Priority cold water flow to combi

[hometech]

I understand that its normal for output flow from a combi to reduce when the inlet cold water flow is reduced (when other outlets are opened) but is there a method/process/configuration that can be adopted to minimise the effect in the the drop in flow and in doing so, provide a priority flow?

It jumps to mind that having a storage tank to feed the combi would provide a solution but as I've never seen this in any property, I'm guessing its not an answer (but would love to know why)

If I replaced the 15mm cold feed to the combi with a 22mm pipe and then had a connection (tee) from this pipe to a 15mm take-off for all other outlets and positioned this tee as close as possible to the combi inlet, would this provide a priority flow rate of any discernible value?

How about reducing the flow in the existing take-offs, is this what you do? (and by how much to make a difference)

Or is there another option that my non expert mindset has yet to consider?

 

[Timmy D]

There are storage tank options. Look up mains accumulators.

In essence, its a tank like an unvented cylinder that charges itself from the mains at mains pressure during periods of no demand.

Then when there is demand it releases its water back into your system to supplement usage at multiple outlets at the max flow rate provided from the street until empty. It will recharge again during no demand.

Say you’ve got 14lpm into the property and coming out one outlet. A second outlet is opened and you’ve now got 2x7lpm. The accumulator would use its storage to boost both outlets back to 14lpm until it ran out of stored water.

You can also pump these tanks to increase flow rate into the 100lpm range. Enough to run multiple showers.

 

[Scott_D]

What’s your flow and pressure from the incoming mains?

 

[Knappers]

There are a few things you can do, this is how I would approach your issue.

1: Fit flow limiters to your outlets, many taps will have optional diffusers rated at different flow rates (6lpm at basins, 8-12lpm shower etc) you can also buy iso valves with interchangeable limiters. Your choices would be lead by your incoming flow

2: Accumulators are a great option to increase flow for a limited time and would likely do the job.

3: If flow rate is poor then there are some wras approved mains boosters to help a bit

4: look at incoming main and meter. Water supplier will charge a fair chunk to change to a 22mm meter but if you've got a restriction at meter it can only help.

5: storage tank and booster is likely an option if all else fails, never seen on a combi so would have to check with boiler manufacturer. Tank should be a proper sealed and vented cold storage tank in my opinion (insulated fiberglass, not a black plastic tub wrapped in a binbag altho it may be compliant) I would use a DAB e sybox as a booster.

 

[Ric2013]

Reason storage systems not generally used is probably due to the fact that a combi is usually chosen to save space and cost rather than technical incompatibility. If an accumulator is part of the original plan, then the balance starts to shift in favour of installing a system boiler and cylinder in the first place.

 

[Knappers]

Obviously you should look at eliminating restrictions in pipework too but I dont think trying to encourage the water to flow where you want with pipe size is going to do much if anything

 

[Knappers]

Reason storage systems not generally used is probably due to the fact that a combi is usually chosen to save space and cost rather than technical incompatibility. If an accumulator is part of the original plan, then the balance starts to shift in favour of installing a system boiler and cylinder in the first place.

A good hw cylinder with integral accumulator is the norm, 100% agree with above and if you already have a combi it doesn't rule one out.

 

[hometech]

What’s your flow and pressure from the incoming mains?

6 bar static and 14 litres per minute
second tap open drops pressure to 2 bar

 

[hometech]

There are a few things you can do, this is how I would approach your issue.

1: Fit flow limiters to your outlets, many taps will have optional diffusers rated at different flow rates (6lpm at basins, 8-12lpm shower etc) you can also buy iso valves with interchangeable limiters. Your choices would be lead by your incoming flow

2: Accumulators are a great option to increase flow for a limited time and would likely do the job.

3: If flow rate is poor then there are some wras approved mains boosters to help a bit

4: look at incoming main and meter. Water supplier will charge a fair chunk to change to a 22mm meter but if you've got a restriction at meter it can only help.

