Sizing a pressure tank « Alternative Technology Assn Forums

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Sizing a pressure tank

(21 posts) (11 voices)

Started 8 years ago by Fateyes
Latest reply from ghostgum

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Fateyes
Member

Hi,

With little engineering know-how I am struggling to understand the ins and outs of some of the posts here on pressure tanks.

What I do know is that I've been told it could be worth getting one.

Our system currently is this:

A brand new (not plumbed in yet - crappy old pump broke) Lowara (Xylem) 2HMS4 pump.
A 20,000ltr tank.
Whole of house supply - 1 shower, 1 wc,1 bath, 1 bathroom sink, 1 kitchen sink, 1 laundry sink, washing machine + outdoor hose.

I cannot find an Australian site with a simple guide on sizing a pressure tank for the pump. Using a US site and doing some conversions suggests at least a 100 ltr tank as a minimum which sounds too big/expensive. My supplier had 8ltr and 18 ltr Lowara tanks in stock - but this sounded small to the point of why would you even bother?

Is there a minimum size I should be looking at for the rating of my pump?
And is there a maximum at which point the additional expense makes it not worthwhile?

Is it just another point in the chain that could fail/break/require servicing, etc?

Thanks for any, simple explanations/assistance,

Cheers,

Laurie

Posted Tuesday 18 Jun 2013 @ 10:14:46 am from IP #
Johnnojack
Member

Maybe your pump can operate without a pressure tank, many new ones are made to do this. However it will still benefit from having a pressure tank. Remember if there is no pressure tank the pump will start even when you fill up a glass. While it may use minimal power when it does it would be better if it only started every 10, 20, or even 100 litres. Pick a number. I would go with the 18 litre model as it is possibly the best compromise between size and price. I would like to get a 100liter tank for my pump as it is not a 'smart' type and the less times it starts the more efficient it is, and the more smooth my shower is. As for adding another point of failure, the most likely point of failure of the pump is the start switching mechanism, and less starts means less wear.

Posted Tuesday 18 Jun 2013 @ 11:25:52 am from IP #
Lance
Key Master

Yes, it does depend on the pump to some degree, some are variable speed and don't really need a tank, although all systems will benefit to some degree with a tank, and the general rule is, the larger, the better.

Small tanks will still result in a lot of pump cycling, remember, you only get to use about 1/3 of the actual tank volume as the pump cycles between a minimum and maximum system pressure.

If it is a simple on-off type pump then I would get a minimum 100 litre tank. We have one a bit smaller (is fairly new and was here when we moved in so can't find the excuse to upgrade it really) and you do notice pressure variations.

Posted Tuesday 18 Jun 2013 @ 12:06:31 pm from IP #
johnmath
Member

I don't understand the "must have a tank" view that is often expressed. My whole house and garden is running on rain water for several years now on a single no-brand Chinese made inexpensive pump I bought second hand. It uses around 80 kWh per annum (or around $30 cost of electricity) so it is hard to see how there would be any "savings" by adding an expensive pressure tank. (When I travel around rural SA I see pressure tanks littered all over the place like discarded drink containers, so it is would appear - superficially at least - that pressure tanks have a high annual amortised cost anyway.)

Every time I have experienced a pump fed house/hotel/motel that incorporates a pressure tank (going all the way back to my grandparent's house in the 1960s) I have found the sawtoothed water pressure that occurs because of tank induced pump cycling, and the consequent pulsating water flow, to be annoying. Even the noise of the pump cycling on and off during washing machine fills and showers, etc, is more irritating than if it is on constantly - at least to me. I can imagine the pressure cycling would play havoc with instantaneous water heater temperature regulation too. These are problems that DON'T exist in a system WITHOUT a pressure tank. YMMV

Posted Tuesday 18 Jun 2013 @ 1:08:14 pm from IP #
yabbietol
Member

Hello Lance and others
I am very interested in getting a better pressure pump i am on tank water and just spent a couple of hundred $ repairing my Davy pressure pump Good repair job, but disappointed in a pump only a few years old cracking its stainless steel impeller cover and the control unit also became a unreliable. A new Davy pump is expensive but it does do a lot of work. Does any one have any recommendations about good reliable pressure pumps. I also have a no name Chinese pump to fill my toilette cisterns and it while giving good pressure has an unreliable control unit.
I agree with johnmath about pressure tanks and think the solution is a reliable reasonable priced pressure pump either 240v or possibly 12 V solar powered (once had 12V pump in weekender and it was noisy, but worked well). I am no longer sure about Davy as I have had two in 10 years and needed repairs also they are expensive. Any suggestions on good value pressure pumps

