Source strangulation for an instant water heater?

[Jonathan212]

Got an electric water heater in my flat, one of those instant ones. It outputs a low flow but this is alright as you mix the hot water with cold water in the shower. Problem is, in the summer you need to mix in too much cold water in the shower and the heater shuts down, probably as it senses low inlet pressure. If the house water pressure were higher, you would be able to push more water through the heater and it would not shut down. Will this be achieved with bigger pipes from the basement to the 3rd floor flat and to the shower? Or would that spoil the strangulation that is in place due to the internal diameter of the pipes being 1/2''?

 

[Spinnor]

With a make and model number you could probably get the manual online which might have a troubleshooting section. I am a little confused by the question, I don't completely get the situation. Could you post a link of the water heater manual?

 

[Jonathan212]

Afraid not. When you route too much water through the cold pipes, the water in the hot pipes slows down. This is sensed by the heater and it shuts down, ie it stops heating but water still flows through. We want it to keep heating, and this could be achieved if the house pressure were higher.

 

[anorlunda]

Is it possible that all your pipes are becoming clogged with mineral deposits?

How old is your plumbing?

Is your water supply hard water or soft?I have no idea why your problem is different winter and summer.

 

[jrmichler]

In summer, the incoming cold water is warmer than in winter. You mix less hot water to get the same temperature, and that hot water is heated less because it enters the water heater at higher temperature. Higher temperature in and same temperature out equals lower temperature rise equals less heat added.

You need to read the owner's manual, but that water heater MAY have a minimum heat rate. If so, there will be a minimum flow rate. That hypothesized minimum flow rate will be higher in summer because of the lower temperature rise. You can test this by running a low flow of hot water from a separate faucet while taking a shower.

The owner's manual will also tell you the minimum incoming water pressure. Is there any place that you could put in a pressure gauge to check water pressure with and without the shower running?

 

[Jonathan212]

I think it is obvious the pressure is too low, there is nothing to investigate. Just tell me if the pressure will rise with a bigger pipe. Or will it fall because the existing strangulation raises it?

 

[anorlunda]

Strangulation is not a bad term. Resistance is better.

If the plumbing has too much resistance, the pressure will be high when there is no flow, then drop too much when there is flow.

Yes, bigger pipes will have lower resistance. If your old pipes are clogged with minerals, then new pipes of the same size will also have less resistance. The picture shows what can happen in real life.

But there is also the bottleneck effect. If I first send the water through a narrow pipe, then a wide pipe, the narrow section will restrict the flow and the wider pipe won't help. That is what "bottleneck" means. To be sure that you do not have a bottleneck, you need to inspect or replace all your pipes all the way out to the street.

So a little bit of investigation first to find out if you have a bottleneck, may be much less expensive than replacing all the plumbing based on a guess.

 

[Spinnor]

The bottle neck could be the heater. Minerals can build up in small heaters like that and reduce flow. Post the model and make, I'll search for the manual.

 

[Jonathan212]

The heater is just 2-3 years old. And of course it is a bottleneck and that is by design, you are supposed to mix its low-flow hot water with cold. Also I was present when the 20-30 metres of 1/2'' piping was installed and know there is this constant size all the way from the meter to the balcony. The idea is to replace this length AND the length from the balcony to the heater AND the length from the heater to the shower. No bottlenecks. At first sight it seems a bigger pipe would increase both the pressure and the flow. But the old plumber that installed the 20-30 meters said 1/2'' piping would give a HIGHER pressure compared to the 3/4'' option! Why? Thought this was what strangulation was about, or maybe there's another term for this effect? Bernoulli or something? If he was right.

 

[jrmichler]

Your use of the word strangulation tells that you do not have a background in fluid dynamics. Your posts all assume a problem, but you have not given any evidence to back that up. We can help, but first we need some pressure measurements. If you are willing to spend the money to remove and replace pipe, you can certainly afford to buy a pressure gauge to find if changing the pipe will really help. The photo below shows what you need in order to measure pressure in a domestic water system.

The cap fits a water hose connection. All necessary parts are in your local hardware store. Drill and tap a hole, screw in the pressure gauge, add a hose gasket, and screw the assembly onto the water hose connection on the outside of your house. Measure the pressure with and without the shower running. Look for a hose connection near your water heater, and measure again. There may be a hose connection on a utility sink faucet. Measure there. After that, reread posts 2, 5, and 7 in detail. Then come back.

I think it is obvious the pressure is too low, there is nothing to investigate. Just tell me if the pressure will rise with a bigger pipe.

