Instantaneous Gas Water Heater... umm, Problemmo...

Hello Everyone,

I’ve been taking a closer look at Instantaneous Gas hot water heaters lately, and they certainly look the goods except for one major flaw - Minimum Flow Rate. Yes, I said Minimum. This is the smallest flow rate required for the electronics inside these things to react and ignite the gas to start the water heating process. At flow rates below this, all you get out of them is unheated water...

The following are the published "Minimum Flow Rates" for a variety of popular models, and I have been told by technical reps from at least two of the companies concerned, that these figures are optimistic if anything:

Rinnai Infinity: 2.4L/min
Rheem Integrity: 2.7L/min
Dux Endurance: 3.0L/min
Bosch Highflow: 2.5L/min
Bosch Hydropower: 3.5L/min
Bosch Pilot: 2.0L/min (except 10P: 5.0L/min)

You can see that the Bosch Pilots (the 13P and the 16P at least) at 2.0L/min, are the best of a bad bunch.

Now tell me this... What happens up here in Sunny Old Queensland if you jump into the shower in the middle of summer and you don’t quite want an absolutely cold shower, but you don’t want a warm one either. You just want it to be lukewarm. I’ve never measured the temperatures involved, but my gut feel is that cold water straight out of the ground in the middle of summer up here is probably about 25deg, and that a comfortable shower with some grease-cutting ability for hair-washing would be at about 30deg in summer - ie. only a 5deg increase above the ambient cold water temperature. Unless you’ve got controllers wired into it, the hot water produced by your Instantaneous Gas unit hanging on the outside wall will be either 55 or 60deg (depending on the brand), unless you purchase one of the models that are factory preset to 50deg. Let’s say you do just that...

Now if your shower rose is flow-restricted to even as much as 9L/min (I say "as much" because some of the new ones are less!), then by my calculations, to have a 9L/min shower at 30deg, you would require 7.2L/min of cold water, and only 1.8L/min of 50deg water out of the hot water system. The result - none of the above units will even turn on! The problem would only worsen for even more efficient shower roses (which the government up here will eventually want us all to get), or for warmer ambient cold water temperatures, or for even more tepid shower temperatures, or for water heaters that weren't preset to 50deg...

Now if you went the way of controllers to get around the problem, you would need at least two of them - one for the shower, and another for the kitchen sink, laundry tub, vanity basin, etc - and you definitely can’t get two temperatures out of the things at the same time if someone wants to use the sink while you’re in the shower. What’s more, wiring the controllers into an existing cavity brick home would be almost impossible to do neatly...

I know there’s plenty of people out there who absolutely swear by them, but unless I’m missing something, I reckon the whole concept is a bit "half-baked"...

Have any of you had this problem with such units during hot weather?

Many Thanks,
Batpig.

I had a problem with my instantaneous HWS, that the hot water would shut off in the middle of the shower. Removed the flow restrictor from the shower head, no problems.. I use a Rinnai Infinity.

I think you're missing the point of the unit - you don't use the shower's cold tap when using one.

Set to 30 degrees and let it supply you with 9 litres a minute of 30 degree water.

Most (I'd say all, but I haven't explicitly looked for the absence of kitchen controllers) have a minimum of a bathroom and a kitchen controller; once the shower control is activated, the kitchen is locked out from increasing the temperature so the showeree doesn't get to resemble a boiled lobster.

There's nothing in the kitchen (barring coffee, and you use a kettle for that!) that can't wait ten minutes for really hot water while someone has a shower. Kinda like how you can't run lots of cold (or hot) water from another tap in the house with storage heating without the person in the shower complaining anyway.

The three after-construction installs that I am familiar with have all looked very neat; the wires for the controller don't need to drop down below the height of the noggins so the installer doesn't even have to play with long drill bits, and whatever modifications have been needed to the plumbing have been made without huge hole in wall dramas.

Put a solar unit on the roof, costs more initially. but it will pay its self off in the long run.:)

Quote:

---

Originally Posted by Batpig

Hello Everyone,

Now if your shower rose is flow-restricted to even as much as 9L/min (I say "as much" because some of the new ones are less!), then by my calculations, to have a 9L/min shower at 30deg, you would require 7.2L/min of cold water, and only 1.8L/min of 50deg water out of the hot water system. The result - none of the above units will even turn on!

Many Thanks,
Batpig.

---

What the sales man doesn't tell you is that in any case a flow restiricted shower head is not reccomended to be installed on an instant hws, you need to get a instant hws shower head or remove the flow restrictor.

Your exactly right in that the unit won't turn on, or it will stop and start.

Quote:

---

Now if your shower rose is flow-restricted to even as much as 9L/min (I say "as much" because some of the new ones are less!), then by my calculations, to have a 9L/min shower at 30deg, you would require 7.2L/min of cold water, and only 1.8L/min of 50deg water out of the hot water system. The result - none of the above units will even turn on!

---

I have a Methven Kiri Hi Rise Shower head (7.5L) connected to my Rinnai 26 which has 2 controllers and it functions perfectly.

Quote:

---

Originally Posted by Master Splinter

I think you're missing the point of the unit - you don't use the shower's cold tap when using one.

Set to 30 degrees and let it supply you with 9 litres a minute of 30 degree water.

Most (I'd say all, but I haven't explicitly looked for the absence of kitchen controllers) have a minimum of a bathroom and a kitchen controller; once the shower control is activated, the kitchen is locked out from increasing the temperature so the showeree doesn't get to resemble a boiled lobster.

The three after-construction installs that I am familiar with have all looked very neat; the wires for the controller don't need to drop down below the height of the noggins so the installer doesn't even have to play with long drill bits, and whatever modifications have been needed to the plumbing have been made without huge hole in wall dramas.