Posted Tuesday 18 Jun 2013 @ 10:39:06 pm from IP #
Greg
Member

Go with a name brand, Onga, Davey etc. They will give you what you want.
One added advantage of a pressure tank - when the power goes off you still have water pressure. Not a problem for most but regular outages made this viable for me.

Posted Wednesday 19 Jun 2013 @ 2:18:46 am from IP #
dlvb19
Member

+1 John Math

Skip the pressure tank and get a good quality variable speed pump.
Grundfos!!!!!
Anecdotal evidence suggests Australian brand pumps are a ways behind in terms of durability. Davey pumps estimated life = 10yrs, Grundfos = 25yrs, Grundfos only 20% more expensive. Everybody knows Australian manufactured products are simply adequate whilst the Germans lead the pack.
Again, only anecdotal.

Posted Wednesday 19 Jun 2013 @ 3:03:14 am from IP #
Fateyes
Member

Hi,

Okay, thanks all, well not sure still! Maybe will save up a bit of cash and go for a 60 ltr one.

Cheers,

Laurie

Posted Thursday 20 Jun 2013 @ 9:32:02 am from IP #
TonyT
Member

Without a pressure tank, the pump starts every time a tap is opened. It may then run for only a few seconds.

The larger the pressure tank the more Litres of water can be used before the pump starts.

With Direct On Line starting (typical) there is a high current (? 7 times the run current) when a motor starts.

Fewer starts each year means that a motor lasts for more years.

Posted Thursday 20 Jun 2013 @ 11:18:49 am from IP #
Johnnojack
Member

Johnmath while you are having a good run with your pump and it suits your needs I will just agree to disagree with your view. I have a Davey pump(not stainless but plastic) which has moved with me from my last house and which is still perfect after 20 years. Blimey that surprised even me when I worked it out, but I got it in 1993. Ok it is on it's second pressure tank. I agree that the tanks are scattered across the countryside. In fact the 20l tank I have had on there for the last 15years is one of unknown age I found dumped in the SE. Haha it is a Grundfos tank so yes they are good quality. While I agree with Tony's points,(for my old pump) the new 'smart' pumps do have variable speed and do not create a current spike when they start. That said I'll stick with what I've got and if the pump or tank ever give up I'll replace with similar, maybe a Grundfos with larger tank. Hope I'll one day be able to install it at the retirement home.

Posted Thursday 20 Jun 2013 @ 11:57:22 am from IP #
yabbietol
Member

Hello again
An update on my above posting. I should point out I live in a rural area and am totally dependant on tank water and pressure pumps.

Since my last post above my toilet cistern filling cheap Chinese pump developed a serious major leak. So I was forced to act quickly I had a 40 psi 17 litre a minute 12v pump as a replacement pump on hand. I had purchased the 12v pump from escaping outdoors on eBay (≈$70) to replace my caravan water pump (the original Avan pump was undersized and very noisy for the work it had to do).
My new toilettes had just replaced my 13 year old Dowmus composting toilettes (which were an expensive disaster) the Dowmus needed 12V exhaust fans which I had converted to 12V solar some years ago. So fortunately I still had the 100W panel, charger and 100 AHr deep cycle lead acid battery still in place.

So yesterday I plumbed in my 12V 40 psi pump. Now my toilettes cisterns are filling quite well from their dedicated 22,000 litre water tank (they have Geberit fill valves which operate on low as 1.5psi, but my gravity feed is way too slow). The time taken to fill at 40 psi is comparable to time taken to fill at 140psi form a 240V 10bar pump. Yeah, got rid of dodgy240v cheapo pump and no worry about toilet flushing in blackouts and hopefully a slightly cheaper power bill.

Now all I need to do is find a suitable 12V solar powered pressure pump for the rest of my house's water supply. My next project when funding and time permit. I doubt if I can find a powerful enough 12V pump to supply pressure for my roof top solar HWS and still have a decent shower.