I have a solid background in fluid flow, and it is not obvious to me. Nor is it obvious to other posters on this thread. This is not the time or place to write a book, so the short version is that if you replace a length of pipe that is not your bottleneck with larger pipe, you will not increase your pressure. Nor is it obvious that you have a pressure problem. That's why we are asking for some simple, low cost tests.

 

[Spinnor]

The heater is just 2-3 years old.

Did it work well in the summer when first installed? Can the temperature output be varied? What is the make and model?

 

[Jonathan212]

This is the one: Siemens DH06101

Didn't notice the problems till now. Maybe it's the weather.

I could write down all the equations and graphs I think obviously apply at various points along the piping, in order to compare notes with others here and find out where I am wrong. If I am proven wrong, this is totally fine. If anyone of your bottleneck statements is proven wrong, a moderator will close the thread.

 

[Asymptotic]

Siemens DH06111 Ratings (from manual)
6 KW
G 1/2A inlet port
Min/Max inlet water temperature: 4°C/20°C (39°F to 68°F)
Min/Max inlet pressure: 1.0/10.0 bar (14.5 to 145 PSI)
Rated output: 3.0 LPM at 37°C
Switch on flow rate: 2.3 LPM
Switch off flow rate: 1.8 LPM

An internal filter is mentioned, but the only detail provided is,​

"Clean dirt and lime deposits from the filter in the jet regulator."​

It's worth checking whether it is partially clogged, and reducing flow rate.​

Installation
20 to 30 meters of 1/2" ID pipe, vertical run.

Observations
1. @jrmichler made several good points in post #5 and #10, and @anorlunda in post #7.
If possible, measure the inlet pressure at no-flow and normal flow before concluding low pressure is at fault.

2. Maximum inlet temperature for rated performance is 20°C. Whatever water is contained in that 20 to 30 meters of pipe run will eventually rise to ambient temperature, and influence the first minute or so of operation. In summer, city water mains temperature rises to match the temperature of the soil around it.

3. The heater element turns on when flow rate exceeds 2.3 LPM, and turns off when flow drops below 1.8 LPM.

My guess is heater flow rate drops below 1.8 LPM after you've adjusted cold water flow setting to get the desired outlet water temperature. One troubleshooting check is to temporarily remove the showerhead (which restricts flow rate, especially a "low-flow" head), and see if you can achieve desired final outlet temperature at the resulting higher flow rate.

A main pressure regulator and usage meter are often located together where the city water main enters a building to reduce unregulated city water mains pressure (which may be in the 120 to 160 PSI range, for instance) to a usable value. If more than one user are in the same building, the main regulator may be set to an outlet pressure somewhat below the lowest pressure seen on the unregulated side, and pressure regulators for each individual circuit set to reasonable usage values (which may be in the 30 to 50 PSI range).

If marginally low pressure at the heater is suspected, and your installation is similar to the one outlined above, increasing your circuit's pressure regulator setting by 5 PSI or so should eliminate or greatly reduce the symptom.

 

[Jonathan212]

Unfortunately there is no pressure regulator. Just a valve to prevent reverse flow. Maybe a pump would be cheaper and guaranteed to solve the problem if it's big enough. But it has running costs.

Back to pipe sizing, why don't you check this old plumber's wisdom here and forget the heater. What is your take on it?

 

[Jonathan212]

Took out the tiny filter from the heater, it has no deposits on the mesh, just a little on the rim. Put it in a little vinegar anyway and back to the heater. No effect.

 

[Asymptotic]

Unfortunately there is no pressure regulator. Just a valve to prevent reverse flow. Maybe a pump would be cheaper and guaranteed to solve the problem if it's big enough. But it has running costs.

Back to pipe sizing, why don't you check this old plumber's wisdom here and forget the heater. What is your take on it?

I have a well system, and even there a pressure regulator (in the form of an adjustable setpoint/differential pressure switch) is used for pump control. If the source is a city water main there has to be a pressure regulator somewhere between it and the user since mains pressure is usually quite high. Water meters are usually installed just downstream of the mains regulator (to save them from being hammered by higher pressure fluctuations present on the upstream side), and is probably outside the building in close proximity to the meter.

 

[Jonathan212]

All I see upstream from the meter is 15 cm of piping coming from inside concrete. And the backflow preventing valve in the middle. No access to any pressure reducer. Could measure the flow at the ground floor if it helps. That tap is after 20 meters of different piping though.

 

[eachus]

I've been thinking about solutions, not problems. I definitely think that you are asking for less than the minimum flow in hot weather, and anyway the inlet temperature is probably above 20 degrees C anyway.

