Flow rate needed

[jamesdiyer]

Hi all

 

I've had a 15kw vaillant aerotherm plus installed. I'm getting a flow rate of 1700l/h, pump set to 100%. Volumiser of 100L, no buffer. Only machine pump. Manual says max flow is 2050l/h.

 

I'm trying to determine if this is enough/acceptable. 

 

We have a heat loss off about 10kw at -7C. Unit does 12kw at -7. 

 

I understand heat = flow x delta t x 4.2

 

Is this delta t at the heat pump? 

 

The issue is, primary pipework was done in 26mm internal 22m total, and I'm frustrated by this, it's a 11.16 kPa loss v 4.2 kPa if in 32mm internal. 

 

Am I right in thinking if we had 32mm internal primary the flow rate would be higher and this we could carry more heat kW maximum for the same delta t? 

 

Is so what would the flow increase by?

 

This isn't my area of expertise so I'm trying to get my head around it. 

 

Thanks in advance. 

Edited Wednesday at 20:38 by jamesdiyer

[JohnMo]

45 minutes ago, jamesdiyer said:

32mm internal primary the flow rate would be higher

Piping size isn't the only thing affects pressure drop, ball valve strainers can kill flow, I took my strainer cage out and added a full flow filter instead, good improvement in flow, I also replaced 3 port valve for a high kvs one again more flow. Poor selection of anything in the circuit will kill flow.

 

11kPa is nothing of a pressure drop, so you are loosing head somewhere else. 11kPa is around 1m head loss. Not sure what size pump you have but it should be 6 to 8m head pump.

 

I have a flow rate of around 1320L/h for a 6kW heat pump. 

 

So things to look at

Valves fitted need to be full bore

Diverter valves look at a kvs of over 10

UFH ensure all flow meters are fully open

Radiators look at how the lock shields are set and balanced

Ball valve strainer, install a proper filter with low pressure drop instead.

 

[jamesdiyer]

Thanks for the pointers on things to look at, I will get on that.

 

Is 11kPa pressure drop in the primary pipework really nothing? It's 1m head, which could be 0.4m head if it were 32mm internal? Or maybe I'm missing something? (We're talking about only the 22m total primary here, not the rest of the circuit). 

 

After the diverter the pipework immediately splits into two circuits of 22mm copper each, which tee off into 15mm tails of about 6 radiators on each circuit. Neither had too bit a head loss as a result of this immediate split flow. All lock shields wide open. 

 

Can I please check the location of delta t, and what flow rate I need? 

 

Here is pump curve. Number 3, VWL155/6

Edited Thursday at 05:32 by jamesdiyer

[jamesdiyer]

This is the 3 way diverter. Is the cv value the K? And this its 9.5?

 

 

Edited Thursday at 06:00 by jamesdiyer

[JohnMo]

You need to calculate the pressure drop of the whole system. 1 bit of it's a bit meaningless without seeing the whole picture.

 

Have a look here for some guidance 

https://heatpumps.co.uk/technical/pressure-drops-flow-rates/

[Conor]

Is your heating system working as expected / adequately?

[jamesdiyer]

That's a good question. I don't know, as it's currently being installed. That's why I'm trying to understand now - if it's acceptable or not. Both before I pay the final bill, and before winter comes and when it's very cold we are not warm. 

 

 

[jamesdiyer]

I drew a plan and did some calcs. I stuck them in chatgpt, with a few prompts and we are both are 35kPa Inc +15% for fittings. Using flow of 1650l/h

 

Can Simone explain delta T, which is assume is at the heat pump. Must it be 5C, or will it be ok at 7C? 

I'm struggling to find answers to these online. 

 

When I asked vaillant they said to aim for flow of 2050. My assumption was that the primary pipe makes a huge difference flow final flow, as it's 11m each way. 

 

Thanks 

 

 

 

 

[JohnMo]

13 minutes ago, jamesdiyer said:

explain delta T,

dT is your flow dT from the heat source and is a variable. The bigger the dT the higher the output flow temperature is for a given mean temperature (heating output at radiators or UFH), so ideal is to run a low dT. But you can run any dT you like within reason. But as suggested by Vaillant, flow is king with with a heat pump. You need to get rid of bottle necks. 

 

So you need to get rid of pressure drops where practical or add additional pumping power to the system to overcome them, such as a second pump on the return piping run, set at a fixed speed to get full system flow where you need it to be.

 

 

[sharpener]

12 hours ago, jamesdiyer said:

I've had a 15kw vaillant aerotherm plus installed. I'm getting a flow rate of 1700l/h, pump set to 100%. Volumiser of 100L, no buffer. Only machine pump. Manual says max flow is 2050l/h.

 

I'm trying to determine if this is enough/acceptable. 

 

We have a heat loss off about 10kw at -7C. Unit does 12kw at -7. 

 

I understand heat = flow x delta t x 4.2

 

Is this delta t at the heat pump? 

 

The issue is, primary pipework was done in 26mm internal 22m total, and I'm frustrated by this, it's a 11.16 kPa loss v 4.2 kPa if in 32mm internal. d

 

Not sure where you are as you post the pump performance in Spanish! In the UK the largest Arotherm plus is 12kW which is what I have. The primaries are in 28mm (o/d) and are about 10 m to the point where the circuits split into 22mm to the rads and 22mm to the UFH. The pump in the outdoor unit is very powerful, we see flow rates >2000 l/hr and it all works fine, I don't think you have fundamental cause to worry with what they are doing. If they have economised by using a cheap filter get it changed to a decent Magflow, Inta or Adey one and keep it clean.

Here is a flow/pressure calculator which may be handy.

 

The other useful relationship is 1 kWh = 860 litre-degrees i.e. 1kW = 860 degree-litres/h.

