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We are getting ready to redo our bathroom, and we were hoping to change our tub/shower from a double-handle control to a single dial control. One of our neighbors (same condo building, different unit) recently made the same change and said that her water pressure dropped significantly when she installed the dial control. What would cause this? Is there a way to install the single-handle control that would not lose so much water pressure?

Update:

Just a quick update. We just had the contractor install the rough-in, dial, and shower head with no specific instructions, and we haven't had any pressure or flow issues! Our neighbor must have just had a bad install.

David K
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  • Did they put in a new shower head at the same time? The change in flow rate may not be due to the valve but to the shower head. Their old shower head may come from the pre-restrictor time or it may have a 2.5 gal/min restrictor and unbeknownst to them the new shower head has a 2.0 or 1.4 gal/min restrictor. The shower head should have the max flow rate written on the face of it. – Jim Stewart Jan 06 '17 at 16:19
  • 2.5 gpm is the max flow allowed in a shower head sold in the US and in my opinion this is abundant flow rate. Some water short states or localities may have lower limits. As far as I know the shower valves themselves do not have flow restrictors in that range; the intended flow restriction is in the shower heads. The fact is that the flow restrictors in the heads are easily removed or modified to increase flow. – Jim Stewart Jan 06 '17 at 16:46
  • The shower heads are tuned for good performance at a particular flow rate. They'll do ok in a near range, but if you are in or past the low end of their range, the stream will start sagging. Just like jetting a carb. – Harper - Reinstate Monica Jan 06 '17 at 17:02

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Does this person have the professional competence to know the difference between water pressure and flow?

Because the new thing in showers is "low flow" showerheads (often by law). Meanwhile, any 1-handle valve must have an anti-scald feature which throttles back the hot water. It's hard to make that work over a wide range of flows. So of course they optimize for the low-flow heads common in new-construction. With a "classic" head, it may come out limp-wristed.

You can do a bucket and stopwatch test on yours and (with her cooperation) hers. I bet the difference is significant.

I prefer a nice deluge, but even more, I prefer a long shower. Low-flow showerheads are not some sad wimpy affair. They feel like normal showerheads. If flow is too low, They can be a little jumpy when used with classic valves.

The dirty secret behind the dial valve is they are being forced down the throat of the American public by government agencies to reduce the statistical occurrence of scalding in showers, that itself is on the rise due to government recommendations to set water heaters to scalding temperatures to kill bacteria which can cause Legionaire's disease. (Thanks Jim Stewart, for the knowledge, not the disease). Sigh. Pull one string, and three other things unravel. In all fairness, Legionaire's is a new disease only first seen in the 80's and has taken quite some time to figure out.

Or, do as many of us have done, and get an on-demand hot water heater set to a sane temperature... And keep your valves.

Oh, and here's another trick. The notion of drywalling up the shower valve is most decidedly non-Victorian. In most Victorians I've lived in, on the wall behind the mixing valve, there's either a tasteful cabinet door with a latch, a closet, or a built-in bookshelf with a false back. That makes it ridiculously easy to gain access to the mixing valve and tub drain linkage, which makes a swap a very practical DIY.

Rockhorse Park
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Harper - Reinstate Monica
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  • The temperature of stored water in a hot water tank should be around 140 F to prevent the growth of Legionella. This does present a scald hazard. I personally have a tankless water heater so I don't have to worry about bacteria in stored warm water. This is a bottom-of-the-line first-generation natural gas fired tankless. When I changed to very low flow shower heads the heater suddenly became erratic--would turn off in the middle of a shower. (And we have the old style double handle shower mixing valves.) I tried drilling out the restrictors, then took them out of the shower heads altogether – Jim Stewart Jan 06 '17 at 16:33
  • Me too with the on-demand. Same problem, my low-low flow showerhead is because of a consumer grade cheapie on-the-showerhead chlorine filter, not my idea. Ijust gave up trying to mix, and use straight hot. – Harper - Reinstate Monica Jan 06 '17 at 16:44
  • That is an alternative I was trying for awhile: reduce the output temperature of the tankless heater and use all or nearly all hot water at the tap. I think this might reduce scaling and lengthen the lifetime of the heat exchanger in the tankless heater, but then if you want 120 deg water from the tap you don't have it. Some modern tankless have a remote control for setting temperature, but ours doesn't. – Jim Stewart Jan 06 '17 at 16:53
  • No, I would not say this person would know the difference between pressure and flow. So if I'm understanding this, the issue is more to do with the flow rate at the showerhead and is not actually caused by the dial valve? And this can be adjusted by removing or widening the flow regulator, as described here? – David K Jan 06 '17 at 18:04
  • I would say it's caused by the dial valve being designed and tested for a showerhead of 1.5-2.5 gpm flow, which is the modern standard. The valve has a huge part in controlling flow, and you can't expect it to be able to deliver wildly beyond its design flow. If you bring too little flow to a shower head, the stream sags and it looks pretty pathetic. – Harper - Reinstate Monica Jan 06 '17 at 18:55
  • Time to get some facts about your neighbors' shower head. What is the flow rate on it? Is it hand-held or fixed? You could tell them that it is possible to increase the flow of the shower head (if the shower head is what is limiting the flow as I think it is) by getting another calibrated flow restrictor or by modifying the existing restrictor or removing it entirely. They have the single handle valve in place so they should focus first on what is easily changed--the restrictor in the shower head. Remember, we have not even established that they put in a new head. – Jim Stewart Jan 06 '17 at 21:57
  • There are lots of different manufacturers and models of these single handle shower valves. Some of them get better reviews than others. Some of them have two concentric handles one for temperature and one for flow rate. What is the brand and model of your neighbors' new shower valve? The old two handle shower valves are no longer allowed to be installed in most jurisdictions, but AFAIK one does not have to remove the old type unless one is redoing a bathroom. – Jim Stewart Jan 06 '17 at 22:25
  • What is the difference between flow and pressure? I am in a similar situation and its clear to me that my pressure is lower. – AdamG Sep 10 '18 at 15:26