understanding secondary hot water returns

[Dave2010]

hey guys,

I have been asked to investigate a problem where a lengthy run is taking nearly 5 mins to have sufficient hot water out of the tap.
The system is unvented and has a secondary return with a pump on the return just before it re-enters the cylinder. The pump is an old BOSS brand and is blue in colour and I am unsure if is stainless steel or brass/bronze internals which I am led to believe is what these system require to prevent rust. There is pressure gauges either side of the pump but there is very little difference in their readings which are around the 2.5bar region. The pump does appear to be working. I feel this pump is just simply not up to the job of pumping to the extremities of the loop. There is also no check valve after the pump/before re-entry into tank.

Is there a definitive way to determine the pump is sufficient and working correctly before splurging money on a new bigger one?
Is there a way of working out what size pump is required?
Should a check valve be fitted?

thanks

 

[swbjackson]

Pump doesn't need to be very powerful, all it is supposed to do is maintain a circulation around the system so that there is minimal delay before hot water gets to the tap.

I assume you have the G£ qualification to be working on an unvented set up.

 

[ctp]

Its not a pump its a circulator its not meant to develop pressure ,iwould not expect to see much if any gain in pressure, if the pipe gets hot behind the pump its working.

 

[Dave2010]

I assume you have the G£ qualification to be working on an unvented set up.

Nope.

Its not a pump its a circulator its not meant to develop pressure ,iwould not expect to see much if any gain in pressure, if the pipe gets hot behind the pump its working.

Yeah my bad, I should have wrote circulator. The pipe is hot behind the pump but as there is no check valve after the pump and I was wondering if it could be the actual tank back feeding past the pump, if you know what I mean? I have seen a setup in the past of a vented system where the pump was working and return pipe was hot about 300mm before the pump but cold thereafter. The hot water was pushing past the pump even though it was running.

thanks guys

 

[Dan Robinson]

I assume you have the G£ qualification to be working on an unvented set up.

Nope.

 

[Dave2010]

how far do you go though here then??

Changing the taps is effectively working on the unvented system so can I not even do this now???
What about general maintenance such as draining down to clean etc??

 

[Norcon]

The secondary return system is not part of the cylinder so no G3 needed.

 

[Dave2010]

hey again guys. Wanted to revive this for further advice a bit

Should this secondary return be "balanced" on a long run? The secondary loop appears to be working but it is only hot to the first drop point, then not far after this its cold.
Also rather than the flow and return joining at/near the sink, the link is made above ceiling meaning a good few meters before outlet.

Depending on the above answer, may raise a couple more questions

 

[Norcon]

Yes when more than one loop they need balancing. Should be balancing valves fitted.

 

[Onetap]

I assume you have the G£ qualification to be working on an unvented set up.

Never a truer word written by a typo.

 

[Onetap]

Should this secondary return be "balanced" on a long run? The secondary loop appears to be working but it is only hot to the first drop point, then not far after this its cold.
Also rather than the flow and return joining at/near the sink, the link is made above ceiling meaning a good few meters before outlet.

Yes. If they're not balanced, all the flow from the secondary pump does a quick short circuit around the loops(s) nearest to the pump. The further loops get no circulation.

The balancing valves need to be capable of regulating very low flow rates. Those used for heating are fairly useless.

Danfoss do some thermostatic valves that are good but quite ££, I believe. They stay fully open until the water in the pipe reaches 50 or 55 deg C and then start to throttle down, directing the flow elsewhere.

http://danfoss.ipapercms.dk/Heating/AutoGen/36503_41875/?ForceLanguage=en-UK

You have to achieve 50 degC within 60 seconds from all hot taps, otherwise you do have a legionella risk that may involve legal penalties in any commercial premises.

 

[Dave2010]

excellent, I thought that might be the case.

The link provided is great and I will have a more in depth look into costs etc. However cant balancing be done via gate valves/lockshields etc as I have seen multiple heating systems done this way?

My additional question now you have kindly answered is......I checked out another system of even larger scale and they do not have balancing valves either but their system works perfectly. The only difference I appear to have found is the "link" is made at the tmv at the end of the "drops" rather than in the roof like mine? How can this be?

 

[Norcon]

Less delay when close to the tmv.
I think the guys who fitted yours just said "rollicks... tmv too far away" so just left the loop connection in the roof space.

 

[Onetap]

The link provided is great and I will have a more in depth look into costs etc. However cant balancing be done via gate valves/lockshields etc as I have seen multiple heating systems done this way?

Yes, in theory. In practice they're both a waste of effort.

Gate valves only regulate over the last turn or half turn and then generate noise. The first time any maintenance is done on the taps, the gate valve will be shut and then fully opened on refill. Double-regulating valves were developed for this application but, the flow rates involved on HWS systems are tiny and below the range of many DRVs.

I've no idea how the system you mentioned worked, but there must be some balancing devices in it somewhere. Reverse return? Flow regulators (may look like ball valves)?