Two questions about a bypass valve

[@TonyC]

I have a central heating system with two zone valves, one for the hot water circuit and the other for the radiator circuit. I have a bypass valve installed between the flow after the pump and the return to the boiler. In case it helps, the bypass valve is a Taconova AV23 SETTER Inline with a scale (measuring range) of 8.0 - 30.0 litres/minute. The valve has a window through which you can view the float (attached to a spring) which measures the flow rate through the valve. The valve also has a screw adjustment.

Question1:

I was always under the impression that the bypass valve remained fully closed until the system enters bypass mode – that is, until both zone valves close, the heat exchanger in the boiler stops, but the pump carries on running for a certain period of time in order to dissipate the residual heat in the heat exchanger. However, recently, I noticed that when I switch on either the hot water, or the central heating, or both, the float in the bypass valve moves from its rest position and measures a small flow through the bypass valve. In other words, there appears to be a flow through the bypass valve even when the system is not in bypass mode. Is this to be expected? Or is it symptomatic of a badly adjusted bypass valve?

Question2:

When you adjust the bypass valve by setting the pump speed to 1, placing the system in bypass mode, and turning the adjustment screw with a screwdriver so that the float is set to the boiler minimum flow rate, what exactly are you doing? Are you setting the flow rate at which the bypass valve will open? Or are you ensuring that, no matter what the pump speed is subsequently, the flow through the bypass valve when the system is in bypass mode will always be at least at the boiler minimum flow rate? Or what?

 

[stevetheplumber]

in an ideal worl there would be no flow through the bypass when either valve was open but there will always be some especially when only one valve is opened

 

[doitmyself]

The Tacanova Setter is not an Automatic Bypass Valve, which opens when the differential pressure across the valve exceeds the set value. It is a flow control valve.

Which boiler and pump do you have?

 

[Nostrum]

Sounds like a British Gas install?

 

[@TonyC]

The Tacanova Setter is not an Automatic Bypass Valve, which opens when the differential pressure across the valve exceeds the set value. It is a flow control valve.

Which boiler and pump do you have?

That is a revelation - thank you. British Gas informed me that it is an automatic bypass valve. So, could you please be more precise as to what is being achieved when the Taconova Setter valve is adjusted by means of the adjustment screw?

If it is important, my boiler is a Worcester Bosch Greenstar 24Ri and my pump is a Grundfos ALPHA2 15-60 130. One of the problems I have is that the ALPHA2 Installation and Operating Instructions refer only to two types of bypass valve - a manually operated bypass valve and an automatic bypass valve (thermostatically controlled). I don't think my bypass valve is either of these. I have sent an email to Grundfos technical support about this but have not yet received a reply.

British Gas service and maintain my central heating system, but I do like to know what they are doing with my system and why. Hence the queries I have.

 

[@TonyC]

Sounds like a British Gas install?

Yes, it is. British Gas installed my boiler, a new hot water cylinder, 2 zone valves, the pump (which has since needed replacing), the bypass valve, TRVs on the 4 radiators in the bedrooms, and all the electrical controls in 2009. They also installed 28mm pipes between the boiler in the garage and the pump and zone valves in the airing cupboard.

Why do you ask? Should British Gas have installed a proper automatic bypass valve instead of a flow control valve? Is it undesirable to have a flow control valve instead of an automatic bypass valve?

 

[doitmyself]

my pump is a Grundfos ALPHA2 15-60 130. One of the problems I have is that the ALPHA2 Installation and Operating Instructions refer only to two types of bypass valve - a manually operated bypass valve and an automatic bypass valve (thermostatically controlled). I don't think my bypass valve is either of these. I have sent an email to Grundfos technical support about this but have not yet received a reply.

I thought it might be an Alpha when you mentioned setting it to speed 1 for adjusting the "bypass".

What setting is the pump on in normal use?

It would be interesting to know what Grundfos advise.

Here is some useful information: Grundfos Alpha Documents

 

[@TonyC]

What setting is the pump on in normal use?

A good question! And the one which started this investigation.

The ALPHA2 Installation and Operating Instructions state that for a "manually operated bypass valve" and a two pipe radiator system (which mine is), the recommended speed setting is AUTO[SUB]ADAPT[/SUB].

For an "automatic bypass valve (thermostatically controlled)", the recommended speed setting is the lowest or highest constant-pressure curve (I've not investigated these fully and I wouldn't know how to choose which one to use).

British Gas, when they installed the ALPHA2 pump last year wanted to put it permanently on speed 3. But the pump made such a loud high pitched hum at that speed and so I complained. British Gas have now relented and appear happy for me to run the pump at speed 2. And that is the speed setting at the moment.

