Proper distribution for two houses & irrigation

[sanaka]

Hi.

I'm helping a friend renovate a new house he just bought. It has extensive irrigated landscaping and we're upgrading the irrigation fixtures and piping. There is a second house on a CPR subdivided lot whose water is supplied via a sub-meter off the main house's supply.

The current layout is like this:

There are service valves for the irrigation all over the place, at least one of which kill the water to house 2. Because of this and because it seems the irrigation going on and off would cause fluctuating pressure in house 2, and since we're reconfiguring things anyway, I recommended to my friend to change the layout to this:

That way, house 2 doesn't lose water when the irrigation is being serviced, and I'd think it would smooth out pressure fluctuations somewhat.

Problem is, water pressure isn't that great, probably 35 psi, and my friend is worried giving house 2 a 'dedicated line' could diminish his pressure in favor of theirs.

Can anyone tell me if this is so?

I couldn't give him assurance, only my feeling that my suggested layout seemed more equitable, less hassle, somehow more 'right.'

Can anyone back me up or say why it doesn't matter? Thanks!

Peace,
Sanaka

 

[Jimbo]

In the 2nd drawing, it all depends on the size of the supply piping from the main meter, and the gpm capabability of the meter and it's feed from the street. If the supply is limited, then if either house draws too many gpm, or if the total gpm to both houses is too high, the pressure at that point where they tie together will drop. 2nd house also has the issue of lenght/size of its supply line. By the way. both houses will then experience pressure drop when the irrigation is running.

35 PSI is a relatively low supply pressure, at least by standards around here. There is a good chance that both houses will have issues. I don't know how you support an extensive irrigation system off of this, unless the house supply is large and delivers large gpm/

 

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[Gary Swart]

You have minimal pressure at best. The main meter should be 1" or larger to be able to get sufficient volume, and the supply lines should be kept at least 1" to keep friction loss as small as possible. Increasing the meter size will not increase the pressure, but will allow more volume of water flow. Both of these drawings show cross connections which require the use of backflow prevention devices to avoid the possibility of water from the irrigation system syphoning back into the house supplies. In drawing #1 you need to have 2 separate backflow preventers, in drawing #2 only one would be necessary. If you use electric valves and a timer, you can irrigate only at night when the supply pressure could be a tad higher and domestic use would not be a factor. You must also take into consideration the gpm requirements of each of the irrigation zones. There are charts available that can help you determine how many gpm will be available with the sizes of meter and pipe, and the pressure available.

 

[sanaka]

Thanks Jimbo and Gary.

The irrigation is on timed zones and programmed around house usage. Thanks for the tips on backflow prevention, I will bring that up with my friend as well.

My main question is about whether House 2 having its 'own' line, which circumvents the irrigation system, as in drawing 2, will impact the pressure or flow to the main house more than how it is now, as in drawing 1.

Thanks!

Peace,
Sanaka

 

[Bob NH]

I would consider the following:
1. Put in a booster pump with a tank to supply the two houses at 50 to 60 psi, AND

2. Connect the irrigation system to the 35 psi line, through a back-flow preventer. Design the irrigation system so it will work at the lowest reliable pressure; 35 psi or less.

 

[Gary Swart]

As far as I know, if water is supplied through any city or town water system, a backflow is required if the water is being used for irrigation. There is a chance, albeit a small chance, that contaminated water from the irrigation system could backup into the main water supply. I am required to have an annual inspection of my backflow device to certify that it is functioning properly. It keeps my house water pure as well as keeping any contamination from getting into the city main. I find it interesting that a city water supply would have such a low pressure.

 

[sanaka]

I don't know the actual pressure. I haven't measured it. I'm estimating based on feel compared to a couple places whose pressure I've measured. I just know it's marginal. There's a storage tank/booster system on the land that could possibly be refurbished and used. I promise to address proper backflow prevention.

None of the above pertains to my actual question.

Can anyone say whether the configuration shown in drawing 2 in my originating post for this thread would result in better or worse flow or perceived pressure in the main house as compared to drawing 1? Thanks!

Peace,
Sanaka

 

[Bob NH]

Case I will give serious pressure problems to House 2 when the irrigator is on.

Case II household demand won't affect pressure very much if the pipes to both houses are adequate.