5: storage tank and booster is likely an option if all else fails, never seen on a combi so would have to check with boiler manufacturer. Tank should be a proper sealed and vented cold storage tank in my opinion (insulated fiberglass, not a black plastic tub wrapped in a binbag altho it may be compliant) I would use a DAB e sybox as a booster.

so with the pump option it could use a regular unvented tank right?

 

[hometech]

Reason storage systems not generally used is probably due to the fact that a combi is usually chosen to save space and cost rather than technical incompatibility. If an accumulator is part of the original plan, then the balance starts to shift in favour of installing a system boiler and cylinder in the first place.

The combi was chosen for its ability to supply low temperature flow around the under floor heating circuit and wall radiators

 

[hometech]

Never mentioned this but it might be relevant. The combi hot water flow is controlled by the phone so if i'm having a shower, I can set @ 35 degrees, a bath @ 48 degrees and so on. As the output is temperature regulated, any drop in flow is detected and the flow from the combi is reduced so as not to deviate from the set point. So the issue to resolve would be to provide a constant flow/pressure that is not effected by turning on taps.

 

[Knappers]

The output from storage tank and booster would become your new 'mains' for hot water.

I don't think you want to go that far as ric2013 said an unvented cylinder becomes more practical before that.

I don't know how your measuring but 14lpm at 2bar would suggest a restriction issue somewhere

I would also consider a pressure reducing valve on incoming main also

 

[Knappers]

Pressure reducing valve
Flow restrictors
Accumulator

 

[Ric2013]

I mean why a combi, rather than a system condensing boiler that can also provide low temperature flow? Assuming that system boiler has controls that can then run at a higher temperature when it knows it is heating the cylinder (not all do - which I think is very silly). Of course, no reason you cannot add a tank or accumulator now, just saying most people don't bother.

Regarding your controls - it sounds interesting.

@Knappers - what if he put a PRV set at 1.75bar before the boiler inlet and then teed the cold supply to any mixers after that - to provide balanced pressures at mixers (would also need check valves). Then he should have a steady pressure at the boiler inlet. Would that work?

 

[Knappers]

Yea could do with taking the edge off that pressure variance and an accumulator would buffer the changes further. Flow restrictors will help to balance flows, allow more outlets to be used simultaneously and increase the effectiveness of the other measures.

 

[Knappers]

I wouldn't fit the accumulator untill after doing the other 2

 

[Ric2013]

@hometech What actual event is causing the worst pressure drops?

 

[hometech]

The output from storage tank and booster would become your new 'mains' for hot water.

I don't think you want to go that far as ric2013 said an unvented cylinder becomes more practical before that.

I don't know how your measuring but 14lpm at 2bar would suggest a restriction issue somewhere

I would also consider a pressure reducing valve on incoming main also

The measurement was obtained by fitting gauge to one tap, turn on another and collect water in bucket and using a stop watch to determine time/volume. So I can collect flow but not pressure at the same time. What would reducing the incoming pressure achieve?

 

[hometech]

I mean why a combi, rather than a system condensing boiler that can also provide low temperature flow? Assuming that system boiler has controls that can then run at a higher temperature when it knows it is heating the cylinder (not all do - which I think is very silly). Of course, no reason you cannot add a tank or accumulator now, just saying most people don't bother.

Regarding your controls - it sounds interesting.

@Knappers - what if he put a PRV set at 1.75bar before the boiler inlet and then teed the cold supply to any mixers after that - to provide balanced pressures at mixers (would also need check valves). Then he should have a steady pressure at the boiler inlet. Would that work?

I did consider the european 4 pipe system but then I'd have to handle mixing temperatures and balancing and so on. If the boiler can do it for you, why guess. I have the same boiler in my Slovenia villa and they are very common over there and I have never had any issues with it.
The other important aspect here is consumption. Why heat water to then cool water? makes no sense does it
This sketch is how things are and the possible solution (if I understand).
Is this what you had in mind?

 

[Ric2013]

Not sure what you mean by the European 4-pipe system, so cannot comment on that.

I have to agree (against mainstream thinking) that it makes no sense to heat water to a high stored temperature to then blend it back down.

