Increasing pressure into tankless water heater

April 6, 2015 5:59 AM

The water supply to my house has rather low pressure, and today I'm about to buy a tankless water heater (haven't decided on gas or electric). Is it advisable to use a pressure bladder tank to raise the water PSI before it reaches the heater? Also, is there a way to estimate PSI without a gauge? (more detail inside)

I'm living in a small town in Costa Rica. There is no way to get greater pressure from the town's water supply. It's way too low. It may in fact be too low for an on-demand tankless hot water heater. So I've looked into this a little, and it seems that a pressure tank with internal bladder might be a necessary addition to my home's water system. In fact I saw some designs where 2 such tanks were used in series. Of course a pump would be needed before the pressure tank(s), or I might find a tank that has a built-in pump . In either case I'd be concerned that the low incoming pressure might be too low for the pump and wear it out, unless it's smart enough to adjust.

Is there some solution other than the pressure tank? Maybe if the water heater can automatically adjust to low PSI it will work? The hardware stores in this area may only have 1 or 2 options for water heaters, so they may not have an auto-regulating one. I'm headed out shortly to look.

Any suggestion about going with gas or electric? It has to be an on-demand heater for reasons I won't go into, not the one with a tank like most homes have in the USA.

Also, is there a way to roughly estimate my home's PSI without a gauge? They may not have a gauge at the stores here, either. I found some instructions at eHow for estimating it by lifting a water hose and measuring the height when water stops. Does that look legit? It says to measure from the height of the faucet to the height of the hose end. But what about the rest of the hose? It seems it would make a big difference whether some sections of it were elevated, and how long the hose is.

posted by TreeHugger to Home & Garden (8 answers total)

In either case I'd be concerned that the low incoming pressure might be too low for the pump and wear it out, unless it's smart enough to adjust.

I don't think this is a real concern. As long as there is enough pressure to refill the feed line to the pump (ie anything more than zero/slight negative pressure) this will not be an issue at all. The pump will just run slightly longer to get the same pressure post-pump if there is a big gap between pre- and post-pump pressure. It won't damage a pump to have to pump more. It would only damage the pump if it was evacuating the feed line as it ran, which is nigh on impossible for a mains feed line in most circumstances - especially as this is not exactly high volume if it is a domestic system.

A pump doesn't need to regulate for the incoming pressure. You'd only get an issue if the pump had to lift water up (so pump it up out of a hole) and didn't have the power to do that and was evacuating the line and running dry. If you have mains feed, I don't see an issue and you'd only see a problem at your end if you were using water faster than the pump could replenish your system (ie if your stored pressurised water volume wasn't enough for average usage).

I find it very, very unlikely that you can't find a water pressure gauge. There're very cheap and easy to fit. But if the pump has a pressure cut off (so it pumps to a pre-set pressure) you don't need it unless the pump malfunctions. If the pump doesn't cut off at pressure, I'm not sure how it would work at all anyway unless it was safe to just stall against a high pressure?

As for on demand heat - if you already have gas, I see no reason why you would consider electric. Gas heaters are much better and more efficient.
posted by Brockles at 6:20 AM on April 6, 2015 [1 favorite]

I found some instructions at eHow for estimating it by lifting a water hose and measuring the height when water stops. Does that look legit?

It looks totally impractical, regardless of how theoretically legit. If you take the hose to the peak of a 2-story house you might reach 30' above the ground level. 30/2.31=12.98. You probably have more pressure than that already, let alone after adding a pump.
posted by jon1270 at 6:21 AM on April 6, 2015

If the pressure is that low, why don't you consider a conventional (tank) water heater. They're not as energy efficient, but simple to install (gas models require no electricity) and as long as there is enough pressure to fill the tank it should work fine.
posted by H21 at 6:28 AM on April 6, 2015

Oops, just noticed that you seem to need a tankless! I still think it would be MUCH easier to use a tank type. No additional pump.
posted by H21 at 6:32 AM on April 6, 2015

That's a good point - you want to buy a tankless water heater, but would need to put in (up to) two pressure tanks to be able to run it... That seems... redundant. Can you not just run a tanked hot water system? If you spent some time and effort insulating it (so insulated tank in an insulated enclosure maybe) would a tanked water heater be just as easy?

Alternatively, do you get enough sun for a solar water heater on the roof as a 'pre-heater' and also give you some extra pressure head?
posted by Brockles at 6:58 AM on April 6, 2015

A friend of mine just bought a house on the top of a hill, and had to replace and upgrade the pressure system. Here in the U.S. it's just a trip to the big box store, say "I have a dribble coming in the main water feed", and they hand you a device that you plumb and plug in, and it all works. They might ask you if you want the small one or the big one.

For measuring it: Liquid pressure is simply a matter of how much height of a liquid there is above the measurement point. As jon1270 observes, 30' of head is 13PSI. In the U.S. 35PSI, or about 81' of head, is generally considered the low end for municipal supplies. If you can get 81' of head just put a storage tank up there and pump to fill it...

However, you can probably estimate using GPM flow through a given pipe diameter. If you have a hose connection right near your input feed, you can see how long it takes to fill up a 5 gallon bucket and estimate based on that. One water flow based on pipe size estimation table.

And, really, when you're sizing your pressure booster pump the GPM of the input is what you want to know to make sure that the pump isn't over-pulling the feed lines.
posted by straw at 8:52 AM on April 6, 2015

I know you were asking about water pressure, but when deciding between gas and electric tankless heaters, the gas ones really require a larger gas line to operate properly - you need a blast of gas to heat the water quickly. If I remember correctly when ours was installed, they installed a 3/4" gas line, as opposed to the regular 1/2" one. Mind you, I'm in Illinois, which is significantly colder than Costa Rica.
posted by sarajane at 12:05 PM on April 6, 2015

Response by poster: Thanks all... Your replies were on point. I ended up buying the Stiebel Eltron DHC-E 12 tankless water heater. It has a temperature control which will be most helpful.

Also, according to the guys at the hardware store (who may not really know), if the incoming water pressure is fairly low, the unit supposedly won't malfunction or overheat. In fact the instruction manual says it will activate (turn on) upon sensing a minimum input flow rate of 1 liter per minute (0.26 GPM). Not sure what that amounts to in PSI, but my inlet rate is obviously more than that, because it works.

Thanks to my fast-moving Tico contractor guy, it's all hooked up and functioning now. And the pressure coming out of the on-demand heater isn't that high, but it's enough for my needs. I had to spend about $500 for this heater, and my friends said I should have gotten the much cheaper propane-powered one which many of the locals use. That one is also on-demand and keeps a continuous flow. But I'm pretty happy with my choice despite the big expenditure. Thanks again.
posted by TreeHugger at 5:16 PM on April 6, 2015