There are tanks sold for water expansion in hot water heating systems. One of these, insulated could solve your problem. Put in a tank with some insulation, and a regulator valve. (The tank goes up from the pipe.) The valve could regulate the water flow, but you are probably best off with a pressure regulator. Put the tank in the hot water line after the heater, with a pressure valve that opens at say 20 pounds of pressure in between. It will let the heater run at full flow rate, some going to the shower some to the pressure tank. When the pressure gets high enough the valve will shut off. You probably want about 2-4 PSI difference so the temperature in the shower isn't constantly varying. (The only gotcha! I see is that some HVAC parts are marked in feet (or meters) of water, not PSI. Just be sure you find out which, and do the conversions.)

The other solution is to turn on the hot water in the sink at a level where the heater just doesn't cut in. Now take your shower. If this solution works, you have diagnosed the problem as the low flow limit on the heater. Then you could ignore all the fancy solutions and buy a better heater.

 

[Jonathan212]

You sure the rubber bubble in that tank is ok with being heated?

 

[Jonathan212]

Can you tell from the specs if the heater is on-off at 6 kW or it can increase from 0 to 6 kW?

 

[Asymptotic]

Can you tell from the specs if the heater is on-off at 6 kW or it can increase from 0 to 6 kW?

My reading of the specs is a 6 KW element turns on when flow is above 2.3 LPM, and turns off when it is below 1.8 LPM. There doesn't appear to be any way to adjust power to intermediate values.

Siemens website lists this model as discontinued. It may be worth investigating whether they or another manufacturer sell a model with an adjustable power option. If 6 KW always requires mixing in a lot of cold water, perhaps a lower wattage rated unit is called for.

 

[OmCheeto]

My reading of the specs...

My reading of the specs/literature says the unit in question is "suitable for kitchen sink / wash stand". [ref: Siemens]
ie. low flow.

So this:

...in the shower...

problem, strikes me as very peculiar.

The unit's low flow cutoff is 1.8 lpm.
The lowest flow I could generate in my house yesterday was 4.3 lpm: @kitchen sink with bathtub filling

Today, I tried it again, and broke the 1.8 lpm minimum: 1.5 lpm. @kitchen sink
I had to turn on nearly every faucet in and outside my house to do that.

 

[Jonathan212]

In the summer low flow is a good thing for the hot water. Still not low enough.

 

[JBA]

If you are choking the flow at the shower hot water tap to below the 1.8 lpm to keep the shower water cool then it won't matter whether there are any other water taps open or not.

 

[dschlink]

A simpler alternative would be to add a water pressure tank. I used to live on a ridge and my house was near the level of the water company's main tank. In the summer, my water presuure would drop so far that I couldn't take a shower, despite the 2" water line.. A 25 gal. tank took care of that. Put the tank ahead of the heater inlet.

 

[poor mystic]

As an electronics tech I suggest actually measuring the resistance of the plumbing. I’d use a static test of pressure with no water flowing, and a further measurement with the tap fully ON.
Does that make sense to the fluid dynamics guys?

 

[Jonathan212]

Was the 25 gal. tank raised to near the roof of your house and had an air valve on top or something?

 

[essenmein]

If you are choking the flow at the shower hot water tap to below the 1.8 lpm to keep the shower water cool then it won't matter whether there are any other water taps open or not.

I bet this is what's going on. Easy way to test, run the shower at 100% hot and see if it shuts off.

I made the mistake of setting my mixing valve limits in the winter where the water coming into the house is practically a slushy, now in the summer can't get the shower cold, point being, incoming cold water temp has a huge influence on hot water usage!

 

[sysprog]

Well, the manual says that typically for 37°, 3 liters flow, and it says in order to increase temperature, decrease flow. That's not true with a conventional system -- the water in the hot water tank is going to be at whatever temperature you set the thermostat on the water heater at, regardless of flow rate.

In winter, the input water is colder, and you want warmer water for the shower. In summer the input water is warmer, and you want cooler water. With a conventional system, in either season you just adjust both the hot and cold flows to the right temperature balance and overall flow rate, and you're fine, and that's what most people are used to.

My guess is that in summertime, when you want cooler water, you turn down the hot water flow, and then when that makes the resulting reduced flow get hotter, you turn it down even further.

Maybe there's a safety feature thermostat that switches off the heater when the flow rate becomes so low that the water might become scalding -- after all, the unit has no way to detect that you're mixing its output with that from a separate cold water tap.

I suggest that for this unit, for summertime you start with only the hot water fully on, lower the flow rate until it feels hot to the hand, and then adjust the temperature with the cold water only. For winter, you would want a higher temperature, so you'd have to use an overall hot-cold-mix lower flow rate. The dependency of the temperature on the flow rate would be an annoyance to me, but for the sake of energy efficiency, I can see it as worthwhile for people to become accustomed to it.

