Flow rate needed

[jamesdiyer]

Fyi taking the strainer out made no difference to flow, so it's not the issue. 

[SimonD]

3 hours ago, jamesdiyer said:

The heating circuit is always going to be lower flow than DHW due to the increased pressure drop,

 

Not necessarily. There are multiple factors at play:

 

• Heat demand
• Delta T
• Flow rate
• Flow velocity
• Pump rating
• System control settings

At the moment you're trying to test and commission the system at an ambient temperature of about 20C. At this temperature your radiators are not going to be able to dump the heat into the atmosphere, which is why I'm slightly surprised you are seeing a delta T of 5C at full load right now, unless the system is modulating its output and therefore its flow rate.

 

Also, with Vaillant heatpumps, output to dhw is managed according to a measured delta T so Flow rate will vary too.

 

[jamesdiyer]

And I confirm those two valves connecting to heating inside after 3way are dn20 so 16mm internal. They shouldn't have been there. 

 

So we will take those out and see then see about upgrading the 25mm plastic. Then the elbows. Then see if the full pipe need be re run in 35 copper/40pñ

[jamesdiyer]

8 minutes ago, SimonD said:

 

Not necessarily. There are multiple factors at play:

 

• Heat demand
• Delta T
• Flow rate
• Flow velocity
• Pump rating
• System control settings

At the moment you're trying to test and commission the system at an ambient temperature of about 20C. At this temperature your radiators are not going to be able to dump the heat into the atmosphere, which is why I'm slightly surprised you are seeing a delta T of 5C at full load right now, unless the system is modulating its output and therefore its flow rate.

.

 

 

It was -2C here yesterday morning. 

 

But yes it's modulating a lot and I'm not doing delta 5 much. It's trying to run aroun 25-30C temps and then cycling. 

[SimonD]

1 hour ago, jamesdiyer said:

 

It was -2C here yesterday morning. 

 

But yes it's modulating a lot and I'm not doing delta 5 much. It's trying to run aroun 25-30C temps and then cycling. 

 

It's looking like you've got multiple things going on with the system and you need a proper design in place that looks at the system as a whole. 

 

Best wait for the Vailant person and see what they say.

[jamesdiyer]

35 minutes ago, SimonD said:

 

It's looking like you've got multiple things going on with the system and you need a proper design in place that looks at the system as a whole. 

 

Best wait for the Vailant person and see what they say.

Yes agreed. 

[MikeSharp01]

Finding pressure drops for components in heating systems can be a pain and while I was looking to do a more detailed analysis of our system prior to install I came across this paper from 2019 that gets well into the weeds of the pex-al-pex pipe and fittings. Well worth a look if you want to understand the details and see the difference a coupler, a union and direct connection. 

 

Link is: https://www.e3s-conferences.org/articles/e3sconf/pdf/2019/26/e3sconf_eko-dok2019_00045.pdf

 

 

 

[jamesdiyer]

On 27/10/2025 at 09:04, MikeSharp01 said:

Finding pressure drops for components in heating systems can be a pain and while I was looking to do a more detailed analysis of our system prior to install I came across this paper from 2019 that gets well into the weeds of the pex-al-pex pipe and fittings. Well worth a look if you want to understand the details and see the difference a coupler, a union and direct connection. 

 

Link is: https://www.e3s-conferences.org/articles/e3sconf/pdf/2019/26/e3sconf_eko-dok2019_00045.pdf

 

 

 

 

Thanks. A basics mental exercise makes it quite easy to understand why a joint with a smaller internal diameter would be bad. 

 

Vaillant here Thursday so let's see what they say. 

[jamesdiyer]

Well. Vaillant have been, they said the flow rate is a bit low at 1750 not 2095. But more that not hitting 995l/h at 50% pump effort, it's doing 960l/h.

 

Their suggestion was to run a buffer tank like planned, and questioned why I'd be running a volumizer at all. And said I need to arrive at 2095l/h the norminal to get guarantee active..

 

Seems very much like, computer says X. 

 

Stuck between a bit of a rock now. Don't really want buffer tank, but need to hit 2095 and also high flow at 50% - apparently. 

 

 

Edited 16 hours ago by jamesdiyer

[jamesdiyer]

Need to speak to installer. I don't see how I won't end up with a buffer really. As he wil say that's how he wanted to install it and it's going to change the pipe to 32 internal (from 28 now)

[JamesPa]

34 minutes ago, jamesdiyer said:

Need to speak to installer. I don't see how I won't end up with a buffer really. As he wil say that's how he wanted to install it and it's going to change the pipe to 32 internal (from 28 now)

Is the heat pump actually complaining.  If not leave as is and allow deltat to rise a bit.  Or fit an auxiliary pump slaved to the heat pump one, but no buffer.  Either is easier, cheaper and will likely give better performance than an unbalanced buffer,which is probably what you will end up with.

Edited 16 hours ago by JamesPa

[jamesdiyer]

How would I know if the heatpump is complaining? 

At the moment I don't believe it will matter, we have a heatloss around 9.5kW at -7C. So the machine is big and doesn't need more than what we have flow rate to happily provide that at delta t 5. The machine is big as I'll add a other 100m2 to the system in a year or two - at which point it will need more flow but I'll have to put a pump on that system as it'll be UFH so will change all 

 

He was talking about minimum flow at 50% of pump and the machine not starting or something, but it made sense. 

 

It's a situation that seems if flow rate is not raised guarantee isn't approved or so. 

Edited 16 hours ago by jamesdiyer

[JohnMo]

I had to add an additional pump for DHW due to distance cylinder is away from ASHP and some existing pipe sizes.

