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HeatherHP
HeatherHP

Posted: Mon Jun 10, 2013 2:06pm

Post Subject: New water pump advice

Hi, Our water pump has finally given up completely so its time to buy a new one. Currently we have a Shurflo Aqua King. 10.6 l/min, 20 psi, 7.5amps On ebay, there are loads of similar pumps which seem well priced but none are exactly the same as our current one and it would be great to get some advice on what is the better option. Option 1: 7 l/min (lower than what we have at the moment) 20 psi (same as what we have) 4.5 amps (lower than what we have) Option 2 10.6 l/min (same) 30 psi (higher) 4 amp (lower) So it seems pumps with the higher l/min rating of 10.6 don't exist at the lower pressure of 20psi. To get sufficient l/min rating, we'd have to up the psi by one third? Would this be bad for the pipes, or for the water heater? Is it just safer to go for the 7l/min pump which is at 20psi or would the water flow be too slow? Is the amps rating important? What do we need to consider regarding this? Thank you for your help. You guys were so helpful before when the pump broke the first time. It ran again flawlessly for 6 months, but now, I think, we've pushed the poor little pump to its limit and its time for a change! Thanks, Heather

Tony-B
Tony-B

Posted: Mon Jun 10, 2013 7:33pm

Post Subject: New water pump advice

Before I can give accurate advice I need to know how your hot water is heated, if its a calorifier then what pressure is the pressure relief valve set to, and what type of taps do you have or what size pipes run to them. I doubt you know some of that so all I can do is to give some general guidance. Calorifier PRVs have a pressure setting and if you fit a pump with a higher cut out pressure or even one very close to the PRV setting then the pump is likely to keep running and pump all your water out of the PRV. Next you need to understand that if a tap allows more water out than the pump can deliver then the pressure at the tap will reduce until the flow matches the pump output BUT the volume supplied will be as stated on the pump. In this situation the pump is likely to run all the time a tap is open but stop soon after the tap is closed. On the other hand if the pump can deliver more water than the tap allows out then while the tap is open the pump is likely to keep cutting in and out as the pressure in the system builds up, the pressure switch turns the pump off so the pressure falls and the pump turns on again.I personally would choose to operate a lowish output pump so it is less likely to cycle (turn itself on and off while a tap is running. If you like a "power shower" then a lower output pump (l/min) would tend to give a reduced pressure from the shower but if you are happy with just getting wet then that would be fine. A further problem that is now cropping up is that people fit modern taps that use 8 or 10mm inlet pipes rather than the normal 15mm. These are designed to operate at mains water pressure so they restrict the flow to the tap and with a lower pressure pump may not deliver an acceptable flow. Don't worry about the amps because the pump runs for such a short time per day the consumption is pretty much minimal unless you have a very inadequate or faulty electrical system. I have just noticed that you talk about a water heater (type unspecified). If this is an instant gas heater then one needs to be sure about how strong the heat exchanger is before fitting a higher pressure pump. Many are old, have been frozen in the past and split so are now repaired with soft solder. Of the two choices you give I would tend to go for option 1 to be on the safe side BUT do not blame me if it turns out to be less than satisfactory. Tony Brooks

HeatherHP
HeatherHP

Posted: Wed Jun 19, 2013 3:06pm

Post Subject: New water pump advice

Hi Tony, Thank you for your helpful advice. Sorry it has taken a while to reply, we have been away from the boat for a few days. We don't have a calorifier. We have a Rinnai instant gas water heater. Given this, please could you kindly clarify what you mean by "If this is an instant gas heater then one needs to be sure about how strong the heat exchanger is before fitting a higher pressure pump." To be honest, I find this difference between LPM and PSI really confusing. Any easy way to explain it? Based on what you've said, I think our taps let out more water than the pump can deliver as when we first turn the tap on, the water comes out quickly for a second before slowing to a decent flow. This means it must be set up correctly. (As you say, if the pressure was too high then the water flow would cut in and out). Our current Shurflo pump has a screw on it which when we turn, changes the speed the water comes out. It says next to this screw "adjusts shut off only, does not change flow or operating pressure". This is confusing as it clearly does change the speed at which the water comes out of the tap. Anyway, my reason for mentioning it is because, could we get the higher pressure pump and turn this screw to make it lower pressure but with the same LPM? With regards to the diameter of the inlet pipes to the taps, the outer diameter of the pipe which is just before the tap itself is approximately 13mm. It is one of those flexible woven metallic looking pipes. The majority of the pipe work is much larger however. For instance, the rubber/flexible plastic red and blue pipes leading to and from the water heater are 20mm outer diameter. Hope this extra information makes sense and helps to solve the problem further. Thankyou again for your time. Kind regards, Heather

Tony-B
Tony-B

Posted: Wed Jun 19, 2013 4:46pm

Post Subject: New water pump advice

Easy part first. Yes you do have an instant gas water heater and the heat exchanger is the part between the burner and the flue where heat is transferred to the water. You see I have to be cautious when giving advice because if anything goes amiss someone might try to make a claim on the magazine or myself. What I may say 1 to 1 to your face may well be different to what I will put in writing. Unfortunately delivery or flow rate LPM is confusing because you rarely have a completely unrestricted outlet and any restriction to the flow will cause the pressure to rise. Lets ignore the fact that your pump actually consists of three or more little piston pump and think of it as one large pump like a bicycle pump. Fill the cycle pump with water and push the handle in. A certain amount of water will be squirted out and the amount depends upon how large the pump is. No imaging a cycle pump twice as large. When this one is pumped twice the volume of water is ejected. This is LPM. If we went back to the first and organised it so it refilled itself with water then if we pumped twice as fast we would get twice the amount of water out of it. Now if we put our thumb over the hole to restrict the flow the pressure inside the pump would rise but as long as we completed the stroke in the same time we would have squirted the same amount of water out but with a higher pressure inside the pump. LPM is simply how many litres a minute the pump will deliver if there is no restriction to the flow. The only thing that the pressure (PSI) has anything to do with as far as the pump is concerned is the pressure inside the pump (and thus in the pipes etc) at which the pump cuts out. When you adjusted the screw on the old pump all you did was to set it to cut out at a lower pressure than that set by the manufacturer. It will not alter the maximum volume of water the pump will deliver if there are no restrictions. The pump will just shut down earlier and at a lower pressure. In fact that is too simple because as the pressure rise the pump motor has to work harder so it will tend to slow down a bit and deliver less water, however this should all be allowed for in the LPM figure. Any pressure inside the system when it is running will be governed by the pump delivery volume (LPM) and the flow from the outlet (LPM) at the pressure at that time. If the pump output is higher then the flow from the tap the pressure will rise until the pump cuts out. If the flow from the tap is more then the pressure will fall until the volume of water it can push out of the tap is the same as pump delivery. Usually the pressure falls so the pump motor cuts back in again so the pump cycles on and off. Your Rinnai heater will have a maximum pressure and required flow rate in its instructions. Use these to ensure that you do not exceed them. With no calorifier you have no PRV to worry about so you have more freedom re the maximum pressure in the rest of the system. From your description I think that your system is well set up as long as you get a decent shower.

HeatherHP
HeatherHP

Posted: Thu Jun 20, 2013 3:15pm

Post Subject: New water pump advice

Hi Tony, Thank you again for such a swift and helpful reply. Good to get some detailed explanation that makes sense! Will check the pressure limitations on the heat exchanger of the water heater and get the new pump based on that. Doesn't sound so complicated after all!... Thanks again, Heather

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