ASHP outdoor unit heating pump Q

[BotusBuild]

I'm looking to remove or bypass the fitted buffer tank in our heating circuit, removing the "extra circuit pump" at the same time.

 

Will the heating pump fitted in the outdoor unit will be capable of circulating the water through the heating and DHW?

 

The buffer tank is 50 litres. The UFH pipes have a volume approximately 100 - 110 litres.

 

Info from docs:

The heating pump in the outdoor unit consumes 2 to 50W.

 

[SimonD]

So what we need to know is which exact model do you have - what is its power output? Do you know the index circuit pressure loss for your heating system? And what is the property heat load?

 

 

[BotusBuild]

 

@SimonD

🙂 I tagged it. Its a VWL 75/6.

 

I don't know the index circuit pressure loss. That's a new term for me. Should it be documented somewhere? Possibly called something else?

 

The worst monthh for heat loss for the property is 3246W daily heat loss power for mean min OAT (as calculated using JHarris spreadsheet)

[SimonD]

1 hour ago, BotusBuild said:

I don't know the index circuit pressure loss.

 

This is critical. Your index circuit basically means the sum of the sections of your heating system that adds up to the greatest pressure loss. For every meter that your heating water flows through a pipe is loses pressure. To know whether the pump can deal with this, we need to know the maximum pressure it has to overcome and at what flow rate. If you look at your specification chart, you have a max flow rate of 1205l/h and at this flow rate, if you look at your chart, it gives a residual head of just over 4m head. To know if the pump will be okay with this, you need to know if the circuit that has the greatest pressure loss is less than 4m head. 

 

Do you have a circuit diagram or do you know how your heating system is plumbed?

[sharpener]

This may help https://johncantor.uk/pressure+flow/simulator.html

[BotusBuild]

Two storey building. Outdoor unit is level with the upper floor.

We have UFH on both floors.

Lower floor has 7 loops to a manifold.

Upper floor has 7 loops to a manifold (in the UGF file attached, the small red loop and the small blue loop to the upper right were left out).

 

 

[JohnMo]

All I can say is they are pretty pictures. But unless both spaces are fully open plan, there is zero design work going on.

 

Pipe sizes is questionable. 1/2" pipe isn't really suitable, you should be using 16mm. Your loop length can be around 100m, so the smaller loops can be deleted and incorporated into other loops if you wanted. For info I am doing 7 loops in total for the same heat load.

[Dillsue]

Might be easiest to suck it and see by bypassing the accumulator with the flow pipe and replacing the scondary pump with a straight section of pipe. Avoid sharp bends/elbows in the accumulator bypass!

[BotusBuild]

44 minutes ago, JohnMo said:

All I can say is they are pretty pictures. But unless both spaces are fully open plan, there is zero design work going on.

 

Pipe sizes is questionable. 1/2" pipe isn't really suitable, you should be using 16mm. Your loop length can be around 100m, so the smaller loops can be deleted and incorporated into other loops if you wanted. For info I am doing 7 loops in total for the same heat load.

Sometimes you comments come across a bit passive aggressive.

 

UGF. Open plan to the left. Large single room to the right. A hall where the small blue loop is shown (but not fitted). A utility and cloakroom where the small read loop is shown (but not fitted).

 

LGF. On the left each loop is a separate room. The 2 large loops to the right are a single room.

 

All pipes are 16mm. No loop longer than 95m as laid.

[JohnMo]

1 minute ago, BotusBuild said:

Sometimes you comments come across a bit passive aggressive.

 

Never meant to be aggressive.

[BotusBuild]

I know you are trying to be helpful on here. There are perhaps better ways of wording some of things you post.

 

e.g. you could have asked what pipe size I used, and sought a better explanation for the pipe layout (which equally I could have provided a better explanation for up front)

[JohnMo]

16 minutes ago, BotusBuild said:

I know you are trying to be helpful on here. There are perhaps better ways of wording some of things you post.

 

e.g. you could have asked what pipe size I used, and sought a better explanation for the pipe layout (which equally I could have provided a better explanation for up front)

I could have asked, but you posted an image that stated 1/2", so would have been a wasted question. But as your using 16mm, all is good.

 

But your loops are really only designed to fit the space, not to match the room heat loss. The loops per room, should be matched to heat loss. Balancing is possible with manipulation of flow rates through each loop, but there is a limited effect, if your circulation pump modulates.

[BotusBuild]

@SimonD, is there anything else you are seeking information about? A bit more below.

 

FYI the heat pump is about 3m from the plant room (LGF where the manifold is), connected by 28mm insulated pipework. The UVC (250l) is in the plant room.