But Case I won't deliver any water to House 2 if a proper backflow preventer is installed, unless you have a backflow preventer on every branch line. You need a setup that will protect both houses and the supply system from the irrigation system. That means only one irrigation tap off the main and no potable supply after the preventer.

 

[Gary Swart]

Bob, thanks for straightening out my original post regard using 2 BFP in Case #1. You're exactly right, of course.
Sanaka's last post illustrates the dangers of trying to diagnois problems online. Often little tidbits of information are left out of the problem description. Sanaka, you can't just guess water pressure when laying out a complex system like this. Go to a hardware store and buy a $10 pressure gauge.

 

[sanaka]

Thanks Bob and Gary.

I have a pressure gauge, just haven't measured yet. The actual specific pressure figure is not that important given the obviousness of the pressure just being pretty low - care will have to be taken no matter the psi reading.

What's basically happening is I'm arguing to change the layout from case #1 to case #2, but am meeting a lot of resistance from my friend the owner, who will live in the main house. The case #1 configuration has been in place for years with no one complaining, so my friend feels a) why fix what has never been seen as broken, and b), most importantly that the case #2 setup might decrease his already marginal pressure/flow (main house).

Now, the backflow argument should trump on it's own, as there's not a good way to protect house 2 from irrigation backflow in the current, case #1, layout.

Nonetheless, it would be very helpful to be able to assure my friend that his pressure/flow won't be diminished by changing to case #2.

Bob, do you mean to affirm that this is so when you say:

Case II household demand won't affect pressure very much if the pipes to both houses are adequate.

?

This is kind of a hydro engineering question, I wish I knew the pertinent maths, but it's the same principle as manifold/parallel distribution within a house, as is seen in pex systems, versus traditional series/branching. The manifold setup is supposed to even out pressure fluctuations among fixtures and alleviate things like the shower going cold when the toilet is flushed.

Going to the case #2 layout, I feel sure the same benefits would apply to house 2. But would this diminish flow or perceived pressure in the main house?

Thanks again!

Peace,
Sanaka

 

[sanaka]

Jimbo, I just reread your reply more closely. You said:

In the 2nd drawing... if either house draws too many gpm, or if the total gpm to both houses is too high, the pressure at that point where they tie together will drop

Does this imply that in the case #1 drawing, pressure to the main house won't drop as much as in the case #2 drawing, under the same total gpm draw? Thanks!

Peace,
Sanaka

 

[sanaka]

FWIW I measured the static pressure at the main house's main cutoff this morning: 32 psi.

Peace,
Sanaka

 

[Bob NH]

If the main supply is too small any of the systems will diminish the pressure at both houses. The pressure loss in the supply line will vary as about the 1.8 power of the flow in the supply line.

There is no significant difference in the delivery pressure for cases where the delivery to the houses is the same for each case.

If you constrict the flow to house 2, that will increase the flow available to house 1.

The intent should be to install a system that will be adequate for both houses. Or is it intended that House 1 will get all he wants at the expense of House 2? You can do that by putting a back pressure regulator in the line to House 2 so that it cuts off flow if the pressure to House 1 drops below a set value.

The owner of house 1 seems to be willing to discriminatingly diminsh the flow to house 2 to ensure that his flow is not diminished. If I were being asked what to do, I would tell him that I am not willing to be involved in what I consider mistreatment of the owner of House 2 who has no reason to be discriminated against.

Fix the supply line so everyone gets decent pressure and flow.

 

[sanaka]

Thanks Bob.

My feeling, the reason I suggested case #2 in the first place, is that the current situation, case #1, could be seen as mistreatment of House 2, due to fluctuations from the irrigation, and especially the lack of backflow prevention. However, House 2 is, perhaps ignorantly, content.

The owner of House 1 doesn't wish and isn't proposing to diminish House 2's service, but would really prefer his not be diminished either. Hence the resistance to 'improving' the system if it diminishes his service, as long as House 2 has been happy all along anyway. Thanks for pointing out that the two configurations will probabaly not effect an appreciable difference in pressure to either house.

Upstream of house 1's meter the supply line is 2", so luckily there is gpm headroom. Just no real psi headroom. House 2 is single story and ~20' lower elevation than House 1, which is 3 stories. So House 2's pressure is probably a fair bit nicer on average as well.

Peace,
Sanaka