 

[Jonathan212]

It completely switches off the heating when you try to mix in too much cold water. What it needs is higher flow in both the hot and cold paths.

 

[sysprog]

It completely switches off the heating when you try to mix in too much cold water. What it needs is higher flow in both the hot and cold paths.

Higher flow through the heater decreases the temperature of the water output from the heater.

In the summer low flow is a good thing for the hot water. Still not low enough.

Lower flow through the heater increases the temperature of the water output from the heater.

You seem to be unclear about the following (from the manual):

To regulate the water temperature: Open the tap fully, the appliance switches on. Now​

increase the temperature by reducing the water flow volume.​

[emphasis added]​

This refers not to the temperature of the water emerging from your shower head, but only to the temperature of the water coming out of the heater.

Please re-read what @JBA said, and also please carefully re-read my post #29,

You seem to not be grasping that unlike in a conventional system, in which the temperature of the hot water does not depend on flow rate, in the instant water heater system the hot water temperature does depend on flow rate.

This is because the 6k watt heating element can either heat less water to a higher temperature, or heat more to a not-as-much higher temperature. It's called 'instant' because it does that 'on the fly'. You're not just drawing however much water from a pre-heated reservoir.

With a flow-regulated hot water temperature, it's a bit trickier to manage the temperature and flow rate at your shower head. If you're not trying too hard to conserve water, and you want a cool but not too cold summer shower at a not too high flow rate, max out the hot water to its body-temperature 3 liter per minute flow, then add some cold water -- maybe 1 liter per minute -- to get somewhere around swimming pool cool. If you want a fuller flow (many Americans, especially in the great lakes regions, use around 8 liters per minute) for your shower, get another heater, and run the two of them in tandem, and merge their outputs before the hot water tap, but be aware that with the resultant 12kw electric power use, if your electricity cost is 25 cents per kilowatt hour, that's a nickel a minute, a dollar for a 20 minute shower, and that's without considering the water bill.

 

[Jonathan212]

I said MIX IN too much cold water, not pass through the heater too much cold water. There is only one place where you mix water. What the manual says applies when you have enough pressure at the place where plumbing separates into cold and hot. Several people have got it, what is going on. No time to explain over and over what others have written over and over.

 

[sysprog]

I said MIX IN too much cold water, not pass through the heater too much cold water. There is only one place where you mix water. What the manual says applies when you have enough pressure at the place where plumbing separates into cold and hot. Several people have got it, what is going on. No time to explain over and over what others have written over and over.

If you max out the hot water to 3 liters per minute, without turning any cold water on, it's going to give you around 37°, i.e. around body temperature. If you then add a flow rate of less than 1 liter per minute of cold water, you'll get acceptably cool water. Would that produce a less than 2 liter per minute flow into the heater?

As @essenmein said:

I bet this is what's going on. Easy way to test, run the shower at 100% hot and see if it shuts off.​

If you try that and it's warmer than you are when it's fully on, then yes, your supply line flow rate is less than 3 liters per minute, and that's too low.

 

[Jonathan212]

No it is not going to give me 37 degrees, it is going to give me burns because the input water is already a little warm and certainly not as cold as when the 37 degrees in the spec was measured. Asymptotic is one of the people who got it:

heater flow rate drops below 1.8 LPM after you've adjusted cold water flow setting to get the desired outlet water temperature

 

[sysprog]

No it is not going to give me 37 degrees, it is going to give me burns because the input water is already a little warm and certainly not as cold as when the 37 degrees in the spec was measured. Asymptotic is one of the people who got it:

I was going by what the manual said. It's still going to less warm when fully on than when more restricted. And if it's fully on then you could presumably add more cold water to make it cool enough without too much lowering of the flow rate. I'll re-read what @Asymptotic wrote.

 

[Jonathan212]

Gosh, it is really simple. There ain't enough water to keep the heater running and the cold flow fast enough to compensate for the heat.

 

[sysprog]

Gosh, it is really simple. There ain't enough water to keep the heater running and the cold flow big enough to compensate for the heat.

Do you know what the combined flow rate from the 2 taps is when you max both of them after the heater turns off? (a wristwatch and a bucket can tell you that) (then you can try them one at a time)

 

[Jonathan212]

How are you going to use such figures? I'd rather have some figures for pressure loss as a function of pipe size and length in order to predict what will happen with a tank or a thicker pipe or a pump of given specs.

