Pump overrun bypass with variable pressure pump

[downthetube]

This topic has sort-of been touched on in a couple of other threads, notably 'Two questions about a bypass valve', but I haven't quite found the answer I'm looking for.

I have a Potterton Suprima 80 boiler (non-condensing, old-school) and recently replaced the old constant-speed circulating pump with a Yonos PICO 25/1-6. I want to operate this in variable pressure mode because of the electricity savings and reduced noise when the TRVs start to shut down. The boiler requires pump overrun and the system is fitted with a Honeywell DU144 bypass valve. This is not suitable for variable-pressure operation because pressure falls with flow rate, preventing the bypass from operating at the point where the zone valves close (S-plan) -- unless, of course, I have it set to low pressure, which means it's permanently open. What's the solution? I can't find consistent advice. I've read that a fixed manual bypass should be used (in a Grundfos document that I can no longer find). But I've also read that Part L requires a bypass valve to be automatic (though I can't find that anywhere in Document L1A or B, with 2016 amendments, nor in Part L itself). I've also seen that constant-flow valves exist, but they are intended for terminal branch balancing.

 

[fezster]

This may help:

http://www.newtonnet.co.uk/permanent/GrundfosDocs/BypassCommissioningAlpha2or2L.pdf

 

[downthetube]

This may help:

http://www.newtonnet.co.uk/permanent/GrundfosDocs/BypassCommissioningAlpha2or2L.pdf

Thanks, fezster. That's the document I was talking about. It says:

If an automatic bypass valve must be used, then a manual fixed bypass valve should be installed in series with the automatic bypass valve.

The only reason I know for including a (redundant) ABV along with the fixed valve is to satisfy a regulation. I really want to avoid this clunky arrangement and I can't find the regulation in the current documents. I've checked Approved Document L1A and L1B, 2010 with 2013 and 2016 amendments and Part L. Can anyone point me to the requirement that a bypass valve must be automatic?

 

[fezster]

The taco valve (or a simple gate valve) in series will ensure that only the minimum flow rate required through the boiler is maintained, thus conserving efficiency. When the pump is operating at the minimum pressure, the ABV will prevent even this flow from occurring, if set correctly.

I'm not sure about the regulation requirement, but the above seems the best compromise when you have a system where all zones could be closed in a pump overrun scenario.

 

[doitmyself]

Can anyone point me to the requirement that a bypass valve must be automatic?

You will find it in Domestic Building Services Compliance Guide, page 15, para 2.0b, which says:

If the boiler manufacturer's instructions advise installation of a bypass, an automatic bypass valve should be provided and the manufacturer' instructions on minimum pipe length followed.

 

[downthetube]

You will find it in Domestic Building Services Compliance Guide, page 15, para 2.0b

Thanks, doitmyself, got that now. I wonder whether a constant flow valve like the Taco can be considered an "automatic" bypass valve for the purposes of the Guide? It is performing an automatic function and, in the case of a variable pressure pump, it is the most appropriate control (a fixed-pressure bypass is not). I really hate the idea of bolting unnecessary stuff into the loop just to satisfy guidance that was written before the current generation of circulators came on the market.

 

[Ric2013]

I can't see the point of the differential pressure ABV with a proportional pressure pump either. It would let by when the flow was already high enough to satisfy the boiler's minimum requirement, and it would close when the boiler needed it. In fact, it would fail to protect the boiler from insufficient flow.

 

[fezster]

With grundfos pumps, the pressure is reduced to a minimum of 50%, so I'd have thought that at minimum pressure, it would perform it's useful function of allowing minimum flow through the boiler. However, because of the ABV, it would never reach minimum pressure, as the ABV would open and the pump would ramp up accordingly. Hence, the use of a fixed valve (or Taco setter) would ensure that the minimum flow is not exceeded unnecessarily.

As said, though, the downside is that the ABV is always letting by the minimum flow, which is inefficient.

That's my understanding anyway.

 

[hallra]

Hi All,

I have the same dilemma as the OP, I've just installed a Grundfos Alpha 3 that requires a manual bypass instead of an ABV to use Auto Adapt mode, however this is in conflict with building regs.

Do I

1) Switch the ABV for a gate valve.

2) Place a gate valve in series after the ABV
- Would this still interfere/confuse the pump modulation?

I have asked Grundfos directly, especially their requirement that conflicts with building regs, the response "its up to the installer, we don't give advice"

I did check with Glow-worm (I have a Ultracom 24 HXI), they recommend to use the alpha3 in AutoAdapt mode with a manual bypass valve, they also stated not to worry about minimum flow rates as the boiler doesn't have a minimum, which I was surprised about as the manual suggested otherwise.

I would like to get peoples opinions as I'm no expert and I can't find conclusive advice anywhere.

 

[Ric2013]

Hi All,

I have the same dilemma as the OP, I've just installed a Grundfos Alpha 3 that requires a manual bypass instead of an ABV to use Auto Adapt mode, however this is in conflict with building regs.

Do I

1) Switch the ABV for a gate valve.

2) Place a gate valve in series after the ABV
- Would this still interfere/confuse the pump modulation?

I have asked Grundfos directly, especially their requirement that conflicts with building regs, the response "its up to the installer, we don't give advice"

I did check with Glow-worm (I have a Ultracom 24 HXI), they recommend to use the alpha3 in AutoAdapt mode with a manual bypass valve, they also stated not to worry about minimum flow rates as the boiler doesn't have a minimum, which I was surprised about as the manual suggested otherwise.

I would like to get peoples opinions as I'm no expert and I can't find conclusive advice anywhere.

I think Grundfos used to prefer an automatic bypass valve that was not based on differential pressure. Trying to remember what they called it...

That said, not entirely convinced you would be in conflict with building regs. I'm assuming the approved document (which is only a guide, remember) doesn't say anything about Autoadapt either, so the whole point (as I understand it) of the ABV is to avoid the pump energy being wasted by a permanent bypass? If so, I'd have thought the energy saved by the pump slowing down at reduced demand would outweigh the energy wasted by the pump having to push a small amount through the manual bypass.

I'd go with what Glow worm said, if they weren't contadicting their written advice with verbal advice, which is what it sounds like!

 

[hallra]

Thanks Ric,

Yeah I will go with what Glow-Worm advised, just will keep a keen eye on it, as you can never trust especially when they contradict themselves...

What are your thoughts on the no minimum flow requirements, rather bizarre especially when a bypass was stipulated...

Obviously, if the boiler starts to complain then I can open up the bypass more, not very scientific/neat but it’ll work.

- With the autobypass in place at the moment, the pump all by itself last night dropped to 0.1 m3/h and the boiler didn’t even blink...