 

Motorised valve switches between heating and DHW. Heating flow goes to the buffer tank (50l), then a pump on the flow to the two manifolds. These have no mixing valves or secondary pumps. All heating pipework is 28mm copper in the plant room, all insulated. About 10m of this on flow and return to furthest (UGF) manifold

[BotusBuild]

John, the current setup works. We are comfortable the way the loops are and by adjusting flows to each.

 

I am only interested in whether I can remove or bypass the buffer tank and rely on the heating pump in the outdoor unit to continue to make the water flow through the UFH.

[SimonD]

13 hours ago, BotusBuild said:

@SimonD, is there anything else you are seeking information about? A bit more below.

 

FYI the heat pump is about 3m from the plant room (LGF where the manifold is), connected by 28mm insulated pipework. The UVC (250l) is in the plant room.

 

Motorised valve switches between heating and DHW. Heating flow goes to the buffer tank (50l), then a pump on the flow to the two manifolds. These have no mixing valves or secondary pumps. All heating pipework is 28mm copper in the plant room, all insulated. About 10m of this on flow and return to furthest (UGF) manifold

 

I think there are 2 options here.

 

1. is to calculate the press loss in each loop to correctly determine the index circuit and then see if you have enough residual head at the heat pump. My suspicion is you probably do. If you wanted to do this then as you have the flow rates already, you might already have the pressure loss figures from the design documentation somewhere? If not, the pressure loss can be calculated from the flow rates. It's a bit laborious.

 

2. is you just take a punt and remove the buffer and additional pump and see what happens! I think you'll probably be okay, we won't know for sure until you get it up and running.

 

Then there remains the question as to why they installed a buffer in the first place. Was it:

 

- just a mindless design that plonked it in there;

- to do with a calculated or feared pressure loss issue;

- to do with system volume as your system doesn't hold the minimum volume per minimum kW output of the heat pump;

- to actually buffer excess output from the heat pump as it's oversized? 

 

I would be worth doing a system volume calculation to make sure you have sufficient volume. That way if you don't, you could plumb in the buffer as a volumiser and this will save you a headache and repipe if you just cut it out and then have issues.

 

The last thing worth doing is to understand your house heat loss co-efficient - this is how much heat your house loses per Kelvin change in outdoor temperature. To do this we need your calculated heat loss at the required design outdoor temperature and design indoor temperature difference. This figure is useful because we can then find out at what outdoor temperature the heat pump will reach minimum modulation to understand where it might start to cycle and then understand if the buffer was put in for this reason, although I doubt this because the buffer is so small.

[Beelbeebub]

Depending on the exact buffer layout and design, it should be easy to bypass the flow side (the flow from the heatpump and flow to the heating) with 2 T and 2 lever valves. 

 

This would leave you with a volumiser on the return. 

 

Leave the pump inline but disconnected electrically 

 

If the Heatpump pump is man enough great. If not you can run the second pump in series to boost the pressure. 

 

If it's all a disaster you can go back to current set up by switching the 2 valves. 

 

 

[Nickfromwales]

1 hour ago, Beelbeebub said:

Depending on the exact buffer layout and design, it should be easy to bypass the flow side (the flow from the heatpump and flow to the heating) with 2 T and 2 lever valves. 

 

This would leave you with a volumiser on the return. 

 

Leave the pump inline but disconnected electrically 

 

If the Heatpump pump is man enough great. If not you can run the second pump in series to boost the pressure. 

 

If it's all a disaster you can go back to current set up by switching the 2 valves. 

 

 

I’d always put the volumiser on the flow with a heat pump, but if this can be chopped to minimise work and needs to stay on the return I doubt it’s a huge issue.

[JohnMo]

 

On 06/03/2026 at 14:03, BotusBuild said:

UFH pipes have a volume approximately 100 - 110 litres

So if you use 2 zone control, you may need the volumiser with a 7kW heat pump, but if you just run as a single zone I would just take the buffer out the pipe work and slot a couple of spool pieces in.

 

Why - General rule of thumb, and a specific requirement for some heat pumps is 20L per kW of heat output at min ASHP turn down. The 20L is calculated on the smallest zone that can call for heat.

 

If you have a well zoned house (lots of thermostats), leave the buffer as it is.

[Beelbeebub]

4 hours ago, Nickfromwales said:

I’d always put the volumiser on the flow with a heat pump, but if this can be chopped to minimise work and needs to stay on the return I doubt it’s a huge issue.

Either/or

 

If you piped a bypass on both the flow and return ports then you could set the system to be a buffer, flow side volumiser, return side volumiser or entirely absent with just a few valve changes... 😁 

[Nickfromwales]

1 hour ago, Beelbeebub said:

Either/or

 

If you piped a bypass on both the flow and return ports then you could set the system to be a buffer, flow side volumiser, return side volumiser or entirely absent with just a few valve changes... 😁 

You and these valves need to get a room pal !