IDM Heat Pump Issue

We have reluctantly inherited a block of 4 IDM GSHP units that have been installed in new build properties.

There are issues with all 4, but after a an initial cursory visit it looks as if balancing and set up will resolve the bulk of the problems on 3.

1 though is having major problems with DHW, in so far that the buffer is loading up to design temp, but then decaying very rapidly, almost as if the buffer plate is absent and is being mixed down by the CH circuits.

I have not fully checked the schematics yet to see if there is anything obviously amiss, but there is no secondary return, seemingly no leak, because the plate pump does not run/cycle, no thermal syphon feasible due to the plate and pump.

The buffer is 1000/50 litres and the HP is a Terra 15s. 2 other units on site are very similar but not subject to the same problems.

BF I know you have some experience of these have you or anyone else got any pointers before I get back on site?

Is it the HGL version or the basic? HGL have 3 pipes leaving the rear of the heatpump. Basic has only two.

It has the HGL technology built in.

The distribution circuit is UFH with load compensating mixing pumps on the manifolds.

The schematics are a little unusual, in that the buffer should have the HGL take off high in the DHW side of the plate, a return from the UFH, a return to the HP but the flow is shown with a tee feeding the base of the buffer 1 way and direct to the UFH the other.

So presumably the base of the buffer only loads when the compensating pump kits are either not running or when they are running low, creating a resistance which diverts flow into the buffer.

BF are you thinking that the HGL take off might have been plumbed in to the buffer base and the UFH flow at the top, which would make a mess of things?

I think separates are a much simpler option.

Jon

The HGL system is circulating a small amount of water through small HX which is transferring energy from the very hottest discharge gas. This water is then sent to the top of the tank to produce hot water up to 60C. The
Discharge gas then continues into a second larger HX where it disappates its heat to the ufh water. Because the HGL system needs a return in order to operate this is taken from the bottom of the tank. So in effect there the bottom of the tank is used as a buffer, where both the flow and the return from the ufh can feed through depending on demand, ensuring constant condensing temps.
There should be a separation plate about 2/3's of the way down the tank in order to prevent the hot water from diminishing rapidly. It is possible the flow was connected on the top side of the separation plate rather than below it. Its also possible that the wrong tank was used and that it has no separation plate. Non HGL tanks don't have a separation plate.

Check it out, I think the plumbing of the tank is the Key. HGL works well because you do no need to run to high condensing temps in order to produce hot water.