[jamesdiyer]

29 minutes ago, sharpener said:

 

Not sure where you are as you post the pump performance in Spanish! In the UK the largest Arotherm plus is 12kW which is what I have. The primaries are in 28mm (o/d) and are about 10 m to the point where the circuits split into 22mm to the rads and 22mm to the UFH. The pump in the outdoor unit is very powerful, we see flow rates >2000 l/hr and it all works fine, I don't think you have fundamental cause to worry with what they are doing. If they have economised by using a cheap filter get it changed to a decent Magflow, Inta or Adey one and keep it clean.

Here is a flow/pressure calculator which may be handy.

 

The other useful relationship is 1 kWh = 860 litre-degrees i.e. 1kW = 860 degree-litres/h.

 

 

I'm in Spain, but British. The 15kw isn't available in the UK, so I used the manual here. I believe it's the same hardware as the 12 but can just do more power. 

 

Maybe something wasn't clear by me. The pipe is 32 od but it's multicapa plastic so 26mm internal - this i what I thought I'd wrong and for a run of 11m should be larger. 

 

Thanks I'll check the filter. 

Edited Thursday at 10:02 by jamesdiyer

[JohnMo]

1 minute ago, jamesdiyer said:

26mm internal

So just about the same as @sharpener 28mm of copper.

[Nick Laslett]

Too much glycol?

[jamesdiyer]

29 minutes ago, Nick Laslett said:

Too much glycol?

No glycol, antifreeze valves. 

[jamesdiyer]

Running the hot water heating, we get 1900l/h. So loose only 200l/h though heating circuit. 

 

 

 

This is my rough circuit sketch of what I believe to be the index radiator, a point D. 

 

 

Edited Thursday at 10:50 by jamesdiyer

[JohnMo]

What the two circles, one must be your cylinder what is the other. How does the return piping from the heating interact with the two circles as you only have return pipe from the two circles?

[jamesdiyer]

15 minutes ago, JohnMo said:

What the two circles, one must be your cylinder what is the other. How does the return piping from the heating interact with the two circles as you only have return pipe from the two circles?

Sorry, I did say rough and I meant it. Good question. 

 

One tank is DHW, the other is a 100L volumiser, plumbed on the return leg to the ASHP, radiator pipe in at top of tank, exit to ashp at bottom. 

 

DHW right, volumizer left. 

 

Edited Thursday at 11:51 by jamesdiyer

[JohnMo]

The issue with volumiser plumbed as is, (if I have understood correctly) during DHW heating cycle, you are also having to heat an extra 100L of water every DHW heat cycle.

 

You ideally have the volumiser in the central heating return only and tee in the DHW cylinder return after the volumiser. 

[jamesdiyer]

8 minutes ago, JohnMo said:

The issue with volumiser plumbed as is, (if I have understood correctly) during DHW heating cycle, you are also having to heat an extra 100L of water every DHW heat cycle.

 

You ideally have the volumiser in the central heating return only and tee in the DHW cylinder return after the volumiser. 

Okay thanks for the suggestion. This wouldn't have an effect on heating circuit or overall flow though no? 

 

 

[JohnMo]

1 minute ago, jamesdiyer said:

Okay thanks for the suggestion. This wouldn't have an effect on heating circuit or overall flow though no? 

 

 

No - no affect on ultimate flow rates through system just energy usage.

[Michael_S]

30 minutes ago, JohnMo said:

No - no affect on ultimate flow rates through system just energy usage.

May be the thought is the extra energy in the volumiser will give the heating a boost when switching back after a hot water cycle and the house possibly having cooled?

 

I think I read somewhere that plastic pipe is less 'smooth' when it comes to flow so 26mm internal diameter plastic has more resistance than the same for copper  (plus it may have inserts in the joints?)

 

The 860 number quoted above is a useful rule of thumb.

[jamesdiyer]

 

10 minutes ago, Michael_S said:

May be the thought is the extra energy in the volumiser will give the heating a boost when switching back after a hot water cycle and the house possibly having cooled?

 

I think I read somewhere that plastic pipe is less 'smooth' when it comes to flow so 26mm internal diameter plastic has more resistance than the same for copper  (plus it may have inserts in the joints?)

 

The 860 number quoted above is a useful rule of thumb.

 

.yes the fixings for plastic pipe are inside the pipe, versus copper which are outside, so each one has a greater effect on flow. 

 

It seems the k of that honeywell diverter is 8. So this might be something to look at changing. And check the filter as said. 

 

I'll wait and see how the system performs when it's really cold (cold here is -7C solid). 

Edited Thursday at 12:55 by jamesdiyer

[JohnMo]

8 minutes ago, Michael_S said:

read somewhere that plastic pipe is less 'smooth' when it comes to flow

Pipe roughness is about 0.0004k (mm) for multilayer. Copper new is about 0.00015 so smoother. By the time it's covered in deposits that could be 0.03. Your clutching at straws going down that route.

[JohnMo]

4 minutes ago, jamesdiyer said:

honeywell diverter

I would have as last on my list of things to improve. A kvs of 8 means it will flow 8m³h of water with a 1bar pressure drop. A kvs of 10 will leave the valve hydraulically invisible at heating flow rates even with glycol (you don't have). 8 is near enough.

[jamesdiyer]

32 minutes ago, JohnMo said:

I would have as last on my list of things to improve. A kvs of 8 means it will flow 8m³h of water with a 1bar pressure drop. A kvs of 10 will leave the valve hydraulically invisible at heating flow rates even with glycol (you don't have). 8 is near enough.

 

Great thanks. Perhaps I'm making a deal out of nothing and we won't have any issue being warm with this flow. 

 

We will see, it'll turn real cold early November. But February is the worst as solar very low and all thermal mass gone. 

Edited Thursday at 13:37 by jamesdiyer