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Installing a higher pressure fresh water pump


blackrose

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I have a spare domestic water pump onboard which at some point I want to install in an attempt to get my Morco gas water heater to work properly. At the moment the flame fluctuates depending on the water pressure/flow, even though my accumulator is set correctly (a few psi below the current pump cut in pressure).

 

My current pump is a Jabsco Par max 3.5 - 3.5 GPM flow rate, 10psi cut in/25psi cut out pressure and the new pump is also a Par max 3.5 but with a 40psi cut out pressure (I can't find the cut in pressure in the pump specs). I've also got an additional Square D pressure switch I can fit if required.

 

The current pump is over 12 years old and still works fine so I'll just keep it as a spare. I've never needed to fit an external pressure switch but I know that's recommend.

 

Anyway I have 2 questions:

1) Will the new higher pressure pump even out the fluctuating flame on the Morco, or is that more to do with water flow rather than pressure?

 

Assuming the answer to the first question is yes:

2) Does a higher pressure domestic water system represent a greater risk in terms of damage to/leaks from the calorifier? My vertical calorifier is 18 years old and I do have a hot water expansion vessel fitted. I think the calorifier has a 3 bar PRV on top so I don't think the maximum 40psi would activate it even if I didn't reduce the new pump pressure with the external Square D pressure switch.

 

Thanks

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and that relates to pump output being higher than the flow rate from the tap/shower. All the accumulator will do is alter the cycling period, not stop it totally. I don't think a higher pressure pump will alter this.

 

2. A 40psi pump and a 3 bar (45 psi) PRV looks very close to me, so the PRV may "bounce" as taps are turned off (sort of minor water hammer).

 

FWIW, the cut in pressure is usually roughly the half the cut-out pressure.

 

Do not Murcos have a flow rate adjustment, probably called the temperature control.

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4 minutes ago, blackrose said:

I've no idea. I didn't think the Morco had any flow rate adjustment but I might be wrong. The temperature control knob has to be set at full otherwise it gets worse but it doesn't stop it fluctuating.

 My thought was that the OPs may have been turned down, which would, as you say, make the flow less and the problem worse. Maybe the OP could experiment with opening another cold tap to stop the cycling.

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56 minutes ago, blackrose said:

1) Will the new higher pressure pump even out the fluctuating flame on the Morco, or is that more to do with water flow rather than pressure?

Is there any way to measure the actual flow rate through the Morco? Pump g/hr, lpm flow rates quoted by the manufacturer are for the pump on its own, not connected to any plumbing. The plumbing drops the real flow rate a lot. On my boat, with all taps open, the actual pump flow rate is half the manufacturers quoted one. Would increasing the pipe diameter too and from the Morco be possible? This would be more likely to improve the pressure and flow rate the heater actually sees.

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2 minutes ago, Jen-in-Wellies said:

Is there any way to measure the actual flow rate through the Morco?

 

I guess you could easily measure the flow rate as going into the Morco**, but its internal restrictions would have a big effect.

 

** Disconnect the inlet supply, get a known sized container and see how long it takes to fill.

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The thing about the Morco D61 is the flow rate and therefore the flow temperature seems to be adjusted by a cold water bypass inside modulated by a metering needle valve which bleeds less or more cold water into the (very) hot water from the heat exchanger.

 

There also seems to be some sort of overheat control which presents itself as the gas flames being turned up or down too. If the flow temp gets dangerously hot, the gas flames turn down to low setting. I've observed this on my own Morcos but I've never been able to work out the mechanism by which it works. I called Morco tech support about it once and they denied it could happen saying I must be simply wrong in my observations, which somewhat annoyed me. 

 

 

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38 minutes ago, Alan de Enfield said:

I guess you could easily measure the flow rate as going into the Morco**, but its internal restrictions would have a big effect.

Easier to measure the flow rate coming out (unless your Morco is hiding water inside it).

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I would fit the Square D and use this, with the current pump to increase the cut in pressure,  not the cut out pressure.  The two pressures are a bit interdependent in the SqD , but it is possible to raise the cut-in pressure without much affecting the cut out.

