Hi, I’ve been advised to switch my ASHP heating and hot water system over to an open loop system. I’m currently on a 4 pipe system, and this would then use the buffer as a volumizer if I understand correctly. The point is apparently that it takes away the need for another pump, and stops mixing in the buffer. I thought I’d ask here first before having the plumbing changes executed. Many thanks for any advice, as always massively appreciated!
Really depends on your starting point. If its lots of zone control and variable flow rates on the secondary side, moving to an open loop is a big step forward for efficiency. However if you already run a fully open secondary side, the gains come from flow matching the primary and secondary sides of the system. So run the primary side in fixed flow rate mode (match flow rate to the heat loss at design) and the run the secondary side at the same flow rate.
Pumping losses, the system pressure drops in a fully open system and one with a primary and secondary via a buffer are very close, so pump gains are very small if any.
I have recently moved from a fully open system with single pump to a fully open with primary and secondary pump via close coupled tees and actually run (in cooling) 15W less via 2x pumps than I did with one. But my main advantage has come from more stable ASHP running and better feedback to ASHP from its changes to flow and expected return temps.
So my advise is dump any thermostats, flow match primary and secondary pumps (removes all distortion from buffer) and leave the rest as is.
In general agree with your response @Johnmo but it’s surprising how much distortion you can get across some 4 pipe buffers even with flow matched pumps!
@johncantor did a good video on this recently here https://www.youtube.com/watch?v=rHl0qPNOkQo.
A before modification example showed a drop in the flow temperature of 5.5K across the 4 pipe buffer.
After modification to a 3-pipe configuration, the flow temperatures match much better:
3-pipe configuration:
A 3-pipe buffer is probably a bit closer to your close coupled tee configuration in terms of the way it minimises distortion/mixing. John estimated in his example that the COP improvement from his example switch from a 4-pipe to a 3-pipe at the example conditions could have been in the region of 3.1 up to 3.5.
I wouldn’t rule out re-piping the 4 pipe buffer as a volumiser as the @ASHPAmateur has been advised, especially if the secondary pumps are not very efficient and there’s plenty of pumping power in the primary pump to run the whole system. After all most of the high performance systems on HeatpumpMonitor are open loop without a buffer and a buffer in many cases just adds un-needed complication.
The other question is how the heat pump is being controlled, if the call for heat is determined by a buffer temperature sensor that can often introduce some other oddities that a simple open loop system avoids.
Lots of options but really need to pin down how the heating system is actually operating or planed to be operated. John Cantor’s example has radiators switching on off so an unsettled secondary flow. If operated in pure WC that should not be occurring.
Then we get in to the bigger conversation of elimination of the vessel all together, as in a pure open system the additional water volume isn’t needed at all.
Will await OP to get back with his full plan of operation going forward. And if UFH or Radiators or a mix of the two.
Sometimes you can’t advoid “radiators switching on off”, eg a B&B where each customer wishes to control their own sleeping temperature. An radator being turned down can both lower COP and lower electricity use.
Good to make the point that actually 2 pumps or one, the pumping power is not much different.
For some buffers, even with good flow matching, there is still lots of mixing.. e.g. the flow outlet to radiators is equal to HP return temperature. It seems like a mixing (top to bottom) happens.
I think the better systems that are ‘open loop’ have well matched radiator size to heat demand, however, in the past, the rad sizing was often inaccurate.. and from what I see, many still are.. e.g. the rad is partly selected by what will practically fit in a certain room. The obvious test is to open all trvs up and monitor room temps over time, of simply see how it ‘feels’. Are there rooms unnecessariy warm?, ot too cold? If not, then open loop should be OK. It also depends on the room layout. It worries me a little that some people increase the average house temp if heating un-used spaces. The other thing to consider (doing this test) is to look for underheated areas, and consider increasing size of adding a rad. You could do this simply to accomodate your needs (e.g.you like a warm bedroom.. or not). Prob best to test at non sunny days. On that point, houses with a lot of windows may not be well suited for open loop.
First of all, thanks so much for the responses so far! The whole thing is a pretty steep learning curve for me…
Let me add first how the system is actually run/set up. I have an 8kw Daikin Altherma ASHP. The heating is almost exclusively run as UFH (with the exception of two small radiators in one room which I kept mainly for visual reasons, plus they fill the space under the window sills perfectly). I have two manifolds. The first one is on the ground floor, and controls two zones of largely identical size. The second one is on the first floor, and controls four zones. Two of these are on the first floor, two on the second floor. I have four Nest thermostats. One controls the two zones of the first floor manifold, one the two first floor zones of the first floor manifold, and then the last two which control one zone each on the second floor. The main reason for the thermostats is to not have warm bedrooms or cold bathrooms. Does this description narrow down the parameters? Thank you!
Will the ground floor self regulate if you remove all activators from it’s loops?
Likely an open loop system can cope with the activators on other floors.
With that setup you will run into issues with a volumiser as you are strangling flow routes for the heat pump as zones close. This is in principle what the buffer does, it allow the heat pump to flow at its own rate.
While you have controlled zones not clear how you can really move from a buffer.
You may be better deleting the zones all together and then balance the heating system, reduce flow in loops where a room is getting to warm. Increase flow where a room isn’t warm enough. If all rooms get too hot reduce flow temperature. Then a good chance to delete buffer all together and remove it, not reuse as a volumiser. You can trial this by just removing the actuators from the manifold, take a note off all the flow rates to each loop so you can revert later if you need too.