New boiler installation lacking hydraulic separation?

[MNW67]

Yeah, that makes no sense to me, but if you look at my attachment/calcs, it looks like you would have around a residual head of 1.99M (pump at 100% speed), at the boilers rated flowrate of 1305LPH, 21.75LPM, not a lot, but obviously the residual head increases with reducing flow demand as the pump head increses and the HEX pressure loss decreases = greater residual head, for example at a typical flowrate, like mine, of 1000LPH, 16.67LPM, the pump head is 5.71M, Hex loss, 1.56M, residual head a fairly healty 4.15M?.

I don't understand that calculation, I'm afraid.

I actually just did a very simple calculation to work out what flow was needed for a 20C dT with a 35kW output. That works out at 1500LPH. And then I looked at the residual head graph and it seemed to show at 1500LPM there was zero residual head.

 

[Johntheo5]

I don't understand that calculation, I'm afraid.

I actually just did a very simple calculation to work out what flow was needed for a 20C dT with a 35kW output. That works out at 1500LPH. And then I looked at the residual head graph and it seemed to show at 1500LPM there was zero residual head.

Yes, I've corrected the spreadsheet, but the (Vaillant) residual graph is, at the vey least, highly confusing, because while the residual heads at 100mbar, 300mbar & 400mbar look fairly correct with the pump running at full speed, they certainly are not if the pump is running in its default mode and setting of constant pressure and 200mbar set point or at 100Mbar or 400mbar.

 

[MNW67]

Yes, I'd agree with that, although the chances of this place actually needing 35kW based on what we've seen so far are minimal

@Johntheo5

I am just posting because I want to make sure I haven't confused you both with my post further up this thread. On further checking, the manual for the 635 states that remaining head is actually 2.5m at 1537LPH. So, I do not actually know what the graph I posted above means.

 

[Johntheo5]

If the HEX dP is "only" 2.5M @ 1537LPM (25.62LPM) then a 6M pump with a head of 4.0M @ this flowrate (1537LPH) leaves only, 4-2.5, 1.5M residual head, very very doubtful if that is sufficient to circulate that vol of water through any normal system, if the boiler circ pump was/is a 8M (or even 7M, then a 8M will run with a 6.6M head leaving, 6.6-2.5, 4.1M @ 1537LPH, enough to circulate that vol through the system, then again, Vaillant's have a internal by pass which comes set to something like a ridiculous 2.5M might still by pass even if fully shut, Vaillant sometimes seem to me to push for a LLH, best avoided if possible, a heat only boiler might be a better choice for someone with high power/flow requirements.

On the other hand a flowrate demand of around 1200LPH, 20.0LPM, will satisfy a heat demand of almost 21kW (20.93), at a dT of 15C, the HEX dP will then be 2.5*sq of (1200/1537), 1.52M, a 6M pump will run with a 4.75M head at this flowrate to leave a residual head of, 3.23M, not bad?.

 

[MNW67]

If the HEX dP is "only" 2.5M @ 1537LPM (25.62LPM) then a 6M pump with a head of 4.0M @ this flowrate (1537LPH) leaves only, 4-2.5, 1.5M residual head, very very doubtful if that is sufficient to circulate that vol of water through any normal system, if the boiler circ pump was/is a 8M (or even 7M, then a 8M will run with a 6.6M head leaving, 6.6-2.5, 4.1M @ 1537LPH, enough to circulate that vol through the system, then again, Vaillant's have a internal by pass which comes set to something like a ridiculous 2.5M might still by pass even if fully shut, Vaillant sometimes seem to me to push for a LLH, best avoided if possible, a heat only boiler might be a better choice for someone with high power/flow requirements.

On the other hand a flowrate demand of around 1200LPH, 20.0LPM, will satisfy a heat demand of almost 21kW (20.93), at a dT of 15C, the HEX dP will then be 2.5*sq of (1200/1537), 1.52M, a 6M pump will run with a 4.75M head at this flowrate to leave a residual head of, 3.23M, not bad?.

I think Vaillant use UPM3. For a boiler this size it will probably be a 7m. So maybe something similar to this. I think there is about a 3.5m head loss through this heat exchanger at 1500LPM, so that seems to work out about right.

I am still confused as to why that other graph shows zero remaining head!

 

[BSM8]

As an update to this, the original installer was unresponsive to any questions regarding boiler sizing and pumps being in series. I asked two other plumbers one of which said the install is fine and another who said that pump separation is needed and that boiler is way to big.

I contacted the gas safe register to ask if they had any guidance about this (they seem like the ones who would know best) and they recommended that an inspection be carried out to check the installation. I will let you know the results if you are interested!

Is there anything specific I should mention to the inspector when he comes? I am not sure how much the gas safety aspect overlaps with building control regulations when it comes to things like boiler sizing and pumps in series - will he raise these as an issues? I am probably overthinking it but after this experience I want to try and be prepared as possible to get the job done right.

Thanks for all your help

 

[Johntheo5]

As an update to this, the original installer was unresponsive to any questions regarding boiler sizing and pumps being in series. I asked two other plumbers one of which said the install is fine and another who said that pump separation is needed and that boiler is way to big.

I contacted the gas safe register to ask if they had any guidance about this (they seem like the ones who would know best) and they recommended that an inspection be carried out to check the installation. I will let you know the results if you are interested!

Is there anything specific I should mention to the inspector when he comes? I am not sure how much the gas safety aspect overlaps with building control regulations when it comes to things like boiler sizing and pumps in series - will he raise these as an issues? I am probably overthinking it but after this experience I want to try and be prepared as possible to get the job done right.

Thanks for all your help

How many rads in your system?, double/singles?

Has the UFH got its own manifold pump?

Do you know how to accesss the boilers diagnostics, D, parameters, if so can see what mode/setting the circ pump is running in, the boiler comes (circ pump) set in constant pressure mode set to 200 mbar, a (only) 2M pump head, maybe thats why the installer stuck in that extra circ pump.