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?

 

[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?

 

[hometech]

Installing a new bathroom has been on the cards for a while now so this is a good time to think bigger. I'm going to have a combi dedicated to each bathroom (2) (I have the space and gas pressure) so now the water flow becomes an accumulator issue (will be requiring good flow to each combi simultaneously) Do I push the water from the tank to the combis using a pump or do I pump into the tank to create internal pressure for the output? Does that make sense?

 

[Ric2013]

You can get instantaneous gas water heaters that might be a bit cheaper than a combi?

I'm not the person with the best experience to comment on your pumping issue.

My understanding, however, is that you can get an accumulator that recharges from the mains pressure, or a mains booster that is a storage cistern with incorporated pump.

You could also just have a cold water storage cistern and then a pump or pumps coming off that, which may be cheaper and easier to maintain than the previous suggestions. Not sure what the downside may be: may be worth a separate thread?

 

[hometech]

Funny you should suggest that. The combi manufacturer does a continuous flow water heater so I've ordered one of those. I replaced the old stopcock which was on the 25mm mains which then dropped to 15mm then back to 25mm. In doing so, the pressure has jumped 2 bar.

 

[Ric2013]

Definitely will help 🙂 From the photo you post, that valve looks like it may be a pressure reducing valve, else I'm not sure why it would incorporate a pressure gauge. Such valves can become a slugish in action and unable to maintain a steady working pressure. If your standing pressure has increased, I'm thinking this was your case.

How's your shower now?

 

[hometech]

It is an adjustable regulator but on inspection after removal, i'm surprised it let anything through, it was so blocked. so now standing is 8bar, working is 6 bar and flow is 29l/min cold.

 

[hometech]

collected the data today. 8 bar static and 6.5 working. All outlets open, still 6.5 bar (how come?)
Method: 13l bucket to collect shower output to fill the bucket
Results:
no taps open, bucket fills in 1 min 20 seconds
1 tap open takes 1 min 22 seconds
2 taps open takes 1 min 24 seconds
More than 2 taps and it takes 1 min 26 seconds
So very little drop in flow or pressure which suggests I might get away with the supply being suitable for the additional continuous heater (which arrived today and is much smaller than I imagined it would be) and should be connected by early evening I'm told.
I'll then compare the two heaters outputs against time/temperature using the shower.