Low return water temp protection methods

Where are you located? You need someone who understands hydronics, and can read a manual. Boiler protection is fairly easy, and can be done a number of ways.

I have a question for @Erin3. What type of radiators are you connecting the replacement boiler to?

If you have a 1940s-era or older home with a large water-content cast-iron radiator system, you need to think about boiler protection. If you have a Monoflo single-pipe system with the wall convectors that were popular in the 1950s, you may not need boiler protection. If your home is equipped with fin-tube baseboard radiators, you definitely do not need boiler protection.

I can explain why if you need me to, but it is easier to tell us what type of radiators you have before we get into a lengthy discussion on the topic.

This is an unusually short answer from me… go figure.

Is this to protect a cast iron hydronic boiler from cracking or something due to thermal shock?

Or is it to prevent condensation issues in the flue?

Or other?

Just a homeowner with a similar system, so let me share my understanding (I think the experts here might have different views!).

If you get a cast iron boiler matched to your actual design day heating load, and if your existing radiators have the capacity to emit that much heat at reasonably high temperatures (160F? higher?) then you'll probably be fine without a boiler protection strategy. Boiler protection strategies, like the 3-way valve suggested above, come in handy when you have more radiation capacity than boiler output, because the way your system will equilibrate is that your radiators will deliver all that heat, but they'll do it at low temperatures. Note, this doesn't mean your house will be cold, it just means that you get the heat you need with radiators that are pleasantly warm instead of scalding hot.

If you want to play with some numbers, I think the formula given in one of the idronics issues for the BTU output of a cast iron radiator is something like btus/hr = 0.3748 * EDR * (avg water temp in radiators - inside air temp)^1.3, where EDR is a measure of how much cast iron radiator capacity you have. If we assume your existing radiator capacity estimate is based on relatively hot water (170F awt) and an inside temp of 70F, I think that might mean your 78,000 btu/hr radiator capacity estimate is about 520 sq ft EDR. If you get a boiler that delivers 88,000 btu/hr, this formula says that in equilibrium, your average water temperature will be (88000 / (0.3748 * 520))^(1/1.3) + 70 ~= 180F. That should be totally fine, and I expect that the experts here would agree with your installer that a protection system is unnecessary. If it turns out that you've got a lot more radiator capacity (lets just double it for example), you would be getting all that heat with 135F water, and things are perhaps dicier, especially since it does take time for the system to warm up, during which you'll definitely see low return water temperatures for a while…

consider replacing the oversized lines. That will help a lot.

old one is but it is being replaced with a conventional ci boiler because of reliability issues with that particular combi.

It's hard to beat the simplicity of a thermostatic return valve. Thermostatic bypass valves have been used in various boilers for many years. Most pool boilers have them built inside. The weil GV gold CI boiler had one built in.

With the large pipe, low head loss system you have a single pump would probably do the job. Check the boilers recommendation for pump.

The color schematic below happens to show the buffer as the load, same concept. Step 2, allowing a portion of the return to blend in may happen in minutes, it may take 1/2 hour, it is all based on the load and boilers delivered output. if the boiler is oversized to the attached radiation, the most common match, then the valve opens faster.

The largest market for these valves is the wood boiler industry. I spotted a handful of manufacturers with these valves at the Mostra show in Italy a few weeks back. So they talk more about solid fuel problems, like creasote formation same physics at work with fossil fueled boilers.

A pump and controller plus flange set. A bit more $$.

I think the VS pump like this color drawing makes sense. This piping concept has been used for years with CI boilers connected to low temperature radiant as it both mixes and protects the boiler, basically called "injection mixing" It's the concept and product that put tekmar on the map

The Taco schematic below this schematic, see the difference??

It's all about the pump placement. Again how does the pump "speed up" to overcome the large thermal load? It's not adding any heat energy. Only if the distribution pump drops off could that happen, the thermal clutch would be the boiler pump. may as well just use a $50 aquastat to hold the pump off.

The ONLY reason to use any return protection method is the load /distribution is much larger that the boiler can keep up with, suppressing it's operating temperature condition. May as well pipe and control it with a appropriate fix as the goal, not a "maybe" That was the whole point of injection mixing!

These 3 port mix valves are fairly simplistic, never are all 3 ports closed. Think of it as a thermostatic tee. As one closes the opposite opens, so flow is always passing through in some direction.

Remember the Holohan parable, whatever goes into a tee must come out of a tee

You need to know the activation temperature and the differential designed into the changeable cartridge.

Or use a high Cv basic adjustable 3 way thermostat valve

Boiler valves are large port and don't need to have the 2-5 degree accuracy of a listed thermostatics mixer.

All valves, especially control valves get sized by their Cv number

Use a formula , spread sheet or online calculator to determine pressure drop through the valve if you exceed the Cv number

the pump sizes by the the gpm and the head, flow resistance of the circuit

The gpm tells the amount of heating btu you can move. A a common 20 delta, temperature drop around the circuit the math is simple.

To move 120,000 you need 12 gpm

The large piping will not present much flow resistance, nor will a three way valve

Estimate 4’ of head, so an 007 would move 12 gpm, 120,000 btu/ hr

choice for high flow low head applications, if you need more than 12 gpm or so.

The amount of water, the volume does not effect the pump selection.

if you place 3 shut off ball valves like this, you can isolate the pump and 280 valve for easy service. A purge valve before the bottom left shut off

Expansion tank and fill valve before the pump.The pump pulling away from the mix valve, pumping into the boiler

Air purger at boiler supply connection.

Here is another way to do it, this is my house, 100% radiant throughout.

First circulator, off the top on the feed is the primary loop. Keeps constant high temp feeding the boiler to prevent low temp return.

Second circulator operates off the a High Limit Aquastat preventing high temps from going through the system keeping it at 120.

So you low temp going through the radiant while maintaining a higher temp returning to the boiler

Don’t mind the mess it’s an older picture lol

Close, one of the pumps would need to turn off when boiler return is below 130. It could be the injection loop pump or the boiler pump.

The primary loop is always the loop that has the expansion tank connectd to it. I think the distribution loop, up top, with the zone pumps is the primary loop?

my wording was off, you’re correct in saying that’s technically the primary where the closely spaced tees are. But First pump(black) is looped up and right back through the return back into the boiler. 2nd turns off when it reaches temp, not when it’s below, it’s controlled by a High Limit

a lot of people here resort to installing their own boilers after what happened when they tried to hire someone to do it. not quite so bad with hot water but finding someone that understands steam in some places seems to be impossible