 

Just add to anywhere in the circuit, on CH side only not in the DHW part and run at a fixed speed. The original pump will then just modulate as needed, the secondary pump just makes up for the lost or missing head of the original pump. If you install near the 3 port valve you can select the secondary pump on only when DHW is off, either via a relay or if valve has it the secondary contacts.

Edited 16 hours ago by JohnMo

[jamesdiyer]

Thanks both, that sounds a good suggestion and saves changing the primary which I feel would be difficult to achieve without baring the cost. It's a shame it's not wider. I'll see if the installer will just do this, the vaillant man loves a buffer than (and running on expanded mode) but doesn't seem to mind how I run it, as long as it hits the flow needed. 

[JamesPa]

27 minutes ago, jamesdiyer said:

How would I know if the heatpump is complaining? 

It will stop altogether if it detects flow rate too low.  Mine did when the filter bunged up. 

 

BTW I suspect the mesh filter, normally fitted near the return inlet, has a fairly high pressure loss even when not bunged up. If yours is like mine a model with a larger area may well assist!

Edited 15 hours ago by JamesPa

[jamesdiyer]

Apparently running a secondary pump inline is not allowed, said something about cavitate.

 

His recommendation is change tubing to 32mm internal, from the 26mm internal now (22m total). And then buffer if needed. 

Edited 15 hours ago by jamesdiyer

[jamesdiyer]

9 minutes ago, JamesPa said:

It will stop altogether if it detects flow rate too low.  Mine did when the filter bunged up. 

 

BTW I suspect the mesh filter, normally fitted near the return inlet, has a fairly high pressure loss even when not bunged up. If yours is like mine a model with a larger area may well assist!

 

 

I did try running without filter mesh and made no difference to flow rate. 

[JohnMo]

11 minutes ago, jamesdiyer said:

 

 

I did try running without filter mesh and made no difference to flow rate. 

Looks like your head losses are pipe related maybe all the elbows used etc. I would put a Wilo 7m head pump in, then you get the ability to dial in the correct settings to get the flow you need and have some flexibility.

[Dillsue]

1 hour ago, jamesdiyer said:

It's a situation that seems if flow rate is not raised guarantee isn't approved or so. 

If your concerned about the guarantee the your pretty much going to have to do what Vaillant suggest! That seems to be increase pipe sizes or add a buffer. 

 

What does the installer say about Vaillants comments?

[jamesdiyer]

I've sent them the vaillant report and await their response. 

 

I want the installer to increase pipe size first (and remove as many connections as possible) and see if we hit flow. If not then we can add buffer and pumps. 

 

I think the installer will just want to install the volumizer as a buffer and be done. Which imo is just masking a bad install. 

 

I don't know why the vaillant guy loved a buffer and expanded mode so much. 

 

Edited 14 hours ago by jamesdiyer

[jamesdiyer]

This is what the vaillant report said: 

 

>Direct aerothermal installation with 2 radiator circuits and a 200-liter return tank. The minimum flow rate for heating, with the pump at 50%, is 9641 l/h, and at maximum, 17171 l/h, and for domestic hot water, 1615 l/h. These are low flow rates because the piping is 32 mm multilayer and everything is supplied by the unit's own pump. It is recommended that you use 1.25 inch piping, the return volumetric tank as a buffer tank with a sludge separator on the return line, and separate pumping units for each circuit because you may have flow rate problems that would not be covered by the appliance's warranty. You have purchased components from Germany, and a different part number is listed. You will speak with the installer to modify the installation and improve flow rates.

[JohnMo]

15 minutes ago, jamesdiyer said:

32 mm multilayer ... It is recommended that you use 1.25 inch piping

32mm and 1.25" are the same nominal size, so I assume the writer is actually saying install 32mm copper pipe instead of the multilayer? Thought you were in Europe, so no idea why they refer to imperial sizes, which aren't specified in the UK any more (not for the last few decades anyway) let alone in Europe. Generally sounds a bit of numpty.

 

52 minutes ago, jamesdiyer said:

expanded mode

Quick search "Expanded mode" on a Vaillant heat pump system is a setting that combines weather compensation, room temperature influence, and traditional thermostat on/off switching. It will aggressively heat to meet a target temperature and then switch the heating system completely off once the target is reached, turning it back on when the temperature drops below a certain threshold.

 

So run it like a boiler - not great for running costs and he's one of the experts?

[jamesdiyer]

Yes 32mm internal. And again, it's a situation of computer says. He doesn't run a heatpump at home so it's only from what the computer (vaillant training) has told. 

 

The installer has said he's happy to change the pipe, though there will be a small cost to me - which seems fair.

 

The issue with 40mm plastic is it's 4m long bars which will need connectors that reduce flow. The alternative is 35mm copper which has full bore connectors. And hope this change is enough to bring the flow rate up and avoid fitting a buffer. My calculations make me feel it will be. 

 

 

[JohnMo]

I would replace as much for copper as you can - multi layer ok for UFH pipes not sure I would allow it elsewhere. Hep2O is bad enough and they have pretty much full bore connectors, the multi layer connectors I have seen all look have reduced bore.

[jamesdiyer]

It's worth noting I've a cop of 4.2 over the past two weeks (heating only). And the house is warm, we haven't been to -7 for a week but have been to -2 overnight. This is also with cycling, given it's mid autumn weather. The HP runs maybe half the day. (I think cop isn't far off accurate as the flow and return temps with heating off but pump on read the same temperature).

 

I think our heat demand is 9kW (with .5 ACH number). I don't think with the current flow we will have an issue being warm now - but we might when I finish renovating and need to heat another 100m2. 

 

Right now I'm actually quite satisfied with how it's working. A couple bedrooms are a bit colder, but their radiators are undersized, which actually is advantagous as downstairs can thus be 21 and upstairs naturally finds itself around 19 in most bedrooms, for the curve flow temp. 

Edited 12 hours ago by jamesdiyer