 

The logic is that as presently arranged  when the pressure falls toward cut in, the water flow rate will drop.  Temperature rises and and, as explained by MtB, the morco reduces the heat to keep the water at a  safe temperature.  If the cut in pressure is raised the pump will start to run sooner and the fall in water flow rate will not happen, so the morco will not need to fluctuate.

 

N

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12 minutes ago, BEngo said:

Temperature rises and and, as explained by MtB, the morco reduces the heat to keep the water at a  safe temperature.

 

 

Careering off at a tangent, have you worked out how it does this? 

 

 

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4 hours ago, BEngo said:

I would fit the Square D and use this, with the current pump to increase the cut in pressure,  not the cut out pressure.  The two pressures are a bit interdependent in the SqD , but it is possible to raise the cut-in pressure without much affecting the cut out.

 

The logic is that as presently arranged  when the pressure falls toward cut in, the water flow rate will drop.  Temperature rises and and, as explained by MtB, the morco reduces the heat to keep the water at a  safe temperature.  If the cut in pressure is raised the pump will start to run sooner and the fall in water flow rate will not happen, so the morco will not need to fluctuate.

 

N

 

Ok that's worth knowing, thanks. I wasn't aware that you could increase the cut in pressure of a pump with an external switch. I'd assumed that was limited by the pump.

4 hours ago, Bobbybass said:

I installed a higher power water pump.....and suddenly I had water gushing from leaks all over the place....☹️

 

Higher power? Was that higher pressure or flow rate - or both?

 

That relates to one of my questions above. I'm wondering if a higher pressure pump is more likely to cause leaks - particularly from the calorifier.

Edited by blackrose
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1 minute ago, blackrose said:

 

Ok that's worth knowing, thanks. I wasn't aware that you could increase the cut in pressure of a pump with an external switch. I'd assumed that was limited by the pump.

The Square D has to be wired instead of the existing switch in the pump else it makes no difference!

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8 minutes ago, blackrose said:

 

Ok that's worth knowing, thanks. I wasn't aware that you could increase the cut in pressure of a pump with an external switch. I'd assumed that was limited by the pump.

 

Higher power? Was that higher pressure or flow rate - or both?

 

 

1. As Tracy says, you have to bypass the pump's own switch. There are two wires coming out of the cylindrical body. One is negative and goes straight to the boat's wiring and the other, the positive, goes to the pressure switch. I cut that wire at the pressure switch and connect a wire from the D Square to it. Others simply set the pump's pressure switch to a higher value than the Square D, but that still leaves the tiny switch contacts carrying the pump current.

 

2. Higher pressure causes leaks, a higher flow rate might result in a higher pressure in parts of the system closer to the pump, but the pressure switch should cut the pump unless it si also a higher value pressure switch.

 

 

Edited by Tony Brooks
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  • 1 month later...

I haven't changed my pump yet, but thinking about this recently, is it definitely flow rather than pressure that's the relevant factor to keep the Morco running consistently?

 

I may well have misunderstood but the Morco burns fine just after the pump cuts in and pressurises the accumulator but it stops burning when the pressure from the accumulator reduces until after the pump cuts in again. So isn't that pressure rather than flow? Or is it both?

 

If I isolate the accumulator and turn on a hot tap the pump cycles on/off once a second and so does the Morco, but it actually provides more consistent hot water than with the accumulator in the system.

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4 minutes ago, blackrose said:

I haven't changed my pump yet, but thinking about this recently, is it definitely flow rather than pressure that's the relevant factor to keep the Morco running consistently?

 

I may well have misunderstood but the Morco burns fine just after the pump cuts in and pressurises the accumulator but it stops burning when the pressure from the accumulator reduces until after the pump cuts in again. So isn't that pressure rather than flow? Or is it both?

 

If I isolate the accumulator and turn on a hot tap the pump cycles on/off once a second and so does the Morco, but it actually provides more consistent hot water than with the accumulator in the system.

 

They are both related. On any given pipe or circuit, a higher pressure will produce a higher flow rate and vis versa.

 

In this case, I think it is pressure because it is dropping too low before the pump cuts back in. A higher cut out pressure is likely to cause a higher cut in pressure on the standard pump.