Posted Thursday 20 Jun 2013 @ 11:27:34 pm from IP #
Gazoola
Member

I am still undecided about pressure vessels (PV).
SCENARIO 24l PV with draw down of say 8l.
Shower : 5 min shower @ 8l per minute. The pump will still come on 4-5 times.
A jug of water in the fridge will negate the pump coming on for each glass of water.
Toilet flushes look like the bigger issue - maybe two flushes per (if lucky).
Yes there are other times like rinsing hands, dishes where the PV would be excellent, but all the pump shops I discussed this with said no need for a PV with modern pumps. I even saw a builders plans, for a whole estate of houses, and all had water tanks and pump (Davey submersible D42) but no pressure vessel.

Some have commented on smart pumps. Are these the pumps that are stated as being "Constant pressure", like the Torrium controlled pumps I've seen in the Davey range? Or are smart pumps the variable speed pumps?

Any other recommendations of variable speed pumps other than Grundfoss?

Posted Thursday 10 Oct 2013 @ 5:15:11 pm from IP #
Gazoola
Member

For my planned setup with tank and mains, I want tank water just for WC, laundry, 1 outdoor tap, and HWS. Note that all other cold taps, kitchen, bathroom are on mains.

With the issue of variable pressure, when using a PV, if I have a shower, cold is on mains and hot is from HWS supplied tank water with a PV, would I feel huge swings in temperature?

One other thread mentioned you can use a regulator after the PV the smooth out the pressure. This sounds like a great idea if it works, but if so, then why isn't everyone doing this. Has anyone fitted a pressure regulator to their PV outlet and can vouch for this?

Posted Thursday 10 Oct 2013 @ 5:22:05 pm from IP #
ghostgum
Member

I've got an 18l pressure vessel with 8l draw down, on a Davey pump with the Torrium controller. When you turn on a tap, the pump remains off for small amounts of water usage (e.g. cistern refill). Once the pump starts up, it remains on until the tap is turned off and the pressure vessel is full (at shut off pressure).

I don't have it connected to the HWS (just the toilet, washing machine and garden), so I don't know if the pressure fluctuation causes temperature swings.

Posted Thursday 10 Oct 2013 @ 11:27:49 pm from IP #
Diver
Member

Gazoola said:
SCENARIO 24l PV with draw down of say 8l.
Shower : 5 min shower @ 8l per minute. The pump will still come on 4-5 times.

The tank (8 litre draw down) will not fill instantly but yes, cycling should be avoided.

A pump will run for much longer when refilling a larger pressure tank that is in use. An 8 litre draw down is not suitable for whole of house use.

Gazoola said:
I even saw a builders plans, for a whole estate of houses, and all had water tanks and pump (Davey submersible D42) but no pressure vessel.

That'll be right...and I'll bet that every pump will be installed incorrectly and not as per the manufacturers instructions. That's why tanks with submersible pumps have less sediment and are 'cleaner' inside.

I regularly see numerous sub standard systems on new homes. Builders are the last reference you should use when assessing the validity of using a pressure tank. Their priority is lower building costs and maximum mark ups; not having good rainwater harvesting systems.

Gazoola said:
Any other recommendations of variable speed pumps other than Grundfoss?

Several years ago, a past acquaintance developed a revolutionary new pump that uses considerably less energy and is quieter than anything on the market... so quiet that the 1665 W pool pump version delivers 530 litres/min at noise levels that are about the same as most lap top computers.

The pump was initially going to be licensed for pool and pressure pump use but the focus shifted to large industrial applications and China manufacture. The household pressure pump versions are a while away yet.

Australia is behind places like California that have strict regulations that, for example, require the use of variable speed pool pumps.

Centrifugal pumps have had little development over many many years.

Posted Friday 11 Oct 2013 @ 1:52:56 am from IP #
Gazoola
Member

ghostgum said:
I've got an 18l pressure vessel with 8l draw down, on a Davey pump with the Torrium controller.

I thought that pressure vessels (PV) needed a manually adjustable pump pressure controller to set the high and low pressure. So I'm surprised to see that you have a Torrium controller which is an automatic constant pressure controller and an 18l PV. Are you happy with the setup? The 8l draw down is pretty good too, being 44%. Is that an accurate figure?

