Tankless Gas Water Heater with low activation flow

Hey all,

I installed a Marey 3.0 GPM tankless propane water heater. I have a 3.0 GPM water pump that pressurizes my system to ~40 PSI. The minimum activation flow rate for the Marey water heater is so high that I have to have the hot water fully open to activate the heater so that if I ease off the hot water in the shower even just a little the water heater shuts off. Any suggestions for a work around or a water heater better suited for low flow situations?

I think I had a similar problem that was solved by turning down the flow rate through the heater.
Mine is activated when water passes through a turbine that spins an igniter and releases propane to ignite.
If the flow was set at max, it would not light at low water flow levels.

It looks it supports 0.5gallon to 3.1gallons

Minimum Activation Rate 0.50 GPM

https://marey.com/product/power-gas-10l-ng-ga10ng/

S Benji, your message seems muddled up!

As a result the recent Texas winterpocalypse and my poorly insulated enclosure housing my water heater I have the the opportunity to revisit finding a tankless propane water heater that works under low flow conditions. I found the following post on the Green Building Advisor website. The following paragraph from the post sums up to solution:

The key to providing hot water at low flow rates with tankless water heaters is to match the combination of flow rate and temperature rise to fall within the range of the water heater's capacity. The other way is to have some amount of stored hot water (generally called a tank!) that can be used to accomodate low flow rates as well as provide for some amount of peaking capacity (which depends on the stored volume.)

I'm going to try out the Marey GAS 5L – 1.89GPM Liquid Propane Tankless Water Heater to see if the small unit works better for our needs.

I hope that you weren't hit too hard by the freeze up.

One solution to your initial problem may have been to turn the burner knob down lower, lowering the temperature of the water coming out of the unit. This would mean that in your shower you'll be asking for more of the flow to be coming from the heater.

I've found it can be challenging to find the sweet spot with these things. Showering can be a very active exercise...


I think my unit is a 4.8GPM "TC-Home" (pretty sure it's the Marey in a different case), 40PSI water. When dialled in it works quite well, though I prefer showering from a hot tank of water as it provides near-constant temperature where as the on-demand unit is more prone to wander higher in temperature --> if you turn the hot water down at the shower the water in the tankless heater creeps up in temperature, leading you to further turn down the shower, which further increases the temperature in the unit... eventually you have to shut if off and wait a minute or so before starting over.

The death of the tankless water heater was the biggest loss. We also had some PVC pipes running into our cistern pump shed crack. It's fixed for now. I'll do some winterizing redesign once the run on plumbing supplies is over.

Our cabin is pier and beam and all the water supply lines are strapped underneath. I haven't checked the hot water lines but all the cold water lines appear to have survived intact. That's a big relief.

This was our first go with a tankless water heater. I agree that the sweet spot is often elusive. We've been experimenting with a batch solar hot water heater that feeds the tankless unit further compounding the issue. On the plus side making showering an active sport seems to really reduce showering times.

I'm thinking about installing a 2 gallon thermal expansion tank inline just before the shower to provide a little moderation between scalding and ice cold.  

Sometimes the pipe diameters effect the operation.
I use 20 or 25mm poly pipe.
But I have seen 12mm pipe and I wonder if that causes issues with the lower pressure and the flow?

Aaron Yarbrough wrote:

I'm thinking about installing a 2 gallon thermal expansion tank inline just before the shower to provide a little moderation between scalding and ice cold.  

Ah, interesting, as long as the incoming and outgoing water gets mixed nicely this could be a good solution.

PEX pipes are pretty resilient to freezing in my experience.

Aaron Yarbrough wrote:As a result the recent Texas winterpocalypse and my poorly insulated enclosure housing my water heater I have the the opportunity to revisit finding a tankless propane water heater that works under low flow conditions. I found the following post on the Green Building Advisor website. The following paragraph from the post sums up to solution:

The key to providing hot water at low flow rates with tankless water heaters is to match the combination of flow rate and temperature rise to fall within the range of the water heater's capacity. The other way is to have some amount of stored hot water (generally called a tank!) that can be used to accomodate low flow rates as well as provide for some amount of peaking capacity (which depends on the stored volume.)

I'm going to try out the Marey GAS 5L – 1.89GPM Liquid Propane Tankless Water Heater to see if the small unit works better for our needs.

Hello Aaron, I too lost my Bosch propane indoor tankless water heater in the Texas winterpocalypse of 2021, in Alpine TX. It's in a small guest house that we're about to rent, so I'm only now replacing it.  What was your solution?  We have low flow and a tiny space built just for tankless heaters, so I can't go the tank-heater route.

Hello Aaron, I too lost my Bosch propane indoor tankless water heater in the Texas winterpocalypse of 2021, in Alpine TX. It's in a small guest house that we're about to rent, so I'm only now replacing it.  What was your solution?  We have low flow and a tiny space built just for tankless heaters, so I can't go the tank-heater route.

The Marey GAS 5L - 1.89GPM tankless water heater is working for us. Part of our issue was that I had solar batch water heater feeding the tankless water heater so the the incoming water temperature could be pretty swingy. Since detaching the solar water heater from the system it's more consistent and I can gradually adjust temperature output throughout the year.

Good to hear! This one has a much nicer price than others I've looked at. Is it OK for indoor? (I suppose that's the vent at the top, to be vented through the roof.)

Does it have a pilot that stays on, and if so is that pilot sturdy? (i.e. do you have to relight it often)

Sorry for the basic questions. Our frozen-&-burst one was a Bosch indoor propane tankless and we had it vented through the roof, so that space exists indoors but with the only access from an exterior panel. It was a pain to access to relight when air flow would blow it out or a guest would tinker with the hot valve on the shower, varying the flow rate across it.  The access situation would be the same, so I'm hoping little regular access is needed. My husband had installed it, but he has since had a stroke and is "leaving everything to" me.

Jessie Kelsch wrote:Good to hear! This one has a much nicer price than others I've looked at. Is it OK for indoor? (I suppose that's the vent at the top, to be vented through the roof.)

Does it have a pilot that stays on, and if so is that pilot sturdy? (i.e. do you have to relight it often)

Sorry for the basic questions. Our frozen-&-burst one was a Bosch indoor propane tankless and we had it vented through the roof, so that space exists indoors but with the only access from an exterior panel. It was a pain to access to relight when air flow would blow it out or a guest would tinker with the hot valve on the shower, varying the flow rate across it.  The access situation would be the same, so I'm hoping little regular access is needed. My husband had installed it, but he has since had a stroke and is "leaving everything to" me.

Yes, it vents through the roof. The pilot lights automatically powered by two D batteries. The batteries lasted us about nine months.