Need help with new install of radiant with Buderus cast iron boiler

Is it the cost of the 4 way that prevents you from considering that route?

The 4 way both mixes down the radiant and gives you boiler return temperature protection. Return protection could be a concern with a high temperature boiler connected to a high mass low temperature slab. A one pump solution. Two pumps if you add the indirect.

If total slab is 2000 sq ft? At 4" thickness that is around 15 yards of concrete, 60,000 lbs of thermal mass. A boiler, even your oversized one, cannot quickly change the temperature of that mass. As such the boiler needs something to protect it from extended cold running cycles.

With an oversized boiler you will have short cycles that could prevent the boiler from warming up adequately.

So the 4 way solution addresses all the conditions you will experience.

Two choices for ODR, add it onto the boiler as @Ironman suggested or onto a 4 way valve.

A tekmar 256 around 200 bucks is an inexpensive way to get boiler ODR with good features. Then a "dumb mix valve, around 230 bucks. The 256 protects the boiler return.

This gives you a safe, efficient, controllable system.

With two zone valves, a basic off the shelf hydronic relay box is all the control you need to have.

The proper 4-way setup with an actuator (mechanical actuator, not thermostatic) and outdoor reset, will aim to protect the boiler, and also aim for the correct mix outlet temp based on the outdoor temperature.

Let's do a hypthetical. Consider how much concrete you have on your floor. Now lets assume that you could heat your space to the exact indoor comfort you are looking for, while the outside temp is -10, by supplying a constant 110f supply to your manifold. Now let's fast forward to a day it is 35f outside. With a thermostatic mix valve you are still going to be supplying 110f water to your slab (a "dumb" miux valve just set by hand will also do this assuming the boiler setpoint remains constant, though it will eventually overheat even more), by the time the temp reaches the desired comfort level, the slab is hot, and the heat in the mass of your slab will continue transferring to the space thus over shooting the temperature you are looking for inside. Now let's say you only need 80f water on that 35f day, with a mechanically actuated mixing valve, and properly setup ODR control, you will be supplying the exact temp water needed to maintain the comfort level in your space. In theory you can match the load precisely and the system runs constantly, to maintain a perfect comfort level in the space.

These are features that are basically built in to modulating condensing boilers, no mix valve needed for those with a single temp system, and no worry of cool return water temp. and all of the above is ignoring the potential damage that overheating a big concrete slab can cause

A 3 way thermostatic can only send the adjusted temperature to the slab. It is a smart valve with a one track mind :)

So if you set it to 110F for example, and the weather warms, heatload goes down, it is sending water that is too warm and you will over-shoot the thermostat setting. Remember the slab needs some, maybe hours to respond to a load change. So it could take 2, 4 maybe more hours for the slab to cool down and bring room temperature down.

This is called the flywheel effect. It works in both directions, warming or cooling down the slab. One of the very few downsides to a high mass radiant emitter.

We try to minimize that by 1) adjusting the supply water temperature based on outdoor temperature, called ODR outdoor reset.

Not to overwhelm you with options, Constant circulation is another nice function. In my shop the pump runs 24/7 durning heating season. The thermostat connects to a 3 way zone valve. It pulses heat into the slab. It also takes the passive solar gain I get in the south end of my shop and moves it to the back, north (colder) part. So my slab across the room stays very consistent. I'm connected to a mod con with ODR, so mostly it sends between 90- 110 temperature. But you add another 150 bucks or so for the 3 way zone valve.

A manual 4 port valve gives you a few options. You can go down and manually turn the knob during a cold or warming spell and fine tune it. Maybe 3- 4 times a season as you get large outdoor temperature changes.

IF you add the tekmar 265 to the boiler it could run maybe 160 on coldest day, modulate down to maybe 140 even 130 with that particular boiler.

As the boiler modulates down, the hot supply to the dumb, manual mix valve decreases by default, so it get a bit of what we call proportional mixing. The manual mix valves tracks a bit with the boiler reset curve and temperature.

While not as accurate as a mix valve with a sensor and actuator, it would modulate water temperature a bit more accurately than a thermostatic mix that only knows 1 condition, the setting on the knob.

The 4 way is always slipping some flow back to the boiler to help keep it in a safe condition. A 3 way does not allow two mixing points as a 4 way can. Price wise about the same.

Pipe it with a manual 4 way. Try and see if it is being controlled well enough for your use habits.

If not add the 256 later to get more control, higher efficiency. The lower the boiler operating temperature, the more efficient it runs.

Viessmann claims for every 3 degrees you reduce the boiler temperature fuel consumption decreases 1%. So ODR controls actually can pay for themselves over a few years based on your energy costs and consumption, of course.

200 bucks or so seems like a good investment for temperature control, fuel savings. Even on a "free" boiler :) Pretty much all boilers come with some version of this now.

As far as the boiler protection, running them cold for extended periods causes condensation corrosion in the flue and at worse case excessive CO from the flame. It's not just about saving the boiler, but allowing it to run safely, efficiently.

I think we are all try to get you a safe, comfortable, efficient system with the pieces you have and the least additional $$

one pumping 4 way is limited in boiler protection and a high mass boiler is preferred.