Posted Friday 11 Oct 2013 @ 7:56:05 am from IP #
Johnnojack
Member

Gazoola, you said "With the issue of variable pressure, when using a PV, if I have a shower, cold is on mains and hot is from HWS supplied tank water with a PV, would I feel huge swings in temperature?" If you are one of those people who like to run from the sauna to the icy pool and back, this system will be perfect. No seriously it won't work. You need hot and cold water from a common supply if you like a constant temperature shower.
If the rainwater is good for the HWS why isn't it good enough for the cold? You are investing in a rainwater system so why pay for mains water as well. Not sure where you are located but if it is raining and your tank is overflowing why not use it. When the tank gets too low, switch the whole house back to mains. Having a dual plumbing system in the house is kinda 1970s now. What with dual flush toilets, front loaders, large tanks and big houses with large catchment areas. Spend the money you save on plumbing on getting a bigger tank. FWIW my last years use from mains water in suburban Adelaide was 6kL That all went on the rose garden overnight.

Posted Friday 11 Oct 2013 @ 11:55:54 am from IP #
Greg
Member

I posted some information on my pressure tank here -> http://www.ata.org.au/forums/topic/2128/page/2

Its giving me a small electricity saving (would you buy it based on payback - no), better quality water flow and prime for me - water when there is power outage. Not for everyone but for me it works.

Posted Friday 11 Oct 2013 @ 10:15:18 pm from IP #
Gazoola
Member

Johnnojack said:
You said "With the issue of variable pressure, when using a pressure vessel (PV), if I have a shower, cold is on mains and hot is from HWS supplied tank water with a PV, would I feel huge swings in temperature?" If you are one of those people who like to run from the sauna to the icy pool and back, this system will be perfect. No seriously it won't work. You need hot and cold water from a common supply if you like a constant temperature shower.

I don't think it matters for hot and cold water to be from a COMMON supply if you like a constant temperature shower. You must mean that the hot and cold taps (even if from a seperate supply - tank and mains) should have CONSTANT PRESSURE, so that when you adjust your shower temp it stays the same.

So would a pressure vessel (PV) feeding the hot tap vary the pressure enough to make a NOTICEABLE difference to a shower if the cold feed was on mains? My gut feeling is that it would...
So there is not just size of the PV to consider, but for those considering mixed supplies, the use of a PV could be difficult.

Options for me to consider:

1. As suggested by Johnnojack - Put the whole house on one supply at the same time, either tank or mains backup.

2. Plumb tank water only to toilet, laundry and outdoor tap - where hot/cold mixing is not needed.

3. Use a constant pressure pump with no PV.

4. Use a variable speed pump.

Posted Saturday 12 Oct 2013 @ 1:00:45 pm from IP #
Johnnojack
Member

. "You must mean that the hot and cold taps (even if from a seperate supply - tank and mains) should have CONSTANT PRESSURE, so that when you adjust your shower temp it stays the same."
Well yes, however unless you have gravity feed from 2 separate header tanks at the same level, how do you get constant pressure from 2 sources?. If the cold is on mains and hot is PV then you may start off with right temp. then as the PV drops in pressure your shower gets colder until the pump kicks in and then it gets hotter.
I once had a house with constant pressure HWS (open solar tank in roof) and a rainwater tank with pump and PV. To avoid problems with the shower I put a cold water feed tank in the roof space for the showers only (open tank with float valve like HWS). You could even have a shower during a blackout. 29 years on it is still working fine I believe.

Posted Sunday 13 Oct 2013 @ 11:28:29 am from IP #
ghostgum
Member

Gazoola said:
I thought that pressure vessels (PV) needed a manually adjustable pump pressure controller to set the high and low pressure. So I'm surprised to see that you have a Torrium controller which is an automatic constant pressure controller and an 18l PV. Are you happy with the setup? The 8l draw down is pretty good too, being 44%. Is that an accurate figure?

You need to pump up the pressure vessel to suit the pump pressure - details in the pump installation guide. "Torrium® cut-in pressure is normally 80% of the pump's last shut-off head." So I suspect that you don't get 8l draw down. We do get enough for one toilet flush which is 3/4.5l. That reminds me I need to do my periodic recheck of the pressure.

Posted Sunday 13 Oct 2013 @ 10:49:20 pm from IP #

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