Sanity check on radiant system install

eadlist

Hi,

So I'm in the process of remodeling my house (including heavily insulating and air-sealing) and changing the hydronic system over from baseboards to radiant floor heating. The boiler is an older Slant-fin 150K. My plan is run the boiler loop through an expansion tank, air eliminator, 4-way motorized valve and then through three zones with each zone having a med-head circulator pump. I had a few questions:

-Does the boiler loop need a separate circulator pump? My "Modern Hydronic Heating" textbook suggests it doesn't. The existing baseboard setup has the circulator pump (a Taco 007) on the return side right up against the boiler. Should I move that pump to the supply side? Remove it? Leave it in place and dormant? Or hook it up to the control system so the zoning circulator and the boiler loop circulator(s) are running at the same time?

-The 4-way motorized valve from taco seems like a reasonable way to get outdoor reset and boiler temperature protection. Their seem to be a dozen ways to do this. If someone can confirm, this makes sense in my case, I would appreciate it. The boiler seems to have a few more years left and at some point in the future I plan to switch to a high efficiency boiler/water heater that doesn't need a minimum temperature.

-After purchasing a bunch of the parts, I stumbled on the idea of the injection mixing block like the Taco RMB-1. Since I would have a restocking fee, my current planned setup would probably cost about the same as injection mixing block. Is it really that much more efficient to go with the RMB-1 in my small (1600 s.f. heated space) residential system? Or would the electricity and maintenance costs of either system likely be of minimal difference over the next decade?

Thanks for any help! Although I have a bunch of plumbing and remodeling experience, hydronic systems are new to me. I've done a heating load calc and have the tube and plate systems figured out. This is the last bit.

Zman

Is your boiler cast iron or copper tube?
Boiler circulation is critical with copper tube.
Do you have a drawing of what you are proposing?

eadlist

It's a cast iron boiler. I've attached a basic plan here,

Zman

Cast iron boilers do not need very high flow rates to prevent boiler damage, so what you are proposing should work without the boiler circ. In fact, the boiler circ may make the the mixing control less stable
I don't love that type of set up if you have very small zones as the boiler could see very low flow rate, potentially shortening it's life.
Siggy is a very smart guy. I am certainly not going to critique his designs.

eadlist

Thanks for the info. I'll move forward with this plan.

One other question, Zman - you mentioned small zones. Ballpark, what length would you consider a small zone? I have the option of making an area of the house 2 smaller (400 ft and 500 ft zones) or 1 larger (900 ft zone).

Rich_49

Those zones are all small compared to what the actual heat loss for the home is and the monster size boiler . Your gas usage right up until this thing dies will be high , hope you were not expecting it to go down a whole lot . That insulation you are adding is a good thing but will further exaggerate what is already probably an oversized boiler . Taco's I Series 4 way valve only comes in an
1 -1/4" variety so head loss through the valve will not be an issue , I believe that valve has a Cv of >18 .

Your drawing posted is perfect for your application and get rid of the pump near the boiler . You may want to check out the Taco SR501 ODR control .
http://www.taco-hvac.com/uploads/FileLibrary/102-431.pdf

The boiler will fire for short intervals and that heat will remain in the boiler as the mixing valve will only sip tiny bits of water to mix with return water . Stack losses will be larger than prior to this work . What type of water heater do you now have also ?
What type of radiant install will you be performing , plates under sub floor , manufactured above floor system ? Have you done a heat loss for the structure in either a block load or room by room (preferred) ?

hot_rod

I'd be surprised if that boiler doesn't end up being about twice the size you need. After you run a load calc the sizing will become clear.

You could add a buffer tank and help cut down on the cycling. it would require two circs. If you use ECM circulators, like the Grundfos Alpha the power consumption would be about the same as one PSC type circ.

That is a great boiler and should last many years if protected and cared for. A simple 3 way thermostatic valve could provide return protection if you go the buffer tank route.

