Flow rate needed

[JohnMo]

2 minutes ago, jamesdiyer said:

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 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. 

All makes sense, but the limited flow rate will naturally cap output ultimately. So ok for now, but may be an issue later when installer has been paid and moved in to other things and may be difficult to re engage.

[jamesdiyer]

Just now, JohnMo said:

All makes sense, but the limited flow rate will naturally cap output ultimately. So ok for now, but may be an issue later when installer has been paid and moved in to other things and may be difficult to re engage.

Yes agreed. So best to get it sorted now. Installer called me right up after sending the report and was happy to engage and work to remedy issue in manner I'm happy with. So we will get there in sure. 

[jamesdiyer]

Can somone point me to a post on how a true buffer tank would work. I'm assuming  it just does it exactly the same way as it would without a buffer (running true WC) and then that heated water in the buffer is pushed around the circuit by a secondary pump and then is returned colder which retunes to heatpump. The inefficiency people talk about comes from lost heated water that's mixed in the buffer and secondary pumps. 

 

I'm still not sure we would hit the magical 2000l/h from just changing the primaries - it would be close and depend on what losses are incurred by the connections and 90s. 

 

At 1750l/h we have a pressure of 70kPa. I calculated the current primary as 11.5kPa and that reduces to 4.5kPa for 32mm internal. Being generous we come to 60kPa which moves is to 1900l/h. This does give a 11kW at delta 5C or 13 at 6. The 15kw machine is capable of 11.3kw at -7C for 45C - so having more flow doesn't seem needed, but perhaps I'm overlooking something. 

 

I'm inclined to think a simpler system the better. But maybe I'm making a mole hill and just re plumbing the volumizer we have as a buffer is just ok too. 

 

Shame this subject seems to be a bit of a (unnecessary) art and argument on what's best with different experts. 

 

 

 

 

Edited 17 hours ago by jamesdiyer

[JohnMo]

43 minutes ago, jamesdiyer said:

Can somone point me to a post on how a true buffer tank would work

Do a search on 2 port, 3 port and 4 port buffers on Google. If I was doing a buffer a big one only which you have. But would only do 2 port nothing else. This way a fully open system mostly flows around the buffer.

 

A port buffer looks like this

 

A video worth watching, talks 4 and 3 port a 2 port is a simplified 3 port

 

[sharpener]

1 hour ago, jamesdiyer said:

Can somone point me to a post on how a true buffer tank would work.

 

Think there is a useful article on the Caleffi web site.

[SimC]

At 1750l/h we have a pressure of 70kPa. 

 

Where was the 70 kPa measured?

If across just the pump, what was the pressure drop across the HP heat exchanger?

[jamesdiyer]

1 hour ago, SimC said:

At 1750l/h we have a pressure of 70kPa. 

 

Where was the 70 kPa measured?

If across just the pump, what was the pressure drop across the HP heat exchanger?

 

Hi, maybe I've made a false assumption, but I read it from the pump graph from vaillant posted above. Flow rate l/h v Residual pressure kPa. Dies it not work like this? If we have flow of X at pump running 100% then it's working against pressure Y? 

 

 

Edited 14 hours ago by jamesdiyer

[JamesPa]

5 hours ago, jamesdiyer said:

The inefficiency people talk about comes from lost heated water that's mixed in the buffer and secondary pumps

Nearly.  The loss of efficiency comes from the liklihood that cool return water is mixed with warm flow water thus reducing the flow temp to emitters relative to the ft from the heat pump.  Since the flow temp to emitters governs their output, you need to increase the ft from the heat pump to compensate.  The cop of a heat pump reduces with increased ft hence loss of overall cop.  

 

It an be made to work with little or no loss, but for that the water in the buffer needs to  be properly stratified, which has implications for buffer design and size, and the pump rates also need to be controlled.  This (it seems) almost never happens presumably because the people who know how to do it properly also know how to design out a buffer in almost all domestic situations.  The typical dumpy 50l buffer is unlikely to have any material stratification.

