperature. I don’t know the balance between higher flow temperatures and longer cycles on COP…
If you force the heatpump not to restart for 30 minutes whenever it stops what would the result be on comfort? (Eg use thermal mass of property/ufh.) TPI thermostat with minimal cycle length configuration should do this.
Running cost wise the best investment maybe a few Fogstar battery to move electricity to Octopus IO rates (assuming you have a EV to qualify for best off peak rates.)
Ideally no space for buffer tanks and feels like a plaster over the issue. I’m quite against hydraulic separation of any kind as you instantly throw away 10% ish efficient (50% if you’re considering COP loss). I did try increasing the minimum stopping time but found in milder weather the house would just be cold as it hits its temperature limit in under 2 minutes and then falls back to a lower mean water temperature.
We’ve got a massive 40kWh battery system and charge off peak, but really, unless the output of the HP can be decreased, I don’t think any amount of coaxing will get it to run nicely.