So the water expands up into the EV from the domestic side without being refreshed. Surely this creates the effects of a dead leg.

3BackflowPreventer

The vessel will be initially empty with a pressure of 15psig (30psia) in it. After the pump cuts out the inside pressure will be 30psig (45psia)

The pressure inside the expansion vessel needs to be ABOVE the pump's cut-off pressure (typically ~30psi) otherwise the pump itself will pump up the expansion vessel's diaphragm and thereby waste some of the effective volume which could have beem used for hot water expansion.

So the volume of water delivered from your accumulator before the pump cuts in will be the difference in the air volume = 4.2 - 3.3 = 0.9 litres

The point I am trying to make (not very successfully) is that if an accumulator is big enough it can and does serve both purposes

Furthermore, the air in the pipe will dissolve in the water over a month or so and thus the system would need some means of relenishing the air.

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My gut feel is that even if the science says otherwise, an EV that's at least 10% of your calorifier(s) will generally be enough. So with 80 litres total, I think you'll get away with an 8 litre EV. They are getting quite big, by 8 litres.

Just to be clear, an accumulator stores pressure to stop pump cycling, an expansion vessel prevents pressure relief valve weeping due to hydraulic locking (hot water expanding). If a non return valve is fitted then an 'expansion vessel' fits between NRV and PRV. If no NRV then accumulator acts as expansion vessel also.

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Chris has calculated this from a theoretical viewpoint, which I accept, although practical experience by several people seems to indicate you normally don't need as much as the physics and maths might imply.

So if we assume that there is 2 litres of expanded water and an 8 litre expansion vessel, the volume of air after expansion will be 6 litres.

Chris will tell you what the maths indicates though, I feel sure - probably more than 10 litres, I'm guessing, (if you can fit it in !).

If you mean you have the extra length tee'd off the main line and this extra length is full of air (which gets compressed) then you will need a section of 15mm pipe that is 13 metres long to accomodate 2 litres of expansion!!!!

Shouldn't they be pre charged to a pressure higher than your water pump can put out, so that the pump isn't capable of using them as another accumulator.

Of course that's true but, if you are able to follow the maths, you will see that the required size of a combined unit would be very large (>10 litres) in order to do an effective job as both expansion tank and accumulator. This precludes its being fitted under a sink in the galley in most cases.

8 wilkinsBackflowpreventer

There are several other performance requirements to which the assemblies need to be able to demonstrate compliance, including:

Once a hot tap is opened of course, the excess expansion pressure is released and the expansion vessel has no role to play unless the water is reheated and expands further.

Only joking (half) but it does semm that Chris W was working out the pressure settings rather than the volume? I need to order one in the next couple of days as it one of my next jobs.

PS. I suspect the position of the accumulator might be a factor... does it help if it is downstream of a tap rather than upstream and beside the pump?

This is because (as the maths shows) the optimum set pressure for the 2 units is very different, ~15psi for the accumulator and ~30psi for the expansion tank.

It comes up every time, but always in the context of "I don't know but I've heard it can happen". You don't get a problem in the hot pipes, which constantly heat up and cool down, nor at the warm interface between hot and cold, wherever it might be. I most strongly suspect it's another urban muyth - but I'm sure it will keep on coming up, time and time again.

Part of the consideration as to which backflow assembly to use for which category is the potential downstream hazard. For example, fire sprinkler systems may use potable water to fill in the sprinkler system lines. These systems are stagnant over time as one huge “dead leg.” This is a non-health affecting or low hazard situation that requires high rates of flow when needed, so a DC or DCDA is appropriate. However, some fire sprinkler system designers add antifreeze or fire-retarding chemicals to the system water. Now that water is a health affecting or high-hazard fluid that requires use of an RP or RPDA.

Wilkins 375DA

I guess we got lucky....and a few factors may have helped here...... A longish distance from galley tap to calorilifier (14 feet), a vertical calorilifier, only heating water to 60 degrees f (old engine and setting thermostat low) , and a low pressure shureflow (approx 20 lbs per sq inch not the 30 lbs sq inch one). We also dose the integral water tank regularly with purifier.

The point I am trying to make (not very successfully) is that if an accumulator is big enough it can and does serve both purposes

PS: Note that in the next post I show that an expansion vessel needs to be set ABOVE the pump's cut-out pressure, so an accumulator and an expansion vessel use widely differing set pressures. Trying to use one vessel to do both is a very poor compromise.

