# Source strangulation for an instant water heater?

## Main Question or Discussion Point

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''?

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## Answers and Replies

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Spinnor
Gold Member
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?

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.

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anorlunda
Staff Emeritus
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
Mentor
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?

sysprog, russ_watters and Asymptotic
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
Staff Emeritus
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.

Steelwolf, sysprog and Asymptotic
Spinnor
Gold Member
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.

sysprog and Asymptotic
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.

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jrmichler
Mentor
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.

sysprog, Asymptotic and jim mcnamara
Spinnor
Gold Member
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?

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 any one of your bottleneck statements is proven wrong, a moderator will close the thread.

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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.

sysprog, jrmichler, Spinnor and 1 other person
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?

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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.

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.

sysprog
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.

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.

NTL2009
You sure the rubber bubble in that tank is ok with being heated?

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

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 web site 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.

sysprog
OmCheeto
Gold Member
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.

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

JBA
Science Advisor
Gold Member
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.

sysprog
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.

Steelwolf and sysprog