Not sure if you've got my idea with your sketch. Leaving the pipe sizing for now, what I was suggesting is that if the working pressure is around 2 bar, you reduce the pressure to your combi to just below that. Then the combi will not suffer from fluctuations in pressure because it will always be reduced to just below the mains pressure anyway. Ignore what I said about check valves - I was confusing with thermal stores!

If you have any mixer taps, then the cold and hot sides will need to be supplied at the same pressure, so makes sense to tee off the cold from the reduced pressure section of pipe (as per my sketch).

I would not expect a PRV to give 100% linear pressure over flow. If you want an absolutely steady pressure when you have a shower, it seems better still to avoid competition for water in the pressure reduced pipework. So you may want to use a second pressure reducing valve from the higher pressure pipe for your cold to any mixers. An outside tap and other non-mixers could come, without pressure reduction, from the high pressure zone, so long as your mains supply is good enough to maintain 2 bar at all times.

However, if the problem is other hot taps being opened (which you haven't suggested it is), this obviously will not solve that problem. Dont take my idea as being an absolute truth - it's more a suggestion for further consideration.

 

[hometech]

This issues relates to the use of a shower. Turning on another hot outlet while using the shower obviously drops the availability of water but it would seem the cold water is a bigger issue. I need to start collecting data through measuring flow rates and pressures and see if I can get an answer for the manufacturer as to pressure or flow which causes the reduction.
My guess is that it has to be flow of water through the heat exchanger rather than pressure. Is that a reasonable assumption?

 

[hometech]

the manufacturer states the impeller on intake measures flow not pressure. Am I right in thinking that I can forget about the differential of the pressure (static/working) as being a factor?

 

[Ric2013]

Well, the pressure will drive the flow so when the pressure drops the flow through both your boiler secondary heat exchanger and your shower will also drop. So if your shower and boiler had 6 bar and then a second tap opens, the pressure may drop and reduce the flow through the boiler/shower. The boiler will then compensate to maintain [a steady temperature, presumably] but at a different rate of flow.

As for the drop in pressure, I'd say 14lpm and 6 bar is good going. Makes sense to me that then opening a second tap would drop the pressure considerably. If all your stopcocks and isolation valves are fully open (worth checking the external one) there's not a lot you could easily do about it even if there were a fault.

Reading between the lines here (because I'm not really sure why it matters to you that the flow reduces so long as the temperature is stable), would I be right to think what you want to achieve is not just a steady flow through your shower, but a steady flow of at least x lpm?

 

[hometech]

Well, the pressure will drive the flow so when the pressure drops the flow through both your boiler secondary heat exchanger and your shower will also drop. So if your shower and boiler had 6 bar and then a second tap opens, the pressure may drop and reduce the flow through the boiler/shower. The boiler will then compensate to maintain [a steady temperature, presumably] but at a different rate of flow.

As for the drop in pressure, I'd say 14lpm and 6 bar is good going. Makes sense to me that then opening a second tap would drop the pressure considerably. If all your stopcocks and isolation valves are fully open (worth checking the external one) there's not a lot you could easily do about it even if there were a fault.

Reading between the lines here (because I'm not really sure why it matters to you that the flow reduces so long as the temperature is stable), would I be right to think what you want to achieve is not just a steady flow through your shower, but a steady flow of at least x lpm?

The steady flow is right but not necessarily a rate per min. I just want to create a situation where while having a shower, if a toilet is flushed it does not slow the shower down to a trickle.

 

[Ric2013]

Then let's take a step back.

What if you reduced the flow to the toilet cistern? As an experiment, you could partially close the isolator that feeds the toilet cistern fill valve. Cistern would take longer to refill, but this may not be a big issue.

At a guess, I'd say the toilet fill valve probably takes 6 litres per minute, which is not that high a flow. You say you still have 2 bar with 14 lpm.

I'm beginning to wonder whether your shower needs a very high pressure to work. If you open a tap instead of the shower and flush the toilet, does the tap also reduce to a trickle?