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My cold feed into bottom of cal. is fed from a tee which also goes to cold taps, cal hot water expands back to tee and therefore taps, this cold feed is lengthened to accomodate enough hot water before it reaches the tee. Has been successful for our setup. Hope I have explained clearly.

Rearranging the equation and assuming that the PRV pressure is 42psi as measured on a gauge (but actually 57psi absolute pressure; ie: 42 + 1 atmosphere, where 1 atmosphere = 15psi)

If you would like more information, feel free to contact ASSE at staffengineer@asse-plumbing.org. You can also find me at the ASSE International Mid-Year Meeting, held April 10-11 at the ASSE International office in Mokena, Ill.

Using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply. This can lead to warm water coming out of the cold tap in the galley, as well as a possible issue with bacterial growth in the warm water. I know nothing technically about the bacteria but it does come up every time.

Therefore the volume of water held in the accumulator will be 2 litres - 1.2 litres = 0.8 litres. If you have a typical 2 litre accumulator measure the water volume that comes out on turning a tap before the pump cuts-in. You will find that theory and practice agree very closely.

I'd have thought the only thing that should be forcing water into them is expansion as the hot water heats. Other than when you first open a tap, and this gets pushed out, I'd then expect them to stay full of air, and that to only be replaced by water if you got into another heating, and hence expansion cycle.

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This caused me some worry when I fitted out our new boat.... in the end I copied what was on the last one as it seemed to work.

RPDAvs DCDA

Using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply. This can lead to warm water coming out of the cold tap in the galley, as well as a possible issue with bacterial growth in the warm water. I know nothing technically about the bacteria but it does come up every time.

Think about it - water will expand by around 3% over the range of temperatures which we find on a boat. The larger the calorifier the larger the volume therefore of expanded water.

On an ASSE 1013 or 1047 backflow prevention assembly, if the downstream #2 check fails, the relief valve vents water until the outlet pressure is at least 2 psi lower than the inlet. If the relief valve fails, the upstream #1 check can still provide protection.

The pressure inside the vessel will therefore rise to 45 x 3.3 = P2 x 2.4 = 62 psia = 47 psig which will cause the PRV to open (assuming it's a normal 42psig type).

Re-running the equation, the pressure inside the vessel will increase to 45 x 3.3 = P2 x 2.0 = 74 psia = 59 psig which will cause the PRV to open widely.

If a bypass line bypasses the checks, and on that line is a water meter and smaller backflow assembly of similar design, now you have an RP detector assembly (RPDA or 1047) or a DC detector assembly (DCDA or 1048). If the bypass line only bypasses the downstream check (check valve #2), and includes a meter and a single check, now you have a RPDA Type II or DCDA Type II. It should be noted that an RPDA can function as an RP and, similarly, a DCDA can function as a DC – the requirements are the same except for the needs of the detector assemblies.

The precise "maffs" is as follows and utilises Boyle's Law of gases where P1 x V1 = P2 x V2 where P1 and P2 are the expansion vessel pressure before and after water expansion and V1 & V2 are the volumes of air inside the expansion vessel before and after water expansion.

I'm not exactly clear what you mean......... water is incompressible, so having an extra length section won't accomodate any expansion.

So a 15 mm pipe run from the water tank ...off this came the galley cold ..then the shower cold ...then accumulator... then bathroom basin cold and then the vertical calorilifier.

RPDAvs rpz

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Of course. That's why I said above, viz: "using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply"

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Therefore at pump cutout pressure, 30 psi, the accumulator will contain about 2.5 litres of water. This seems to allow about a litre to be drawn off before the cut-in pressure of the pump is reached and the pump restarts.

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Once a hot tap is opened of course, the excess expansion pressure is released and the expansion vessel has no role to play unless the water is reheated and expands further.

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Backflow Preventer Failure Mechanisms When we refer to backflow protection, we are protecting against two scenarios in which backflow could occur, where the outlet pressure exceeds inlet pressure. How can this happen? Some examples:

The bottom line is that allowing for tolerances, your PRV is not guaranteed NOT to open even when heating from 20 to 60 degC. It probably will open. From 10 to 70 degC it will definitely open. A single vessel to do both jobs is a compromise and will never be perfect. To optimise it (it can never be perfect) requires careful measurements of all the pump's pressures and the real heating range and the adjusting of the pump's cut-out pressure. Even then, there is no guarantee the PRV will not open.