Here is an example, minus the mixing device for the radiant. This shows a boiler protection device with the pump and valve built into one package.

What about DHW?

eadlist

@Rich - That's a good idea for the setback on the boiler controller. Right now their is a very basic controller with only a high limit setting (no low limit and factory set differential). I am a bit confused how having a setback on both the boiler and the 4-way valve would work. Would they complement each other or fight against each other? I imagine the jacket heat losses for the boiler are pretty heavy.

@hot rod For DHW, there is a separate 50-gallon 40K BTU water heater. It's only a few years old and in good condition.

I like the idea for a buffer tank for reducing short cycling. I am wondering though, if the boiler will just be too oversized for the remodeled house with much smaller loads. Given that the boiler doesn't appear to have been well-cared my med-term plan is to replace the boiler with a properly-sized condensing DHW/Boiler. I think I'd rather put the costs of the buffer tank towards that. Does that make sense? Although the long-term goal is clear (high-efficiency, condensing friendly boiler), I am still a bit confused as to how long to nurse along the old boiler. But that will probably depend on gas usage under better insulated conditions, how bad short-cycling is and if it starts to leak.

Rich_49

My thought was also that the expense of a buffer would be better set aside for new unit . The mixing valve and control would be an either / or suggestion . Mixing valve would allow for longer calls but would require some good math , example , 120* SWT requirement , use the boiler mass like a buffer (stack losses) and hold it out as long as possible and use every BTU in there before firing . High limit at a higher temp and whatever differential is appropriate to keep the boiler from condensing (140*) for sake of insuring no condensing and longevity of existing boiler .
I would study what is available on the market now and keep up with advances in the combi market as it is rapidly doing so . Using a combi with plenty of mass and few stack losses is advantageous .

Zman

I like all the advice you are getting. Solid stuff.
The thing that rattles around in my mind is, water (and energy) will only circ back through the boiler at the rate that it is being absorbed by the system.
The exact amount of flow at the boiler will be a moving target depending on the number of zones calling and water temp to the radiant.
In your case, less zones is a good idea. This will also help reduce short cycling, although as pointed out above, given the size of your boiler, this will be an issue either way.

Carl

hot_rod

Depends on how lopsided the load to boiler output as far as keeping that cast boiler, I suppose.

Cast iron boilers can last 30 years and are less $$ to maintain compared to mod cons. Maybe a couple grand for an installed buffer? How much for a mod con or combi boiler installed with new venting, etc? Probably 3 or 4 times that. It will be a long payback even at a 10% increase in efficiency for the small load I think you will be looking at.

What are the past energy costs? How much would you like to shave off the heating energy consumption? And how much would you spend to accomplish that?

In a few weeks Idronics 17 will be out and we have some new and unique ways of designing and piping buffers. We talk about Extergy and Thermocline and how to utilize them for optimum storage of energy.

It's bests to keep those stored BTUs in a well insulated container, the boiler is not one For the same reason you are adding extra insulation to the building.

Unless that boiler is 15 years old or more it is probably a low volume, and low mass, less that 200 lbs. 80 gallons of water will give you a lot more, and much better insulated energy storage.

I agree with Rich, an ODR control will allow you to pull down the tank or storage to the lowest possible temperature, leverage the BTUs, and dampen the cycling.

So many choices.

eadlist

@Hatterasguy That's a really good idea. It's a gas boiler. Sounds like downfiring is a bit controversial but I will definitely give it a try later on (making sure to use a combustion analyzer, get it double-checked by a professional and not kill myself from CO). That would solve a lot of the potential issues. I'll have to replace the aquastat also because the current one only has a high limit and 8F differential.

Wow, lots of good ideas. I can see the long term benefit of making this boiler work and a storing the heat in buffer tank seems like a good idea. I'm not sure what the energy costs will be in the newly insulated house. Last winter (first in the house) which was unusually cold, we had gas bills in the $350 to $400 range. I'll have to see how much insulation and airsealing cut that down.