Im looking for budget options for the OP. An oversized high temperature boiler presents additional options.
Setting it, operating a 4 way manually you need to never close it completely. They used to have an adjustable stop screw to limit the stroke, when used as a manual valve.

I remember doing these systems with older Burnham and Weil boilers when the still had mass and some 8, 10 or more gallon water content. It may not be an option for todays small low water content cast.

I think that Buderus would work fine. It was always promoted as being ideal on low er temperature systems.
Im more concerned that the load on that 800 sq ft could be in the teens or single digits (btu/sq ft) Then the boiler is massively oversized. More suited for a 50 gallon 35,000 btu tank water heater🤔

With 7- 1/2” loops he is looking st some low flow rates, less than 4 gpm when all zones are flowing. I think that boiler can handle that. Ill look for the spec sheet

Here is the single pump option from the tekmar manual , and the essay describing its operation and limitations

https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/35980-source/638705598180000000/e0021-06

I see that note on the drawing in Modern Hydronics

Again thanks for all the info!

One thing I didn't mention about the 800sq/ft living space is that it's a much higher ceiling with a loft and a LOT more glass! There will be a bedroom in the loft so in reality it's more like 1,200sq ft of living space. What's that do to the heat load?

Also, my 1,500sq foot well insulated house house has been running on a 100k/80k CI boiler for 20 years with forced water baseboard without a hitch and there's never been any signs of short cycling.

A classic example of why you want a heat load calculation.

Exposed wall, volume, glass, infiltration all have to do with the load.

Here is a recent home built near Utica, NY. Note the 10 btu/sq.ft load! Looks like you have 2x4 walls, double pane windows? So your loads will be a bit higher no doubt.

You design for worse case condition, but you may never see that condition, or rarely.

The BIN data for Syracuse shows how seldom you might expect design conditions. About 70 hours at design. Both Buffalo and Syracuse get lake effect weather, high winds, lots oif snow potential. BIN data for Buffalo is available if you want to do some more pencil pushing to get a better picture of what you may be up against.

If you have an oval livestock watering tank for a buffer tank you will have much more thermal mass to make use of.

Short of putting a coal stoker stove in the living space you are going to have a very cold home.

That valve is yet another way to protect a boiler from getting cold return water. It blends some output from the boiler with the return.

In this pic it shows a non electric valve being used to protect the boiler, the 280, and a 3 way motorized valve to mix down the radiant.

There are probably a dozen ways to pipe a system like yours as far as temperature mix down, boiler protection, buffering the over-sized boiler, etc.

Some use electronic controls, like the tekmar options. So use thermostatic valves shown here.

Some use motorized valves. Some use a combination of the above.

Over the years we have tried various mixing ideas, show below are yet more injection mixing ideas using valves and a VS pump. And a pic of 3 valve based mix options.

All have pros and cons.

Trying to KISS yet check all the boxes for what you want or need for your system. If it is a temporary system (boiler), I hate to throw too much technology at it.

I have seen CI boilers connected directly to radiant with a couple ball valves for temperature control, last years before failing. That is an option if this is a throw away boiler?

But keeping it safe also is a concern with the boiler within the boiler within the living enevelop.

the bargain brand glycols have weaker inhibitor packages and lower protection rate. Stick with a name brand

I would buy a 30% premix, pump it out of the bucket into the system

If you're looking to keep the cost down, M Copper is perfect. Using L on heating is overkill in most cases. Mad Dog

Here are 3 options on one page to better show the difference.

Option 1, my choice, a 4 way valve with only 1 pump

Option 2 a 4 way valve in a primary secondary circuit, requires 2 pumps, one for the boiler one for the radiant loops. Basically the Taco i-valve drawing

Option 3 variable speed pump injection. This will require 3 pumps. A boiler loop (primary loop) p1.
The injection pump, p2 yes an 006 would work, and a balance valve in that cross over piping.
Then the radiant loops pump p3

You want isolation flanges at every pump also for future service

Not shown is the indirect, it would need a pump also as a parallel loop

so add the cost of the 3 options with all the required parts

If you want to keep it as a simple as possible "forever" an overhead gravity system eliminates the need for circulators and the open to air expansion tank can have its overflow pipe piped to a laundry sink or floor drain.

There is a great deal of information here on the forum about gravity hot water systems that you can read about or you could purchase Mr. Holohans book CLASSIC HYDRONICS from the Heating Help Forum Bookstore and learn how the gravity hot water system works with no circulators.

The open to air expansion tank can be a vertical tank simply placed on the upper floor in the loft out of the way and the drain line could be pex to a laundry sink or floor drain.

The open to air expansion tank would have a sight glass in it to show the water level in the tank or you could occasionally add water to the manually through the boiler to maintain the needed water level for it to heat and rise into the tank and drop back down through your heating loops.

I helped a friend in New Mexico repair and replace his neighbors rusted out open to air expansion tank with a new galvanized expansion tank mounted in its original saddle mount configuration and his neighbor is very happy as she has plenty of heat in her hot water radiators now.

You run the boiler at 170 degrees and the water will rise naturally to the tank and then drop back down to your heating loops without a circulator.