When designing a plumbing system (or a product), part of good design practice is looking back and performing a Failure Modes and Effects Analysis (FMEA) to see where a potential problem may arise. Central to this is evaluating the severity, occurrence, and ability to detect the problems should they arise. The severity of the ramifications of having a backflow incident is by default the most severe given the health concerns. Prior to backflow incidents happening, backflow causes are generally undetectable, even though there is the Authority Having Jurisdiction’s (AHJ’s) initial inspection and annual in-service tests required for backflow preventers. For example, consider how easy it is to leave the end of a garden hose connected to concentrated herbicide. The occurrence of backflow on a system can be high as cross-connections can be made by the general public. Even trained professionals can make an honest mistake. Hence, it is absolutely critical that the assemblies be compliant with ASSE standards and be annually tested by certified individuals.

I'd have thought the only thing that should be forcing water into them is expansion as the hot water heats. Other than when you first open a tap, and this gets pushed out, I'd then expect them to stay full of air, and that to only be replaced by water if you got into another heating, and hence expansion cycle.

Rearranging the equation and assuming that the PRV pressure is 42psi as measured on a gauge (but actually 57psi absolute pressure; ie: 42 + 1 atmosphere, where 1 atmosphere = 15psi)

DCDABackflow

With the pump cut-off, if the water is heated from 20degC to 60degC, the water will expand by 1.44% (from water expansion tables) or 0.9 litres.

Of course. That's why I said above, viz: "using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply"

Kayak are you asking about the dometic water (hot water which comes out of your tap) or the heating water (water which is heated by boiler/engine/fire and runs through the coil in the colorifier.

Install, Test, and Repair A key complementary portion to these assemblies is the ability to properly service the products in the field to help ensure safe delivery of potable water. ASSE/IAPMO/ANSI Series 5000, Cross-Connection Control Professional Qualifications Standard, defines the requirements for the individuals who install, test, and repair backflow protection assemblies. Both the UPC and the IPC reference following ASSE 5013, 5015, 5047, and 5048 for testing RPs and DCs (UPC 603.4.2, IPC 312.10.2). In each of those standards, it’s stated that at a minimum, backflow prevention assemblies shall be tested upon installation, annually, and immediately after repair or when returned to service. The UPC further states that the test “shall be performed by a certified backflow assembly tester or repairer in accordance with the ASSE Series 5000 or as approved by the AHJ.”

Note that low-hazard refers to water pollution (smell, taste, turbidity, etc. are affected) and high-hazard refers to water contamination (resulting in negative health effects). The decision as to which substances or situations are low- or high-hazard and the backflow protection that is permissible is decided by the model codes’ technical committees and local AHJs. In the UPC, table 603.2, and in the IPC, table 608.1, hazard levels for each backflow prevention assembly are defined.

RPZbackflow

The result is interesting because it shows that with most pumps, which are set at about 30psi (gauge) cut-off, the PRV may still just blow if the water expands by its maximum. That's why I always turn my pump cut-off pressure down to about 27psi.

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In use we are not getting warm water in cold galley tap , calorilifier(vertical) stays hot /warm for 18 hours, no signs of bacteria , pump comes on /off with about 5 second delay and prv does not release.

Is there anyone out there who can tell me how I should size an expansion tank on my hot water system. I have 2 twin coil calorifiers at 40 lt each connected (or will be!) in series. Any help is gratefully received as always.

Innisfree has confirmed his remarks actually applied to accumulator(s) fitted on the cold side, which is not what I understand OP to be talking about.

You could of course fit two smaller ones, we have 2 x 5lt with 2 x 50lt calorifiers and works very well, ages before pump cuts in (and ages before it cuts out!)

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If the water is heated instead from 10degC to 70degC, the expansion is 2.2% or 1.3 litres. Therefore the air volume will decrease from 3.3 litres to 2 litres

Approximately 8 litres per 60 litres of calorifier, so about a 10 litre expansion tank for 2 x 40 litre calorifiers combined.

