fezster

I have a 32mm mdpe incoming. Static pressure can be between 3.5 and 6 bar, depending on time of day. Did a small test today. My PRV is after my water softener. With the shower running (approx 20 l/min), dynamic pressure is 2.5 bar. I bypassed my softener - given everything discussed above, I expected the dynamic pressure to increase, albeit slightly. It didn't budge. So a little confused, as there's no way a softener can't have some pressure loss across it.

Can you explain what this means? A PRV is pretty basic in it's operation - a spring loaded diaphram controlled valve. The force of the spring maintains the outlet pressure, regardless of inlet pressure. Maybe if you could provide an example of a dynamic pressure PRV vs a static pressure PRV? The "drop tight" PRV's guarantee that as well as controlling dynamic pressure, they will not allow the static pressure to creep up under no-flow conditions. That's the only difference I'm aware of. Yes, you are correct. I'd definitely gotten myself confused with my own setup. This video here clearly explains the difference in pressure with varying pipe sizes: https://practical.engineering/blog/2021/4/6/flow-and-pressure-in-pipes-explained What I would say, though, for the example of the water softener and a PRV either side is that this would only be beneficial if the dynamic pressure is higher than whatever you would set your PRV to. In my case, as the dynamic pressure is already under 3 bar, a single PRV before or after the softener would maintain anything up to 3 bar. As it happens, my PRV is actually after my softener. Do any domestic dwellings have a dynamic pressure over 3 bar for a flow rate of > 20L/min? Certainly in London, you'd be lucky to get anywhere near that.

My PRV is "drop tight", so the static pressure won't creep up. I'm not sure what a "dynamic prv" is? Mine is a Reliance 28mm Predator 315i. I do agree with your point that the dynamic pressure may be limited by the incoming supply pipe. What I'm not sure about is whether the dynamic pressure can ever differ anywhere in a pipe run - ie. If you have a 100m pipe with 20l/min flowing through it, then surely the dynamic pressure is the same at any point in that pipe run? If that is true, then having a prv at any point in that pipe run will surely not matter. The pipe run itself has a pressure loss, just as any fitting or water softener has losses, and therefore a prv before or after a fitting/softener shouldn't affect the overall dynamic pressure the pipe run is capable of providing. Not sure if I've explained that very well, but that's how I make sense of it in my head.

I'm not an expert, so happy to be corrected. However, I have done a lot of reading on this topic and my testing on my own setup as well seems to suggest (to me!) what I described above. As an example. My incoming static pressure is up to 5 bar during off peak. Measured using a gauge at an outside tap. My prv (with gauge) is set to 3 bar. If I run a shower (approx 15 to 20 l/min), the gauge drops to 2.5 bar dynamic. If I increase my prv to 4 bar, I still get 2.5 bar dynamic. If the pressure loss was downstream only, I'd have expected the prv to have a fixed drop across it (ie. 0.5 bar). However, my understanding of it may be incorrect. That's what I've posted - to get input from more knowledgeable people.

The PRV will try and maintain 3 bar regardless. The pressure at any given point in the system with water flowing through it (notwithstanding pressure loss due to change in height), should be the same. So in your example, if there is 0.5 bar pressure loss due to the softener, but the incoming mains can provide 3.5 bar, then the PRV will open to maintain 3 bar downstream of it. It is odd, though, that the manufacturer instructions say otherwise.

I have a Tapworks AD11. It has 22mm inlets/outlets and I believe they quote >50L/min. Having a dual chamber seems unnecessary to me, given the softener recharges overnight, with no outlets running.

What is the advantage in doing this? I'm not trying to be facetious here - but based on my understanding, having 2 PRV's like this is pretty pointless, and in fact detrimental because the pressure drop across the PRV itself is significant, so you're compounding that by having two of them. Taking your example of 6 bar incoming (I assume this is static?) with a 5 bar PRV before the water softener. When you open outlets, the dynamic pressure will reduce as the flow rate increases, depending on pipe size. Let's say it drops to 4 bar (which would be significant at > 20 L/min, but for arguments sake). The pressure drop across your water softener takes this down to 3.5 bar. Your second PRV at 3 bar after the water softener would reduce this down to 3 bar. So you have 3 bar going to your outlets at 20 L/min. Now let's remove your 6 bar incoming PRV and replace it with a single 3 bar PRV (remove the PRV after your softener). At 20L/min, your incoming pressure is 4 bar. The 3 bar PRV will reduce this down to 3 bar. You have a 0.5 bar loss through your water softener, but the 3 bar PRV will maintain 3 bar, because you have sufficient pressure behind it to do so. So you have 3 bar going to your outlets at 20 L/min. The end result is the same - except you're not losing additional pressure through a second (unnecessary, highly restrictive) PRV. Of course, the PRV itself, in either scenario, should be as large bore as possible to maintain adequate flow rates.

Yes, this is exactly what I did. Reliance 315i pressure reducing valve. Except I teed my balanced cold in 28mm and upgraded to 28mm in and out of the UVC. I also had a separate 22mm relief valve fitted. The pressure loss across the 28mm PRV is still considerable at higher flow rates, but there's not much you can do about it.

What size is the pipework internally? There are calculators online you can use to try and predict what the pressure drop will be, although it is not entirely easy to predict how this will translate to real world. In general, though, you want to try and have 22mm pipework all the way to shower valves, and with a 32mm incoming mdpe, I went for 28mm pipework between the mains and the PRV (which was also upgraded to 28mm) to the unvented cylinder and out, before reducing to 22mm.

I'm sure I've read that plastic pipe has less friction, although I don't have reference to where I saw that right now. However, plastic pipe does have a smaller internal diameter than the equivalent copper and the fittings are far more restrictive, so that negates any benefit of less friction.

Where in the country are you? 70 L/min sounds very optimistic! I am in the SE and my 32mm MDPE has been measured at around 50 L/min off peak, but at any peak time it drops to around 20-25.

You'd be better off with HW priority. Having HW and CH running at the same time seldom works as you'd expect. The flow temps required by your heating will be lower than that required to fully heat up your cylinder to 60-65 degrees and having both running will mean water is unnecessarily circulating through your HW circuit as the CH slowly ramps up to temperature. Instead, with HW priority, the HW tank is given full power to heat up as quickly as possible, with modern UV cylinders having very fast recovery times. The added advantage being that if your boiler supports it, you can run the CH at lower flow temps (including using weather compensation), whilst still allowing your HW cylinder to utilise maximum flow temps.

This only works if your incoming dynamic pressure (ie. With outlets open, rather than static/no flow) exceeds 3 bar / 3.5 bar. The prv will always maintain the pressure it is set at (but not exceed it). So yes - if you have > 3 bar at your upstairs shower with the shower running, you'll get that extra pressure by having the pressure maintained 2 stories up. Have you measured the dynamic pressure at 2 stories up with outlets open? I'd be surprised if it exceeds 3 bar (though, it's not impossible).

Generally, it is better to pipe the EV on the return to the boiler. If on the flow, there is a chance of reverse circulation. Also, if you require an external EV, I'd recommend a heat-only boiler - this has no integral pump or EV. This makes future maintenance far easier, as you do not need to open the boiler to change either of these and / or check the EV pressure. You can also size the pump and EV to your requirements.

@rikakka One easy way of checking if the accumulator is holding water and working is to allow time for it to charge (see if you can hear it filling and/or watch the pressure guage on the inlet to see it slowly rise from dynamic to static pressure), then switch off your main stopcock. You should still have water from your taps until the accumulator depletes.