---

Dear Master, I did realise that running a controller and (mainly) one tap in the shower was one kind of solution, hence my comment:

Quote:

---

Originally Posted by Batpig

Now if you went the way of controllers to get around the problem, you would need at least two of them - one for the shower, and another for the kitchen sink, laundry tub, vanity basin, etc

---

but I still don't think this is a cast-iron solution, because the minimum selectable temperature with any of them when a controller is hooked up is about 37deg. Given that you have different temperature showers at different times of the year - no, heck! make that different times of the day for any given time of the year! - and that the ambient in-ground cold-water pipe temperature changes throughout the year, I reckon that you could still run into trouble even with a controller, due to the 37deg minimum. And when I then think about dialling one temperature after another from different ends of the house several times a day, instead of just using the tap to do both temperature and flow - as is the case with Mains Pressure Storage - I just can't help thinking that the whole Instantaneous concept is still half-baked. Wouldn't be the first time in recent history that substance lagged behind hype. Needs more thought, I reckon...

And don't forget, I'm cavity-brick, so no neat 'n easy retro-installs...

Quote:

---

Originally Posted by bricks

What the sales man doesn't tell you is that in any case a flow restiricted shower head is not reccomended to be installed on an instant hws, you need to get a instant hws shower head or remove the flow restrictor.

Your exactly right in that the unit won't turn on, or it will stop and start.

---

Thanks for confirming that Bricks. I've never heard of those "Instant HWS Shower Heads". Pray tell who might make some?

Thanks to all so far,
Batpig.

Quote:

---

Originally Posted by Batpig

...minimum selectable temperature with any of them when a controller is hooked up is about 37deg...

And don't forget, I'm cavity-brick, so no neat 'n easy retro-installs...

Batpig.

---

The ENVSMT unit starts at 31 degrees: http://www.rinnai.com.au/hotwater/ho...?whs=home&pg=5

And for cavity brick, there's wireless: http://www.rinnai.com.au/hotwater/ho...?whs=home&pg=4

Pull the restrictor out of the shower head you have, its fairly simple.
They are a pain in the neck, I put in a fancy Grohe shower rose for a lady the other day, cost her around $300 and i asked her if she wanted the restrictor removed because she wouldnt get the full benefit from it.
"Oh no" she said, save water and all the rest.
She called the next day describing the same symptoms while trying to have a 'warm' shower. She could only have a hot one.
Pulled the restrictor out and good as gold.

Dear Wonderplumb,

Did she have controllers on her system?

Thanks,
Batpig.

I am not sure about the other brands, but Rinnai at least has I R wireless controllers available for some models

Messing about adjusting the temperature of the hot water would be a bit irritating in my opinion. I like being able to run the washing machine on warm, fill the sink with genuinely hot water or whatever while the shower is going. Also I like having the temperature control inside the shower rather on a wall somewhere and I do adjust it during the shower most times - I want it a bit cooler when I put my head under than when it's just on the rest of my body.

All works wekk with a storage water heater of any type (electric, gas, solar, heat pump) - instant would be OK but I'd want the hot water at least 50 degrees, preferably 60, and adjust it at the tap.

I've heard of people in Tas having trouble with minimum flow with the instant units so presumably there would be even more troubles in Qld etc during summer.

Quote:

---

Originally Posted by Batpig

Dear Wonderplumb,

Did she have controllers on her system?

Thanks,
Batpig.

---

No mate it was a Rinnai 26 factory set at 50 degrees.

Quote:

---

Originally Posted by Smurf

Messing about adjusting the temperature of the hot water would be a bit irritating in my opinion.

---

Amen Brother Smurf!

Quote:

---

Originally Posted by Smurf

I've heard of people in Tas having trouble with minimum flow with the instant units so presumably there would be even more troubles in Qld etc during summer.

---

Amazing... Just goes to show that mankind has this nasty never-ending habit of concentrating on the glossy brochure for each new technology that comes along well before he nuts out the essential technical issues (happens with everything nowadays: every new version of Windows, DVD formats, even with the way that Mulching Mowers clag-up so quickly underneath compared to the old-style Catcher Mowers...)

Quote:

---

Originally Posted by wonderplumb

No mate it was a Rinnai 26 factory set at 50 degrees.

---

Thanks Wonderplumb. Yes, that would be the best bet. 50deg preset will increase the flow rate being asked of the unit for any mixed water temperature at the shower, compared with the otherwise 55 or 60deg output. Combine that with removed Shower Rose restrictors and you might be okay...

Still can see some problems though... You get up in the morning and turn the mixer lever all the way to "hot" until the pipe clears and heats up, then you turn it down a bit and add some cold water. Opening the cold water line will cause a pressure drop in the cold water feed to the Instantaneous, and since there will always be a delay for the existing hot water in the line to come through, you wouldn't therefore know straight away whether or not you'd inadvertantly dropped the flow rate through the Instantaneous to below the required minimum. Would be a case of repetitive, delayed "cause and effect"... Controller would be a pain in the R's for reasons already mentioned, and would only get you down to 37deg for the smaller size of unit that I need anyway, so you're still playing Ducks And Drakes with the delay in finding out whether or not the Instantaneous unit has actually switched off on you... Will be more of a problem up here too, of course, due to the ambient heat in summer...

Thanks guys,
Batpig.

Just point about the flow restrictors.

The supposed benefit of the instant units is that they use less energy. Unless you don't have somewhere to put a decent sized tank, that's really the only benefit.

But if you're going to increase the flow through the shower in order to make the instant unit work properly then that will undo much or all of the energy savings from having the instant unit in the first place.

Obviously they do work well in many situations but they're not a perfect technology that suits everyone. Likewise electric, solar etc also have their limitations and aren't always the best option.