Note on air gaps: Creating or installing an air gap is the safest, most inexpensive, and most conservative means of backflow protection. (Air gaps comply with ASME A112.1.2 and air gap fittings comply with ASME A112.1.3). They are generally mentioned in the same breath as a 1013 or 1047 assembly when citing proper backflow protection. However, while these are excellent for drainage or filling open tanks, they are open to the atmosphere by design and therefore do not have the ability to maintain line pressure during no-flow situations.

If it's an expansion vessel (hot water) rather than an accumulator (cold water) being talked about, then I'm not convinced they should be having a massive effect on pump cycling times, in the normal state of things.......

Standards in the Model Codes Assemblies that comply with ASSE 1013, 1015, 1047, and 1048 are required for various locations in a plumbing system and fire sprinkler system. RPs and air gaps are the most common and best methods of backflow protection. On the following page is a chart that covers the various locations for which RPs and DCs are appropriate for use as referenced by the Uniform Plumbing Code® (UPC) and the International Plumbing Code® (IPC).

I know Chris argued strongly that the maths and physics show that what I have was not sufficient, (5 litre EV with 55 litre calorifier), but in practice I've not seen anything come through the PRV yet. Several others claimed the same for EVs that Chris would say are not large enough.

The precise "maffs" is as follows and utilises Boyle's Law of gases where P1 x V1 = P2 x V2 where P1 and P2 are the expansion vessel pressure before and after water expansion and V1 & V2 are the volumes of air inside the expansion vessel before and after water expansion.

375 rpbackflow

Since these products are present as containment assemblies at the point-of-service to a premises at the water meter and internal point-of-use protection known as isolation, pressure loss needs to be minimized across the assembly with minimum flow capacities defined by ASSE standards. If the products are installed on fire lines, AHJ’s may require a detector assembly to be present by way of an RPDA or DCDA so that the fire department is informed if a sprinkler is activated. ASSE 1047 and 1048 cover the requirements for that scenario.

Only joking (half) but it does semm that Chris W was working out the pressure settings rather than the volume? I need to order one in the next couple of days as it one of my next jobs.

Requirements ASSE’s standards for both RPs and DCs represent the cumulative knowledge from a diverse set of backflow prevention experts, along with consideration for other standards and documents. This ensures that ASSE has the strictest backflow prevention standards for these assemblies. For example, all assemblies are first required to undergo a hydrostatic pressure test before even going through any of the dynamic tests. This is particularly conservative, as most standards for other plumbing products consider hydrostatic tests to be safety tests, checking for catastrophic failures. In our case, a catastrophic failure is a backflow situation in the field.

But if NRV is fitted then exp vessel is needed between NRV and PRV to prevent hydraulic locking which would open PRV whenever hot water expands slightly. Instead of NRV I have installed an extra length of cold water inlet pipe to accomodate expansion, works well, simpler and more reliable than NRV. I find that two 5lt accumulators set at same pressure (2x50lt calorifiers) has worked very well for 4 years without any pressure check or adjustment

If you mean you have the extra length tee'd off the main line and this extra length is full of air (which gets compressed) then you will need a section of 15mm pipe that is 13 metres long to accomodate 2 litres of expansion!!!!

The bigger the calorifier(s), the bigger that potential expansion is, and hence the larger an EV needs to be to guarantee being able to contain it all.

This is because (as the maths shows) the optimum set pressure for the 2 units is very different, ~15psi for the accumulator and ~30psi for the expansion tank.

The precise "maffs" is as follows and utilises Boyle's Law of gases where P1 x V1 = P2 x V2 where P1 and P2 are the expansion vessel pressure before and after water expansion and V1 & V2 are the volumes of air inside the expansion vessel before and after water expansion. Pressures must be input as absolute pressures not gauge pressures (absolute pressure = gauge pressure + 1 atmosphere where 1 atmosphere = 15psi). So 12psig = 27psia and 30psig = 45psia

This is a topic that I followed with interest when it first appeared. I followed all the recommendations and ordered direct from surecal. To allow for the expansion of my horizontal 65 litre tank to the formula of pythagerus squared x the inside leg measurement of grey squirrel the product arrived and I couldn't find any space in the forkin engine room!!!! Throw away all the calcs get an eight litre one as the calorifier has lasted twice as long as its predecessor so it must be doing the job within the limitations I have.