Edited 12 hours ago by JamesPa

[SimC]

2 hours ago, jamesdiyer said:

 

Hi, maybe I've made a false assumption, but I read it from the pump graph from vaillant posted above. Flow rate l/h v Residual pressure kPa. Dies it not work like this? If we have flow of X at pump running 100% then it's working against pressure Y? 

 

 

Assuming the pump is performing correctly at 100% speed, yes you can use the chart to lookup the reported flow value to determine the pressure generated by the pump.

 

I would have expected the Vaillant technician to have measured the pressure drop of the HP heat exchanger, or at least the residual pressure difference between the flow and return connections to the HP unit with the pump running.

 

The 70 kPa developed by the pump is all lost across the whole circuit including the HP heat exchanger, so if the heat exchanger loss is say 25 kPa then only 45 kPa is available to generate the required flow in your heating pipes and emitter system. The published Vaillant manual should at least provide the expected HP HEX pressure drop at design flow rate, and the residual pump pressure for your system.

 

My work involves interacting with air and water balance specialists usually in commercial buildings who validate by measurement with calibrated instruments that a system is performing as per the design criteria. By just relying on a unit display value, and not by seperate measurement, Vaillant have not proved that their unit including pump is actually performing to specification.

 

Obviously you have also highlighted concerns with sections of the distribution pipework sizing, that should also be resolved to improve the resulting flow rate when heating, but you could be chasing your tail if it turns out there is an issue with the pump not actually generating 70 kPa pressure.

[jamesdiyer]

6 hours ago, JamesPa said:

Nearly.  The loss of efficiency comes from the liklihood that cool return water is mixed with warm flow water thus reducing the flow temp to emitters relative to the ft from the heat pump.  Since the flow temp to emitters governs their output, you need to increase the ft from the heat pump to compensate.  The cop of a heat pump reduces with increased ft hence loss of overall cop.  

 

It an be made to work with little or no loss, but for that the water in the buffer needs to  be properly stratified, which has implications for buffer design and size, and the pump rates also need to be controlled.  This (it seems) almost never happens presumably because the people who know how to do it properly also know how to design out a buffer in almost all domestic situations.  The typical dumpy 50l buffer is unlikely to have any material stratification.

Thanks for the explanation. And yes it seems a buffer was initially specified here, because it just makes up for other inadequacies in planned inital install.

Edited 5 hours ago by jamesdiyer

[jamesdiyer]

6 hours ago, SimC said:

Assuming the pump is performing correctly at 100% speed, yes you can use the chart to lookup the reported flow value to determine the pressure generated by the pump.

 

I would have expected the Vaillant technician to have measured the pressure drop of the HP heat exchanger, or at least the residual pressure difference between the flow and return connections to the HP unit with the pump running.

 

The 70 kPa developed by the pump is all lost across the whole circuit including the HP heat exchanger, so if the heat exchanger loss is say 25 kPa then only 45 kPa is available to generate the required flow in your heating pipes and emitter system. The published Vaillant manual should at least provide the expected HP HEX pressure drop at design flow rate, and the residual pump pressure for your system.

 

My work involves interacting with air and water balance specialists usually in commercial buildings who validate by measurement with calibrated instruments that a system is performing as per the design criteria. By just relying on a unit display value, and not by seperate measurement, Vaillant have not proved that their unit including pump is actually performing to specification.

 

Obviously you have also highlighted concerns with sections of the distribution pipework sizing, that should also be resolved to improve the resulting flow rate when heating, but you could be chasing your tail if it turns out there is an issue with the pump not actually generating 70 kPa pressure.

 

Thanks for the explanation. That's interesting. 

 

The graph, which I didn't copy well I re attach. It is 'residual.pressure available'. 

 

Page 39 here https://www.vaillant.es/product-images/0020326641-03.pdf.pdf

 

Let me have a think about your point, though I think it would be a case of pushing vaillant to confirm that, after having changed piping. 

 

I'm just concerned about pushing the installer to change piping, but not for a buffer, and then still not hitting this magic flow rate. And then having the fit a buffer still, or just not fitting a buffer if we hit a flow rate I deem ok.