Pressure reducing valves not allowing flow on lower settings

[Engpat]

Hello,

About the system...
I am working in a large hotel and we are installing new domestic pipework to all bathrooms. The domestic hot water is constantly returned back to the boiler using a pump on the return in order to allow constant circulation of hot water.

The problem...
Each riser in the hotel serves 2 bathrooms per floor and there are 3 floors. At the bottom of each riser we have installed PRVs on the hot (HWS-F). The PRVs range from 1 to 5 bar. When the PRVs are set at anywhere between 1 to 4 bar, circulation is NOT achieved. The only way of achieving circulation is by full opening the PRVs. This is the case on 9 out of 10 of the risers.

Could there really be a fault with the batch of PRVs or can anyone think of anything else? Thank you

 

[dbill74]

Ditch the PRVs. What you need is a "circuit setter" to set the flow rate through each loop.

 

[PEDARRIN2]

I agree with dbill74.

The only time a PRV should be used on a domestic system is if the supply pressure exceeds the code allowed maximum (80 psig in IPC).

I have never seen putting them on the return piping.

What was the reasoning?

 

[Engpat]

Thank you for your responses.

Sorry if I was not being clear: the PRVs are positioned on the domestic flows (HWS-F) at the bottom of each risers. An example of the PRV in question is circled in the drawing below:

 

[PEDARRIN2]

I still think the PRV is wrong, because why would you not have one on the cold water riser if you are concerned about pressure to the plumbing fixtures.

I am familiar with pumps using psig and ft hd of water - so you will have to convert to units you are familiar.

5 bar setting would be 72 psig on the outlet, which would be an acceptable outlet pressure. Do not know the inlet pressure requiring the PRV so do not know the actual pressure loss across the PRV.

4 bar setting would add a minimum of 14.5 psig additional pressure loss to the system which the recirculating pump has to overcome. At ~2.3 feet head per psi, that would be to the 33.5 ft hd added to system.

Since recirculating pumps are typically low TDH, since they only have to overcome pipe friction (including the PRV) maybe the PRV is creating too much artificial pressure loss and exceeds the maximum churn (no flow)on the pump curve.

You might be churning the pump.

If you are concerned about pressure, why not put the PRV near the water heater, on the cold water connection, upstream of the recirculating pump loop. That way the recirculating pump is not trying to overcome the pressure loss of the PRV.

 

[LittleInch]

Yes. Tour ignoring the hydrostatic head that three return pipe needs to get back up 3 floors and then whatever the static pressure is at the end of the return line. You need to ļ at the entire system and loop.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PEDARRIN2]

Little Inch,

The supply pressure does need to account for this due to a faucet opening and making it an open system, but typical hot water returns are close looped so the pump sizing does not need to include the elevation.

 

[PEDARRIN2]

This article explains what I was trying to explain in a pictorial way.

It indicates two zones, which you might not need, but if you do, you need two water heaters.

http://jmpcoblog.com/hvac-blog/dome...roper-application-of-pressure-reducing-valves

 

[Engpat]

Thank you,

I have done some further investigations...
When the PRV is on lower settings (eg 3bar) and the hot tap is turned on, the water is choosing to travel up the return pipe rather than the flow.

When the PRV is fully opened (5bar), the circulation flows in the correct direction ie the pump is definitely pumping in the correct direction; when the hot tap is turned on the water travels up the flow.

I have isolated 9 risers (manually prevented any flow/circulation) to try and force flow around a single riser: flow is still not achieved in this one riser! Surely if the problem was all the PRVs were offering too much resistance for the circulation pump to overcome it should be able to overcome 1 PRV?

Please view attachment to see the full system:

 

[LittleInch]

Engpat - I can't work out how your system can actually work.

If, as you are currently doing, you try and lower the pressure in the hot water line, then the flow will just come via the return line - as you've proven. Only when the resistance to flow is high will the pressure valve release and the flow go the way you think it will go. either that or add non return valves to your return line connection on the header. Then you will get flow through the PRV, but you still won't get a return line flow....

Hence, IMHO, your PRVS are not only not restricting the pressure, but with multiple branches each with their own PRV and multiple flows from the users, the pressure in the return line will go up and down accordingly.