The pressure also needs to be set BELOW the PRV pressure (which is usually around 42psi) otherwise the PRV will still blow as the water expands. Therefore the expansion vessel pressure needs to be set somewhere in the range 30 - 42 psi.

This consensus-based working group is comprised of various manufacturers, contractors, the University of Southern California Foundation for Cross-Connection Control and Hydraulic Research, and ASSE International. The working group’s goal is to compare the current set of standards with that of the existing body of standards and documents to ensure that ASSE has the strictest criteria. These revisions are going to be a major upgrade to align and improve requirements for the benefit of the public. Look for them in the coming months.

So if we assume that there is 2 litres of expanded water and an 8 litre expansion vessel, the volume of air after expansion will be 6 litres.

I was assuming we are discussing accumulators, tho expansion vessel was quoted so perhaps I have got the wrong end of stick (again) If we isolate one accumulator, pump off time is approx halved, with two it's handy in the night if one wants a drink/handwash

If it's an expansion vessel (hot water) rather than an accumulator (cold water) being talked about, then I'm not convinced they should be having a massive effect on pump cycling times, in the normal state of things.......

If the water is below 10 degrees, the pump has cut off and no water drawn off, and the calorifier heated to 65 -70 degrees, I may get a bit of leakage from the PRV, but I can live with that.

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Is there anyone out there who can tell me how I should size an expansion tank on my hot water system. I have 2 twin coil calorifiers at 40 lt each connected (or will be!) in series. Any help is gratefully received as always.

I'm equally sure that you can't get hot water coming out of your cold taps (as has been suggested in the past) although clearly it would be possible for the expanding water to drive some warmth back down the cold feed pipe if you were to open a cold tap that was very near to the hot tank. However the amount would almost always be negligible and usually zero; we've certainly never had any warm water from any of our cold taps.

The pressure also needs to be set BELOW the PRV pressure (which is usually around 42psi) otherwise the PRV will still blow as the water expands. Therefore the expansion vessel pressure needs to be set somewhere in the range 30 - 42 psi.

Distance from accumulator to calorilifier about 10 feet, distance from cold galley tap T off to accumulator about 4 feet. Calorilifier 55 litre, accumulator can't remember but normal blue job from midland. Did not fit a NRV.

An accumulator must be pressure set BELOW the pump's cut-in pressure (this is typically around 15psi). If not, one is not maximising the volume of water that can be held in the accumulator before the pump cuts-in. For space reasons below a sink, a typical accumulator is usually around 2 litres in size. The typical pump's cut-out pressure is around 30psi.

Upcoming Revisions In the coming months, there will be a new set of revisions to these standards such that these requirements will be reviewed again to ensure they are the strictest of any backflow standards or documents. At time of writing, the working group to revise ASSE 1013, 1015, 1047, and 1048 is reviewing the final set of revisions.

Backflow, IMI Hydronics, Balancing Valves, Combined Control and Balancing Valves, Control Valves, Actuators, Differential pressure controllers, IMI Relief Valve ...

I'm not exactly clear what you mean......... water is incompressible, so having an extra length section won't accomodate any expansion.

The result is interesting because it shows that with most pumps, which are set at about 30psi (gauge) cut-off, the PRV may still just blow if the water expands by its maximum. That's why I always turn my pump cut-off pressure down to about 27psi.

I took the question to be about adding an expansion vessel to the domestic hot water from the calorifier, to avoid water being ejected through the PRV as the system is heated.

Furthermore, the air in the pipe will dissolve in the water over a month or so and thus the system would need some means of relenishing the air.

Shouldn't they be pre charged to a pressure higher than your water pump can put out, so that the pump isn't capable of using them as another accumulator.

Why would an expansion tank have anything to do with the capacity of colorifier? Unless as other have stated you are talking about an accumulator.

The calorifer can then heat from about 20 degrees say up to bout 60 degrees with the expansion being taken up by the accumulator and the PRV doesn't release. Of course drawing off a little water takes out some of the expansion and allows more expansion to occur if the water heats further.

The pressure inside the expansion vessel needs to be ABOVE the pump's cut-off pressure (typically ~30psi) otherwise the pump itself will pump up the expansion vessel's diaphragm and thereby waste some of the effective volume which could have beem used for hot water expansion.