Personally, I would love to use AUTO[SUB]ADAPT[/SUB]. The pump is so quiet on that setting and it uses only 8W of power.

I've already download all the ALPHA2 documents from the Grundfos Web site. Does the link you provide refer to any additional ones?

 

[doitmyself]

British Gas, when they installed the ALPHA2 pump last year wanted to put it permanently on speed 3. But the pump made such a loud high pitched hum at that speed and so I complained. British Gas have now relented and appear happy for me to run the pump at speed 2. And that is the speed setting at the moment.

I suspect the BG person didn't understand Autoadapt etc, so he set the pump to something he did understand.

I've already download all the ALPHA2 documents from the Grundfos Web site. Does the link you provide refer to any additional ones?

Yes. The docs are not available on the Grundfos site, which is a pity.

What is the current setting of the Tacanova?

 

[@TonyC]

What is the current setting of the Tacanova?

First, to answer your question, the Taconova AV23 SETTER Inline valve is currently set at D (I hope that means something to you?).

So what I have I learnt so far?

The Taconova valve is not a valve in the sense that it opens and closes. The valve is continually open and the rate at which water can flow through the valve is controlled by the screw adjustment. And the spring mounted float, in conjunction with the scale on the window of the valve, simply measures the rate of flow.

I guess this means that, where the ALPHA2 Installation and Operating Instructions distinguish between a "manually operated bypass valve" and an "automatic bypass valve (thermostatically controlled)", what I have is definitely the former. Furthermore, in the document entitled “Bypass commissioning Alpha2 or 2L”, which I downloaded from the link provided, my valve falls into the category of “Fixed Manual Bypass”. Indeed, in the section with that title, it states, “…Grundfos recommends the use of the Taco Nova Setter valve with flow gauge.”, which is probably one reason why British Gas chose to use that valve in my system.

I have read through the relevant documents referenced by the link. They all have the Grundfos logo, so I assume they are Grundfos documents. Whilst they have provided a little more insight into adjusting a bypass valve in the presence of an ALPHA2 pump, I didn’t understand everything. But I am not sure that it matters. My primary objectives are simply to check that I have a correct configuration and that the bypass valve is adjusted correctly.

In summary, the procedure for adjusting the Taconova valve is as follows. Set the screw adjustment on the valve to the maximum flow setting (I presume this is the F setting on my valve), switch the system on with the ALPHA2 speed set to I, after a little while switch the system off so that the system enters bypass mode, and then turn the screw adjustment so that the float on the valve is set to minimum flow rate of the boiler. After the valve has been adjusted, for my system, the pump can be set to AUTO[SUB]ADAPT[/SUB] or the highest proportional-pressure curve (PP2).

Assuming this procedure is correct, I have further questions. According to Worcester Bosch, the minimum flow rate of my Greenstar 24Ri boiler is 5 litres/minute (about 0.083 litres/second). However my Taconova valve has a scale (measuring range) of 8 - 30 litres/minute. So the obvious question is, how do I adjust the valve to 5 litres/minute? Has British Gas installed the wrong Taconova valve? (There is another Taconova valve in the same series with a scale of 4 - 15 litres/minute.)

I have asked these questions in an email to Tacotherm Technical Support (Tacotherm are the UK marketing agents for Taconova) but I have not yet received a reply. If I don’t receive a reply, and no one reading this post knows the answer, I will simply turn up at their office, which, fortunately, is only a couple of miles from where I live.

 

[doitmyself]

I have read through the relevant documents referenced by the link. They all have the Grundfos logo, so I assume they are Grundfos documents.

Just to put you mind at rest, I found this on Linkedin:

Paul Carne

Current
Technical Sales Engineer at Grundfos Pumps Ltd
Past
Product Applications Technical Support Engineer at Grundfos Pumps Ltd


I will try to reply to your other questions tomorrow.

 

[doitmyself]

According to Worcester Bosch, the minimum flow rate of my Greenstar 24Ri boiler is 5 litres/minute (about 0.083 litres/second).

Where did you get that info from?

 

[@TonyC]

Where did you get that info from?

I got it direct from Worcester Bosch Technical Support at [email protected]. I won't reproduce the whole reply I received, but here are the relevant sections:

The Ri boilers' minimum flow requirement is 5l/m, ...

The minimum flow rate across the primary heat exchanger in the boiler is 5l/m. It is this figure which is also used for the minimum system bypass flow rate, as this ensures there is enough water flow to stop the boiler from overheating after the boiler has shut down, at the end of a heating or hot water demand.

Do you think the information is wrong, or do you find it difficult to believe?

 

[doitmyself]

I got it direct from Worcester Bosch Technical Support. ...

Do you think the information is wrong, or do you find it difficult to believe?

