Here in San Miguel, sometimes the flow rate on your shower heads and faucets isn’t enough to keep your on-demand heater on and thus allowing “slugs” of cold water to enter the system. If you are handy you can choose to use more water by removing the constrictors in the shower heads and drilling a small hole (when necessary) in the orifice to increase the flow rate so your on demand heater doesn’t keep turning itself off thus allowing the cold water to enter the line. If your pressure is a bit low (you are up high in the house and you have a gravity feed tinaco without a pressure boost pump) then you may be signing up for frustration with an on-demand heater.
Solar preheaters save energy and can work with a wider range of pressures. It is a great combination and will reduce your propane gas consumption.
I have heard from enough owners and plumbers on this subject to say that in San Miguel de Allende you need to check your system carefully before installing an on demand heater:
- Do you have a 3/4″ gas supply line from your propane tank to the heater (needed for a 185,000 btu heater)? Long distances from your water tank (tinaco) to the heater may cause insufficient flow and tempermental heating if your line is the typical 1/2 inch copper. A recommended minimum heater size of 185,000 btu’s needs a good flow rate from the propane tank.
- Do you rely on street pressure to fill your water tank (cistern or rooftop tinaco)? Some houses have no cistern and no pressure pump. So they are stuck with gravity pressure from the rooftop tinaco. This means low pressure on the top floor to showers which improves as you go down to lower floors in the house. On demand heaters need a minimum flow rate of around 7/10 gallon per minute. You can test this by just running your hose or shower into a bucket for one minute.
- Do you have a boost pump to increase water pressure to your water lines, filtration system etc? Do you have good pressure everywhere in the house?
- Will you calculate your high or peak demand useage to correctly choose your heater size and does this demand match the delivery rate capacity of your water line (1/2 inch lines with rooftop pressure may not be enough unless your tinaco is perhaps 2 to 3 floors above the heater)
- Does your filtration system (whole house type) maintain enough pressure to avoid cold water “slugs”?
I cannot improve upon David Grubbs information on tankless hotwater heaters here.