Welcome to Motorhomefacts, we are a thriving motorhome community (Largest in Europe). Why Not JOIN NOW and get instant access to more of the website. It costs nothing to join and only takes a few minutes. We have 200,000 different people visiting our site monthly and this amount of motorhomers in one place guarantees a fast response to any questions you may have. We also have unique facilities not found elsewhere such as our Online Logbook, stopover tracker, Motorhome directory with Ebay type feedback and the largest repository of motorhome campsites reviews found anywhere
Last edited by sallytrafic on Fri Mar 13, 2009 8:24 am; edited 2 times in total ______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Thank you I started with what I knew but as the afternoon has wore on and I've been collecting photos etc I had started to realise I need to write some more, especially as some pumps are now run dry and don't need accumulators.
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Thank you I started with what I knew but as the afternoon has wore on and I've been collecting photos etc I had started to realise I need to write some more, especially as some pumps are now run dry and don't need accumulators.
Good start though Frank, diaphram pumps will run dry and you can get the spares for them.
All pumps really need an acumalator to stop the pump 'surges'
Dont think centrifs need accumulators because the system is not pressurised and any excess flow just bypasses through the pump top back into the tank.
At least thats what seems to happen in our van.
Main problem have always been non return valves post pump. They usually stick open and the system drains back down to the reservior.
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
2nd draft please fee free to comment I am working on the photos and illustrations at the moment and they will be added later as and when I get permission to use copyright material.
Most motorhomes have pumped water. Some very simple caravans have a hand or foot operated pump. I did meet a self-builder who said he was going to put a tank in his luton to remove the need for a pump but most ‘sensible’ motorhomes have electric pumps so as not to rely on gravity or manual labour.
Pumps are fairly simple devices; an electrical motor is connected to a pumping device which moves the water through the pipes. There are two main subdivisons ‘plumbed in’ or ‘immersed’. Most motorhomes have the plumbed in type but the immersed ones are often used by caravanning people to pump water from an external tank into their caravans or by motorhomers to pump from a portable tank into their onboard tank to avoid moving from their pitch to refill. The rest of this FAQ will assume a ‘plumbed in’ pump.
There are three types impeller, centrifuge and diaphragm. The first two are rotating types but in the diaphragm the rotation of the motor is turned into a back and forth vibration to act on the diaphragm see illustration
animation copyright KNF Neuberger AB
Most rotating pumps have a recirculating feature ie more water is pumped by the impeller than can easily pass through the pipes this means that if you only open the tap a little way the pump can operate at normal speed and any excess water merely goes around in a little loop inside the impeller housing. All pump (less the smaller impeller types) also have a one way valve to prevent the water flowing back to the tank and keeps the water in the pipes under pressure if the taps are not opened. The diaphragm type also depends on these valves for the pumping action.
There are two main ways of switching a pump on. A switch inside the tap or pressure switch in the line. For a simple installation with just one tap a microswitch that is physically switched on by opening the tap is often the solution employed. Microswitches or their operating cams can fail and they are often fiddly to replace. If there is a toilet, two sinks and a shower with hot and cold water, that quickly becomes a lot of switches to go wrong and a lot of wiring. Under these circumstances a single ‘demand’ pressure switch is employed instead.
The inline pressure switch senses the pressure in the cold water feed and, assuming the hot water is pressure fed from the cold, will sense a lack of pressure anywhere in the system hot or cold. This low pressure will turn the pump on until the pressure rises again. So if you imagine starting with the pressure high and the pump stopped, opening a tap anywhere will allow water to come out under pressure. This will result in a loss of pressure the switch will close and the pump will run until the tap is shut off and pressure rises again. Note that some manufacturer’s incorporate this switch into their pump.
This operation has an inbuilt problem. Water is not compressible. So if the tap is opened only a little way the pump will operate but almost immediately the pressure will rise and the switch will shut off but the tap is still open so pressure falls the pump starts again and this process repeats, making a lot of noise and possibly damaging the pump. There are two ways of dealing with this problem the first is to have a pressure switch with a large hysteresis. This means that the switch closes at a falling pressure but it wont open (switch off) until a much higher pressure is reached. However in practice this would be difficult to specify for the wide range of motorhome water layouts that exist. Method two involves adding something that can be compressed into the water pipe system. It is normally called an accumulator. This acts as a spring. When the tap opens the spring extends maintaining the pressure for longer. When the pressure falls enough the pump starts up and runs for long enough to feed the water out of the tap and re-compress the spring. The result is that the pump doesn’t stop and start so often for example filling a kettle involves only one stop and start instead of twenty or more. The pump will last much longer. Most accumulators are pre-pressurised and come with a valve so you can add pressure with an airline or hand pump after a time if necessary.
As a small aside and another example. My van didn’t have an accumulator. I couldn’t understand why at the beginning of a holiday my pump worked normally but after a few days this rapid on and off switching started . The answer was simple. The pipe connecting to the shower would be drained down after the previous holiday. So when the pump was first operated it compressed the air between the pipe junction and the shower head this compressed column of air would act as as spring. As soon as the shower was used this pipe filled with incompressible water and stopped working as a spring.
There is another problem. If the tank becomes empty the pump will run dry, continuously trying to pressurise fresh air, which pumps designed for water are very bad at, so the pump doesn’t stop. Eventually the motor will burn out or the impeller melt, diaphragm split etc. The same thing happens if a leak occurs in the pipework. First of all all your tank is emptied over your carpets then the pump runs dry. For this reason there should be a separate off switch (or relay) to overide the pump which can be turned off when driving or when the van is stored. At the very least you are preserving the charge in your battery.
As well as not letting the pump run dry the other precaution is not to let water freeze inside. This may split the pump housing or attached pipe outlets. Worse still if it does have frozen water inside do not run the pump 'just to see', this may strip the blades off the impeller, break the diaphragm or burn out the motor.. So always ensure the pump is drained down if exposed to freezing conditions.
Nowadays some diaphragm pumps are marketed which don’t need an accumulator (they have bypasses like the rotating types) or state they will ‘run dry’. One other feature is whether the pump will self prime, that is if the pump is empty at the start whether it will draw water in especially if the pump is mounted higher than the tank.
As always the devil is in the detail and if you have to replace your pump you either need a like for like replacement or need to understand the implications of replacing with one with different features.
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Mind you if I can't get permission I'm going to have to spend a long time in my sketching program and in Photoshop's animation suite.
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Its not an ego trip its just as we have lost the author column
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum You cannot attach files in this forum You cannot download files in this forum
UK
Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray 
