Radiant zoned by circulators of large buffer tank.

Brad White_147

Generally speaking, the main circulator upstream of the mixing valve will force backflow through the return port as you suggest. I have no idea why it was done that way.

I say "generally speaking", because I do not know the hydraulic characteristics of the system. It may be a low head/low flow priming function and that the zone circulator flow exceeds this to draw away from the valve when operating. However, the first circulator is likely to see a slug of hotter water than needed.

I do see backflow as you suggest, and this will hold back the zone circulators as another side-effect.

Any mixing valve wants to be on the inlet side of a circulator. If downstream it becomes a diverting valve, in short.

If the prime circulator is installed downstream of the mixing valve on the supply side, I see the need for a DP bypass as you suggest.

<i> Idle thought: If you installed the mixing valve and prime circulator on the return side of the tank; mixing would occur upstream of the prime circulator and would dilute the tank. If you were running only one temperature, this would not be too bad. I will have to think about this more, if the higher temperature circuit's return can be assured to provide the needs of the lower temperature circuit reliably. </i>

At the zone level is it constant circulation with local mixing to meet room temperature or is it circulator control (VS or constant on-off)?

(Full disclosure, I am not familiar with "Truflow" equipment, if it has internal balancing or what-have-you. I will take these as being ordinary circulators. If there is something critical or applicable about this, let me know.)

What I am asking here is, do the local zone arrangements correct for variations in pressure differentials across the main supply and return lines? Is there any Primary/Secondary circuiting not shown at the zone level?

If circulators are variable speed or on-off, with the variation in flow you will affect the other circulators and the prime circuit as well, without P/S piping. Even if constant circulation, if there is any local blending to space temperature needs, there will still be variation.

My ideal solution is to use 4-way mixing valves instead of 3-way, to separate the buffer circuit from the emitter circuit. 4-Way would respond to a fixed or indexed outlet temperature to the emitters and would automatically conserve the unused HWS, back to the buffer tank. The prime circulator could go on the supply downstream of the 4-way with a bypass loop downstream of all zones, to prime the zone circulators. I know you said to not add any significant equipment, but the 4-way I think would save you some agita in other areas.

Random thougths, not terribly well-structured ruminations.

Cheers!

Brad

Darin_6

INTRO:

I have taken over a job on a new home in which the previous contractor was fired. I am working around a system in which 90% of the materials were already purchased by the home owners through the previous contractor, and 60% was already installed.
Note: Before you voice an opinion and start adding equipment and controls, just realize that the homeowners are in a court battle and have a ton invested in this system already. My standing orders are to work with jack S---! So, I also have very little margin to add more equipment. I have squeezed out some, but not much to get excited about.
The diagram attached was drawn by the previous contractor, I have scanned it in as a .BMP file. Note, this is only one drawing of many. This system is not your run-of-the-mill whole house radiant jobs. The Drawing does however depict the section that I am concerned with. If you need to see more to formulate a sugesstion please let me know.

SYSTEM OVERVIEW:

This system incorporates three staged boilers. The first staged boiler is a large 200k BTU Waste Oil unit that is housed in a large building 300 feet from the home. The second staged boiler is a 150K outside wood boiler. The third staged boiler is a 199M Munchkin Boiler that is in the new homes utility room. All three boilers are interconnected via a transfer loop system between to large buffer tanks. One tank located in the large machine shed, and the other in the home. This configuration was established by the homeowner and previous contractor so the 12000 SQ/ft machine shed and new home could be heated simutaneously and to maintain good redundancy should one boiler fail or be down for maintenance.
The large machine shed also has radiant heat in half of its space and was built 5 years ago, so the owner could store and maintain his excavation business equipment. The machine shed drive apron and all the new homes outside concrete(Everywhere!)has snow melt installed. The new home has a ulta-store tankless coil for DHW, and a Heating coil in the air-handler so forced air heat is used in the mildly cold fall months intead of constantly starting and stoping the whole house radiant system. I hope that covers it?? Yes I am on T&M.

Section of the system In Question:

I have completed most this job already, the boiler staging and trasfer loop is completed. I have added some Tekmar equipment for the boiler staging and that has tapped out the budget of the owners, but it was well worth it. This leaves me very little funds to finish the whole house radiant portion of this project.
The attached Diagram shows this last portion of the system. It consist of a buffer tank and two seperate primary radiant loops drawing from said tank. One for the main floor, and one for the basement. The Mian floor loop feeds the upstair Zones and is tempered to 140 F. The upstairs tubing is installed via Warboard subfloor. The Basemnet loop is tempered to 115 F before going to the Zones and the tubing is slab on grade. All Zones are fed by circulators on call for heat via relays.
The previous contractor unfortunatley completed this portion of the system first. So, just like the diagram he installed the loop circulators before the thermal mixing valves on both the Main floor and the basement loops. At present there is flow checks on the outlet of each zone circulator prior to each manifiold. (Diagram does not show these). There are no Diferential bypass or flow controls installed anywhere.

Finally the Questions:

Why would he (Previous Guy) have put the pumps before the thermal mixing valves in this type of system configuration?

Won't this push 165 F back through the cold feed side of the mixing valve?

Should'nt these pumps be on the down steam (Mix side of the mixing valves?

Or am I wrong?

My next set of questions stems from the first.

Would it not be smart to put a differential bypass prior to the Zone circs between the supply and return headers on these loops, or at the end of the header runs?

Do you need a differential bypass at all?
-- My concern is the loop pump drawn before the thermal mixing valve will prime the zone circulators. Just like when you stack any circulators, it would make the flow to fast in the zones causing noise and decrease the a zones diferential on the return header, which will mess up the performance of the tempering valve.

Last Question. Do you need the Loop Pump before or after the the mixing valve at all in this system?

I am like most guys out there. I am used to coming off boilers directly, off a primary boiler loop, or with with injection mixing. Not buffer tanks in this configuration and with only bone simple hardware as my only resources. Any help would be terrific.

Thanks,
Darin