 

[sysprog]

How are you going to use such figures? I'd rather have some figures for pressure loss as a function of pipe size and length in order to predict what will happen with a tank or a thicker pipe or a pump of given specs.

I want to know how much water you get from each pipe -- you said it was simple, and that "there ain't enough water", so I wondered how much deficiency there might be. You still haven't acknowledged that the temperature of the hot water increases when you decrease the flow rate through the heater, and I was wondering whether if you max out the hot, does it still go below 2 LPM if you then cool the mix with cold, or did you first try to reduce the flow through the hot pipe, and then get too slow a flow after adding enough cold to cool the hot.

 

[Jonathan212]

You start with just hot at maximum flow which is rather slow. You wait a little for it to stabilise to its maximum temperature. Then begin to add cold in the mix till you can stand it. Which is fine in the winter, and fine in the summer too most of the time. But occasionally it is not, the heater switches off as you increase the cold water in the mix. Must be because the public water is a little warm when the weather is warm. Another theory is that too many people are having a shower or watering gardens. Either way, the solution is to increase both cold and hot flow.

 

[Jonathan212]

the temperature of the hot water increases when you decrease the flow rate through the heater

The temperature of the hot water increases, the temperature of the mixed water decreases. This is equivalent to pumping more water through the heater while the cold tap is closed.

 

[OmCheeto]

I'd rather have some figures for pressure loss as a function of pipe size and length

Have you looked at the Hose Water Flow - Pressure Loss graph at The Engineering Toolbox?

I digitized it and generated the following equation, as I didn't see one listed:
pressure drop[psi] / 100 ft = 10^(1.848 * (log_10(gpm)) + ( -2.1041 * ln(pipe diameter[inches]) - 1.3211))

It seems to generate numbers consistent with the graph, so I think it's correct.

 

[Jonathan212]

What is the formula at 90 degree turns?

 

[OmCheeto]

What is the formula at corners?

I do not know what your sentence means. Is English your first language?

 

[Jonathan212]

I meant 90 degree turns of the pipe.

 

[JBA]

If I understand your question there are factors for determining the added loss at elbows in terms of equivalent pipe length to be added. According to Crane Fittings the factor for a std 90° elbow is L/D = 20-30 so the maximum equivalent pipe length to be added to your current pipe length is L = 30 x pipe I.D.

 

[OmCheeto]

I meant 90 degree turns of the pipe.

View attachment 247523

A quick google points to an old PF thread: Pressure drop in elbows & bends in a pipe

I'm having trouble finding an answer in the PDF listed in post #2.
Perhaps tomorrow, I'll devote more time to this.

 

[Asymptotic]

No it is not going to give me 37 degrees, it is going to give me burns because the input water is already a little warm and certainly not as cold as when the 37 degrees in the spec was measured. Asymptotic is one of the people who got it:

It is one of the possibilities. I'm a proponent of measuring things then sorting out what is happening based on the results.

@sysprog's "timed bucket" flow measurement method is cheap and easy, and one of the things I'd be doing if investigating a similar problem. How to go about pressure measurement depends on pipe accessibility and similar peripheral issues ("I want to measure pressure at X location, but can't get at the pipe, so what can I do?").

Assuming the shower head is threaded, I might cobble together a tee fitted with a pressure gauge, and ball valve to go between the pipe and shower head. Measure static pressure with the ball valve closed. Measure pressure (and flow rate; temperature too, if you have a way to measure it) with ball valve open for cold water only, hot water only, and at desired mixture.

Caveat: Static pressure will be the only high quality measurement possible in this configuration. With the pressure gauge located nearly at the water outlet, it will read considerably lower values than if it were located just upstream of the heater unit.​

Solving a problem is a lot easier if you've made a bunch of measurements and developed a sense of what really is going on. Going at it the other way isn't generally as effective.

 

[sysprog]

To what @Asymptotic just said, I would add that cooking and weather thermometers are also inexpensive, and that with the right set of measurements, and @Jonathan212's self-identified comfort zones, one could establish sets of linear and non-linear inequality constraints, formulate LP (Linear Programming) problems, and run Simplex on them, and then do the sensitivity analyses to learn how far one can push one or more of the constraints without departing from a feasible region.

 

[sysprog]

The temperature of the hot water increases, the temperature of the mixed water decreases. This is equivalent to pumping more water through the heater while the cold tap is closed.

If you put 3 LPM through the heater the temperature of the water coming out of it will be lower than if you had put 2 LPM through it. The calorimetric output should be about the same. Is there any tap in your place, cold or hot, that doesn't have what you would think of as a slow flow compared to what you've encountered at, say a hotel room sink?