Rinnai B26 instant HW functions? temp sense or pressure drop?

[rgormley]

have a question for plumbers and or people in the know!

I have a Rinnai B26 instant hot water unit that i had plumbed in line after a solar hot water storage tank.

NOTE: the solar hot water part is a evacuated solar tube heater that circulates the water to a 135 litre ex gas storage
cylinder, so it being used as a "pre heat" for the B26 instant unit.
i have full temperature monitoring on the pre heat system so i know the temp in the header and the water in the storage cylinder.

The water in the pre heated storage tank on a good sunny day can go as high as 40/50/60 degrees.
the B26 Rinnai instant HWS is pre set to 50deg C

Now the questions /observations.

I thought that if the water coming from the "pre heat" tank was say at 55deg C, the B26 Rinnai would not need to turn on the gas burner at all and just pass the water straight through as it was already hotter than 50deg C , thus if i turn the tap on i would get a flow of 55deg C from the pre heated water through the B26 rinnai and thus not need to use any gas to heat it any further....


What i have noticed is that regardless of the temp of the pre heated water the gas burner on the B26 always comes on.for the duration of the hot water tap being turned on.


Is the B26 a little more simple than i thought and simply works on a pressure drop and thus turns the burners on regardless? ie: NOT temperature sensing.
(although this does not make sense as the water if pre heated to say 60 deg would NOT need to be heated as it`s past the pre set temperature of 50dec C


The idea of a pre heater was do that during summer none or little gas would be needed to heat the water.

New photo by Richard Gormley - https://photos.app.goo.gl/wwqH78uZWybBTZnE8

any ideas?
Thanks..

 

[Gasmk1]

its an instantaneous water heater so as soon as you call for hot water it will sense a flow and turn on but it should modulate to the correct gas rate to keep the hot water temperature correct.

https://www.rinnai-uk.co.uk/files/9514/4231/2495/32E_Service_Manualsmallpdf.com.pdf

 

[rgormley]

thank you for the reply, that's what i thought it should do "modulate" the gas flow so as to have a 50deg output based on the water inlet temp.
so the burner will come on regardless, just that it will use the correct amount of gas to keep the water output temp at 50deg c (in my thinking)

 

[John.g]

Is the water supply to the Rinnai taken from near the top of the pre heated cylinder?, have you measured the rise in temp after passing through the Rinnai?, you could reduce its SP temporarily to its minimum of 40C and see if it fires up or not. IMO it certainly should not fire up under the circumstances you outlined.

 

[Gasmk1]

The temperature of outgoing hot water is CONSTANTLY MONITORED by a BUILT-IN SENSOR. If the temperature of the outgoing hot water rises to more than 3°C above the selected temperature shown on the Digital Monitor (or the pre-set limit when Remote Controls are not fitted), the burner will automatically go out. The burner will ignite again once the outgoing hot water temperature falls below the temperature shown on the Digital Monitor (or the pre-set limit).
• The burner lights automatically when the hot water tap is opened, and goes out when the tap is closed. IGNITION IS ELECTRONIC, therefore there is not pilot light. When the hot water tap is off, no gas is

 

[oz-plumber]

Look like an ok installation....but.

1/ Have you got a non-return valve on the outlet side of the pump to stop new cold water being drawn back through the solar collector and mixing with the water from the cylinder.

2/ If it's a B26 set to 50C, It the wrong unit for the installation. The water must be heated to a minimum of 60C to kill any legionella that may breed in the cylinder.
Most Solar booster for this installation heat the water to 65C - 70C - for that fact.
The tempering valve is going to cause you grief, because that operates to 50C and you only have 50C coming out of the unit.

3/ Whenever water passes through the Rinnai unit, the fan will always operate. The gas may not come on, due to the temperature of the water, but the fan is always ready for the ignition process.

4/ This unit may go to fault if the inlet water temperature gets above 50C.