 

If you fitted a square D pressure switch, you can adjust the cut in and cut out pressures independently.

 

 

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The Paloma uses pressure difference across an (effectively variable) orifice to move a diaphragm. The variable bit is adjusted by the temperature knob and seems to involve a cold bypass rather than a real variable orifice.  The diaphragm controls the gas.  If there is insufficient pressure difference,  the  diaphragm "knows" there will not be enough water flow so the heat exchanger/hot water is going to get too hot. The gas valve then cuts off.

 

So, from a standing start with the gas lit,  the water pipes pressurised and the water pump  on and all taps shut:

 

Open tap. Pressure at outlet falls.  Maximum pressure across diaphragm.  Gas valve opens, water heats.

As the accumulator empties, pump runs.

If the inlet pressure falls too low (because the pump has not cut in soon enough, or cannot keep up, )  there cannot be enough pressure across diaphragm.  Gas valve shuts.

 

If the pump keeps the  inlet pressure up all is well. If not hot water happens in stits and farts depending on the various pressure settings.

 

Tap closed.  Outlet pressure rises.  Differential pressure across diaphragm falls.  Gas valve closes.

Pump runs to cut off pressure.

 

I think the later versions work much  the same way, but have extra CO and oxygen detectors which can also shut things down.

..

....

.

 

The pump motor gives not a monkey's about it's switching points.

  Supply electricity, it runs and  pumps.  Switch off, it stops.  The supply/no supply decision is entirely up to the pressure switch.  The cheapo ones built into the pump typically only have adjustment to the cut out ( switch off)  pressure.  The variation between cut in and cut out is fixed by the pump switch  construction. So if the cut in pressure  is not enough to keep the gas on, turning up the cut off pressure will usually raise the cut in pressure too, and 'cure' the problem.  Except that the pump now has to work harder and the 'all taps shut' pressure is higher, risking joint failure and leaks, more wear and tear on the motor and greater power usage..

 

A stand alone pressure switch will enable the cut in and cut out pressures to be set individually.  Often, the problem can then be cured without raising the cut off pressure to risky levels.

 

N

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On 02/04/2023 at 17:41, Tony Brooks said:

1. As Tracy says, you have to bypass the pump's own switch. There are two wires coming out of the cylindrical body. One is negative and goes straight to the boat's wiring and the other, the positive, goes to the pressure switch. I cut that wire at the pressure switch and connect a wire from the D Square to it. Others simply set the pump's pressure switch to a higher value than the Square D, but that still leaves the tiny switch contacts carrying the pump current.

 

Looking at this switch I'm not really sure which terminal connections I should be using?

 

Also I assume the two holes at the top in my second picture are for cable glands which I won't bother with, but what's the threaded female connector at the bottom for? It looks like a connection for the water supply?

 

The instructions supplied with the switch are far from clear.

 

IMG_20230527_123142.jpg

 

IMG_20230527_123211.jpg

 

As for setting the thing.... Some instructions in plain English would have helped. I thought these pressure switches were supposed to be easy to fit?

 

IMG_20230527_124446.jpg

Edited by blackrose
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13 minutes ago, blackrose said:

As for setting the thing.... Some instructions in plain English would have helped. I thought these pressure switches were supposed to be easy to fit?

 

You obviously got an Australian pump and its manual.

Maybe the water will go down the plug-hole the wrong way as well ?

 

 

IMG_20230527_124446.jpg

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Well I've ended up fitting the higher pressure pump without the Square D pressure switch because I don't understand how to fit it or set it.

 

Anyway the higher pressure pump (which has same flow rate as the old pump) has sorted the problem and the Morco now runs without cutting out. So it seems that it's water pressure as well as flow that are the defining factors when it comes to running these Morco heaters.

 

Obviously as Tony says, pressure is related to flow, but as I understand it pump specs for the flow rate are given as if there was no pipework resistance, so I think the old pump the wasn't achieving its flow rate and the new higher pressure pump is allowing a better flow rate even though it's rated the same as the old pump.

Edited by blackrose
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