No, I think it is quite correct. It's just that the info does not appear in the installation manual, so naturally I was curious as to where you obtained it. It's approximately the same as the flow rate you would need at minimum output with a 20C differential.

I think British Gas have supplied the wrong valve. Your one would be suitable if you had to maintain a minimum flow at Max output (24kW) which is approx 18 l/m. However, I'm not surprised as many Installers think that minimum flow means at max output. This is because the manufacturers' only supply this figure in the manuals - if they supply any flow info at all.

It might be worth asking Grundfos or Taconova which valve they recommend.

What flow rate do you get when the heating/hot water is on?

If you want to eliminate flow through the Setter, except when the boiler is in pump overrun, you would have to fit an Automatic Bypass Valve in series with the setter.

 

[@TonyC]

The required minimum flow rate of Greenstar Ri boilers is not published in the Installation, Commissioning and Servicing Manual. I did point this out to Worcester Bosch Technical Support and they replied (verbatim):

However we have informed our management team of this request for a minimum flow rate and have suggested this may be published in future manual updates.

I didn’t understand what you meant by:

It's approximately the same as the flow rate you would need at minimum output with a 20C differential.

But please don’t take the trouble to explain unless you think it is important for me to know.

I still haven’t received a reply from Tacotherm and so I need to chase them up. As I stated in a previous post, I live near to their office and so I could just turn up on their doorstep. I’m a little suspicious about Tacotherm’s support for Taconova valves. The sheet containing the instructions for adjusting Taconova bypass valves, which the British Gas engineer used, appears to suggest that Tacotherm support only two valves, one for 15 mm pipes with a scale of 3 - 12 litres/minute and second for 22 mm pipes with a scale of 8 - 30 litres/minute (this is the one I have). And yet Taconova have a far greater range of AV23 SETTER Inline valves.

You asked about the flow rates in my system. My ALPHA2 pump, as I explained previously, is currently set at speed II (2). With this setting, when there is a demand from the hot water cylinder, the flow rate through the bypass is about 6 litres/minute. (As already mentioned, the scale on the bypass valve is from 8 - 30 litres/minute. So, to estimate 6 litres/minute, I had to extrapolate by noting the distance between divisions on the scale and how far below the 8 litres/minute mark the float is when the demand is there.) When the system enters bypass mode (pump overrun), the flow rate increases to 12 litres/minute.

I don’t know whether I want/need to stop all the flow through the bypass when there is a heating or hot water demand. From a simplistic point view, it would seem very inefficient if a significant amount of heated water is being diverted from the flow from the boiler, through the bypass, and directly into the return to the boiler instead of being used to heat the radiators or the hot water cylinder. Is there any way of assessing how inefficient this arrangement might be? (One crude way might be as follows. 6 litres/minute flow through the bypass when the hot water cylinder is demanding heat. 12 litres/minute flow through the bypass in bypass mode. Therefore, with the pump at constant speed II (2), only 6 litres/minute is flowing through the coil in the hot water cylinder. Therefore 50% efficiency only - that doesn’t seem too good to me!)

I’m still mindful that Grundfos, in one of the documents you referred me to, recommend the Taconova Setter valve. But those documents were written in 2011, so it might be worth checking whether their recommendation has changed.

I’ll report back if and when I get replies from Tacotherm, Grundfos and, possibly ultimately, British Gas.

 

[doitmyself]

I didn’t understand what you meant by: It's approximately the same as the flow rate you would need at minimum output with a 20C differential.

Basically it's because many installer seems to think that the minimum flow rate refers to the rate at maximum output, which is the only figure shown in the manual. This probably explains why BG install a valve with 8-30 Range.

You asked about the flow rates in my system. My ALPHA2 pump, as I explained previously, is currently set at speed II (2).

You said earlier that the flow rate was set with the pump set to 1. This is OK if the pump is then set to Autoadapt, PP1 or PP2, but then setting fixed speed 2 will immediately negate the setting. Why do you not use Autoadapt?

If a fixed speed is essential you don't need the setter valve; it can be replaced by an Automatic Bypass valve.

I don’t know whether I want/need to stop all the flow through the bypass when there is a heating or hot water demand. From a simplistic point view, it would seem very inefficient if a significant amount of heated water is being diverted from the flow from the boiler, through the bypass, and directly into the return to the boiler instead of being used to heat the radiators or the hot water cylinder. Is there any way of assessing how inefficient this arrangement might be?

At max output the boiler needs about 18 l/min. If 6 litres is being returned, unused, to the boiler, the return temperature will be increased, which reduces the efficiency of the boiler. It also means that only 12 litres are being used to heat the cylinder, which is about 16kW instead of 24kW.