The problem with your 1 riser might be that you've just set your PRV too low and it doesn't have enough pressure to get round the loop on the top floor. either that or your valve is closed.

your only way to get any flow is to set all the PRVS the same or, as everyone else is saying, ditch the PRVS and put in manual control valves (on the return line connection back to the header??).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Engpat]

I agree that the PRVs in the risers seem to adding undue resistance to the circulation system…
The idea was to have PRVs, set at 5 bar, positioned on the boosted cold water inlet to the calorifiers. This protects the calorifers. Then I would have just under 5 bar of pressure in my main leg of pipework. This would ensure that a high pressure would be distributed to each riser and then it would be reduced down as it enters each riser using the PRVs.

How would you recommend regulating the pressure in the bathrooms?...
If I set the PRV entering the calorifers lower, the bathrooms that are closest to this PRV would have ample pressure but the bathrooms that are the other side of the hotel (100m run then has to travel up 2 floors) would have insufficient pressure by the time it arrived at these bathrooms.

Balancing the risers…
If you look again at the 1st drawing there is a double regulating valve (DRV) on the return (orange pipe) before it connects back into the main leg which returns to the header. This is designed to control the flow in each riser and allow me to the balance the risers. People have recommended circuit setters and manual control valves to be fitted on the return legs but the DRVs will do the same job?

 

[LittleInch]

your problem is that the return system pressure is governed by the pressure of the highest set PRV minus a bit created by the pump , but it is not clear whether the pump is sized to raise the pressure from this now lower pressure back up to the inlet pressure. E.g if your inlet cold water pressure is say 7 bar and your PRVs set to say 3 bar, then the pump will need to make up at least 4 bar to pump any water back into the hot system.

Thus even with all of the PRVs set to the same pressure, the variance between them will mean one is actually higher or lower than the rest. Therefore there will be no flow through that the return pipes other than the one set or operating at a higher pressure. e.g. if one actually works at say 3.5bar, ones regulating to the set pressure of 3 bar will be static when there is no flow into the bathrooms as you can't make water flow without a pressure difference

It also means that flow will try to go backwards through the return pipe first , as there doesn't seem to be any non return valves on the return piping, until the pressure drop through this smaller pipe is such that the PRV on the main hot pipe opens up.

These PRVS mean that your system is basically inoperable and they all need to be thrown away or moved to downstream of the tee off the hot water pipe but before the bathroom tap. That is the only way that such a system can actually work. This means one per bathroom and remove them from the risers.

The only other way you can make such a network be relatively indifferent to distance is make the header pipe several sizes bigger than you need for friction alone so that in essence it is the same pressure within say 10% regardless of flow or distance from the inlet. Costs more and can't really be retrofitted, but means you don't have the problem of different pressures.

I didn't realize what DRV meant - domestic water systems aren't my day to day business - I thought it meant Drain valve (!) So yes, these DRVs as you describe and vendors describe them is exactly what you need, but you need to sort out the pressure issue first before you can get any flow at all through your return pipes

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PEDARRIN2]

I guess i missed the fact you have a domestic booster pump to boost water pressure

Typical plumbing fixtures (and code) have a minimum and maximum pressure allowed. In my neck of the woods, it is 20 psig and 80 psig. If I cannot meet the 20 psig at the top, I have to boost the pressure. If the available static pressure exceeds 80 psig, I use a PRV.

When there are multiple floors, it gets tricky. But I never put a PRV immediately downstream of a domestic booster pump - sort of like pushing on the accelerator and brake of the car at the same time. I use variable speed drive pumps and do away with the PRV downstream of the pump.

With multiple floors, boosting pressure can be tricky. You have to do a pressure loss analysis and may have to have two systems, one on non boosted pressure to the lower floors and one with the boosted pressure for the upper floors. with this you would have to have two separate water heating systems as well (never mix them)

Your schematic indicates PRV on both the cold and hot water, which serves all floors. It also indicates a PRV at the water heating system. You are boosting the pressure with the pump, reducing the pressure at the pump with the PRV, at the water heating station, and then further reducing the pressure of both the hot and cold water at each of the risers. This makes no sense - at least with the information given.

If the DRV are the same as flow controllers, you have two in series (one where each floor of the riser and one at the base of the riser. This is bad design because they will work against each other.

There are many issues with this design as indicated and you will have problems getting it sorted out.