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Recirculation Pump for Water Tank

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Jack Benson

Jack Benson

Industrial
Jul 11, 2023
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I have posted about this system before, but not specifically the problem i am posting about now.

We have a small instant water heater that has a small water thank (1 litre) attached to it.

The water heater is has a volume of about 0.5 litres.

Its for a very specific application.

When we need to use the warm (not hot max 40C) from this system, we press a button that starts a pre-heat cycle where the water heater comes on and a small pump starts, and heats all the water in the water tank and the water heater to 40C.

There are temp sensors to cut the power off to the water heater when ready.

This is the schematic of our system that i have posted before when asking about where to position the flow sensor:

Recirculation_Pump_Diagram_With_Flow_Meter_-_Copy_rustvv.png




I very very nervous about choosing a pump that would be ok.

We were looking for something 12v, about 20 watts, as we had tested with this power level and it was adequate for our needs.


I was discussing with various suppliers in China and settled on this pump after lengthy discussions with them (they did pressure tests / temp tests for me).

Desun_DS4505HF_ybhisa.png


This is a link to the full manual: Link


We got the delivery about a month ago, and it was working well in our test environment.


This week we got a new sample PCB and instead of the temperature maxing out at before 40C, it was going to about 45C. This is a bug that will be fixed.


The problem was that the pump developed a leak on one of the screw threads (see video): Link



I found it very difficult to source this part and it was the item that i was most worried about when sourcing.

I was able to find something suitable in the EU market to benchmark the specification.


I do not know if the issue here is the water pressure (max 4-bar) or the increase in temperature or that that the pump is just not suitable for our application (which i worry is the most probable).

I realised that I do not have the specific material the pump is made from in my file so I have asked for the information.


I should be able to limit the pre-heat temp to 35 C but i do not want to have a part that is prone to failure in the system.


Your insight is always appreciated.
 
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The pump looks rather cheap and cheerful and made from some sort of ABS plastic?

Have you looked at 12V marine pumps or RV type hot water pumps. Usually you're better off with hoses and hose clips.

Something like this
Google 12V marine RV hot water pump. Dozens of pumps come up so maybe you looked for the wrong thing?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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We do use a pump like that in another part of the system, but we do not need anything with that level of power.

the spec on the link you sent also ABS

I am not arguing that the pump may be too cheap.

We need (if it is possible) to find something that is around 20 watts, that has a smaller footprint than the diaphragm pumps

the diaphragm pump we use specifically says that you cannot connect it to mains pressure - which we need for our application
 
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Why reinvent the wheel? There are plenty of cheap, small water heaters readily available, and they do not use unnecessary pumps. The heating element is in the bottom of the small (or large) tank and rely on natural convection. Who would even think to put the heater above the tank, with the controlling temperature sensor far from the heater. So many things could go wrong.

 
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You may be able to utilize convection to achieve your goal without a pump. Install the heater below the water tank. Hot water should return to the top of the tank, cold water flows from the bottom of the tank into the heater inlet.
 
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This is a small part of a larger system that combines the water heater with a heat recovery system.

Most of it is working perfectly

We only heat the water to 40C so. Cannot keep it perminantly warm due to legionella.

I had thought about convection but it will not be quick enough.

I need to heat all the water in 1-2 min (depending on how cold it is)

Before we had this leak, we were very happy with the performance - but now I need to find a small pump that is reasonably priced that does what we need.

In a different part of the system we already use this pump which I pay $28 ex-china

But you can't connect it to mains pressure and it's much too powerful/physically big for the recirculation.

I think that the type of pump I bought can work for what I need it for, but I need to know how to source the right one. Right material for the temp & pressure.
 
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A pump will not cause the water to heat any faster. That is determined by your heater power. You complain of cost sensitivity and insist on an unnecessary pump?
 
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I do disagree with this statement. Flow rate increase the efficiency of a heat exchanger due to thinner boundary layers and turbulence. We don't have enough information to quantify the effect.

But, OP has fluid in two separate chambers so there does need to be some exchange of fluid between the two.

I do believe OP is underestimating the effects of natural circulation. I've operated marine boilers that were able to naturally circulate enough to evaporate 110,000 lbs per hour of water using natural circulation.
 
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"I do not know if the issue here is the water pressure (max 4-bar) or the increase in temperature"

I have identified your problem:
"I was discussing with various suppliers in China"

You're welcome.
Good luck.
 
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I know this isn't the question, but have you not thought about a thermostatic mixer valve instead so you can keep your water tank at 60C to avoid legionnaires and forget about this pump? Then insulate the heater tank and just keep it hot?

You issue is probably that these cheap pumps are simply not made to have up to 4 bar / 60psi on them constantly and some of them will leak.
Some will be rated for higher, but maybe not on the inlet / suction side which is where your leak is happening I think.

Those rated for higher pressures will cost more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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It looks the same thing tbh as you have now.

Note Max head 5m (0.5 bar). No static pressure rating.

I don't think any of these ABS type plastic bodies are going to be able to cope with mains water pressure up to 4 bar. They are just not strong enough. IMHO

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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"I do disagree with this statement. Flow rate increase the efficiency of a heat exchanger due to thinner boundary layers and turbulence. We don't have enough information to quantify the effect".

We are talking about electric resistance heating. Flow rate has no effect on heat transfer efficiency, which will be 100 percent.

And having two separate chamber is the first fundamental mistake. Both "chambers" can simply be one wide spot in the pipe.
 
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Without flow only the water in the heater will get warm, at least over short periods.

Perhaps the word "effective" would cause less offense?
 
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