Minimum return water protection valves - is it needed?


Minister of Fire
Jul 11, 2008
Northern NH
Challenging the need for minimum return protection valve on boiler with non pressurized storage

Pretty much standard practice and required by vendors is a thermostatic return water low temperature protection to a wood boiler. The operation of the valve bypasses return water away from the building heating demands until the boiler is above the setpoint of the thermostatic element. This probably made sense at some point for systems without storage but IMHO its overkill and “belt and suspenders” for those with non pressurized storage. I don’t have pressurized storage so I can’t comment on its needed. Before folks get out the torches and pitch forks let me lay out my reasoning.

The intent as I understand it of return water protection is to keep the boiler from operating at a temperature that is below the dewpoint of flue gases to limit dewpoint corrosion on the flue gas side of the boilers heat transfer area. Starting with cold storage and cold boiler there will be a period of time where the boiler will operate below dewpoint. My boiler and other vintage units originally were set up so the circulator pump ran as long as the boiler was powered as the concept of storage wasn’t very popular. With this setup it made sense to have return water protection as during warm up it took a long time to bring the water temperature up.

I believe current practice to reduce the electric cost is to use a temperature sensor on the boiler jacket or boiler outlet that senses the boiler water temperature and cycles the pump on and off until the temperature exceeds a set point (I use 140 F). IMO this is effectively doing the same thing as return water protection. The only advantage I can see with return temperature protection is it cuts down on temperature stratification in the boiler jacket from top to bottom. My added on aquastat is at the level of the OEM combined temperature pressure gauge. During initial warm up the pump will cycle on and off for a few minutes until the boiler and the piping out and back from the storage is up to temp. As far as I am concerned during startup, return water protection would actually be encouraging operation below dewpoint on startup as it would be circulating cold water through the jacket while the my pump with aquastat method would not.

So what am I missing on why someone should spend an extra $200 bucks to put in return water protection?


Minister of Fire
Dec 5, 2005
Sand Lake, NY
$200 bucks? Chickenfeed when doing the boiler thing.
Now, I just have a pellet boiler with a measly 120 gallon tank, so what I have is a miniature, storage wise.

As I recall, I have a sensor for the single speed--I'm sure new stuff has continuously variable-- boiler pump. I also have a boiler protection valve. But, what I am most proud of, lol, is an electrically controlled three-way valve that isolates the tank. My thinking was, now this is a pellet boiler, to think of the tank as a place to store excess heat. The three way valve isolates the tank so that the boiler circulates its hot water through the zones, rather than dumping into a cold tank which supplies cold water, initially, along with the cold zones, to the boiler. Now these are baseboard emitters (low volume compared with a slab).

In the real world, with a wood boiler burning hot and maybe not modulating, and your storage having to keep up to absorb it all, and with unpressurized, you can't do anything with the heat until it's in the tank. I don't know.

Those are my musings.


Minister of Fire
Jul 11, 2008
Northern NH
Surprisingly not. The actual volume of the liquid in the storage coil and piping is quite low compared to the volume in the boiler. By the time the boiler temperature switch is 140, the top most half of boiler where the firetubes are located is most likely stratified to higher temperature than 140 the outlet. The colder return water come back at the bottom of the boiler so there quite bit of buffer betwen the return connection and the aquastat.

I am not saying the temperature vale and associated piping is not a good belt and suspenders option, I just think its overkill if the rest of the deign takes return water into the design. I generally try to get rid of stuff that can go bad and a thermostatic return valve fits that profile.


Feeling the Heat
Jan 20, 2009
Central Ma
I don't have an answer to your specific question, or any other question related to return protection for that matter either. However, it's a subject I've thought about a number of times in the 10 years since getting the wood boiler, so I'm going to toss in my .02 (which is actually mainly questions), and hopefully it won't trash up your thread too much.

I'll start by saying that I've never really understood the flue gas thing, primarily because I'm the kind of person who needs to have my hands on something, in order to really comprehend it. And, although I understand dew point, I can't fully wrap my head around how flue gas is in proximity and stays wet long enough to cause significant rust to the internals of a boiler. Perhaps if I could physically inspect a rotted out unit, including the flue gas travel path, the whole thing might click in for me.

However, one thing I do understand completely is condensation caused by the interaction of hot and cold, simply because it's something I've seen examples of many times in everyday life. So if instead of flue gas, the condensation problem was described/defined as hot-cold differential issue, then it would make complete sense to me. The boiler is producing heat which primarily rises upward, and the cold water return to the bottom of the unit is capable of producing the hot-cold differential required to create condensation on internal boiler components. And those conditions would exist for some variable period of time: either until the total overall temp gets hot enough to make condensation impossible; or the return water temp is modified with hotter output boiler water, minimizing the hot-cold water differential temp. So, if the need for return protection was described simply as the minimization of hot-cold differential, it would be completely logical to me.

One other thing related to this that's also been very puzzling to me is the operation of my oil boiler (and I assume most other older fossil units as well). I never thought about this stuff before installing the wood boiler, but as we all know, you learn many new things along this journey. And one of those things I've read a few times is the opinion that all fossil boilers should also have return protection for the same reason as my wood boiler does.

However, the oil boiler in my house is now 30 years old, and has no indication of any rusting taking place, even though the system was designed with no return protection. I installed a number of simple probe thermometers in order to help me better understand the wood boiler operation, including some on the oil burner, which is in series with the WB. On the oil boiler return, I can see 65F return temps for some number of minutes, until the heating loop eventually brings the return up over 140F. So my question is, if return protection is so important, why hasn't the oil boiler rusted out after 30 years of this ongoing shock treatment?

So again, apologies for taking a fork in the road, and hopefully the follow-up discussion from the folks in the know will give me a better understanding of what I'm missing about this whole thing. (and hopefully answer your question as well)


Minister of Fire
Aug 22, 2010
Northern Canada
Instead of a protection valve...
Go with a near boiler pump instead.Valve on each side of the pump then if you need to you can use that pump if one of your others fail.
But i would not go without any protection valve or near boiler pump.


Minister of Fire
Sep 15, 2011
Nova Scotia
I guess I can't relate since I have no actual experience with open storage and a coil. It has always been my assumption that with depleted open storage, you would still get an extended period of 'too cool' return water coming back to the boiler at a gpm rate of whatever your boiler circ would flow. If not, that would seem to give even more advantage to pressurized storage than I was thinking.