Primary/Secondary Temps question

Gooch

So I picked up a copy of Primary/Secondary Pumping Made Easy and I've got a few questions.

I understand the 'What goes in must come out'

Using this the closely spaced tees that connect the Primary to Secondary circuit, If there is more flow on the primary than secondary all secondary return and some of the primary supply will go to the primary return, Raising the return temp correct?

Flip that to more flow on the secondary than the Primary. Some of the secondary return will go in the Primary return with the balance going back to the secondary system and mixing with all of the primary supply, Lowering the secondary supply temp, Correct?

I understand with an CI boiler that the first concept is a good thing to protect the boiler against condensing.

The second is what my boiler requires on P/S system.

My system consists of 3 zones. Basement slab/First floor Joist Trac/In-Duct Coil(backup) The two floor systems I want a 10* Delta T. Which raises a few questions that I have.

If I'm mixing primary supply with secondary return do I need to move my CH supply sensor from the boiler to the Secondary supply?

My boiler doesn't offer a flow rate and head requirement for a 10* delta T. How do I determine the needs here? The boiler is a HTP UFT-80W.

For my secondary and i was looking to build a Header to feed the 3 zones. I need roughly 11 GPM flow at a 10* Delta, Research tells me this should be done with 1.5" pipe correct?

For the Boiler Pump would this be a good place to use a Delta T ECM pump? that way depending on load it can adjust it speed rather than selecting a speed for full load and applying it to partial load?

Zman

Unless you are going to create some very small radiant zones, I don't think primary/secondary is needed in your system.

I would not get too excited about design delta t. In reality, you have little control over these deltas and they don't matter that much. Your slabs probably run at 25 delta at startup and 15 or so once they get rolling. The under floor will probably be less than 10 no matter what. In reality, they will both work fine. AWT is far more important that the exact delta.

The minimum flow rates and deltas in the HTP manual are misleading. They are based on calculated deltas at high fire. In the real world, and especially in your system, that will never happen.
They are based on the universal hydronic formula BTU/hr=gpm x delta t x 500.
On low fire, the numbers for the UFT-80 at 10 degrees delta would be approximately 7,500 btu/hr= 1.5gpm x 10 delta x 500.

I am not sure how HTP would feel about a flow rate of 1.5. That is how the math shakes out...

john walsh_2

Just anecdotal here, but even with the Ts within 4 " of each other and a good 12" ,drop, I still experienced ghost flow and overheating in one of my secondary zones. I had to add the Flo control valve to fix it. Primary/Secondary is great, but You may find that you still need Flo control valves on the secondaries.

Gooch

I guess my thought on primary secondary was to reduce the flow through the boiler as I'd be trying to push ~11 gpm through it and maybe even 15 if the coil would run for.

john walsh_2

If your secondary is going to be using all the primary from the boiler and even mixing it with some return secondary, you're defeating the purpose of the primary loop aren't you? Can you not use a mixing valve between the secondary supply and return past the Ts and just before the secondary circulator supply?

Gooch

I was under the impression the point of primary secondary was to be able to input(or remove) bus regardless of flows in the two separate circuits.

HTP says to have more flow in the secondary than the primary.

john walsh_2

The reason for primary/ secondary is to protect the boiler from cold returning water and to prevent flue gases from condensing causing premature corrosion. You can do what you need using manual or automatic mixing valves for desired supply water temp on the secondary branches with what ever flow rates they require using the correct corresponding circulators on them.

Zman

john walsh said:

The reason for primary/ secondary is to protect the boiler from cold returning water and to prevent flue gases from condensing causing premature corrosion. You can do what you need using manual or automatic mixing valves for desired supply water temp on the secondary branches with what ever flow rates they require using the correct corresponding circulators on them.

He has a condensing boiler.
I see no need for primary/secondary based on info provided.

hot_rod

Primary secondary main feature is hydraulic separation. It keeps multiple and various size circulators out of trouble.

It doesn't necessarily provide return temperature protection, unless you monitor that and respond with pump modulation or a valve that responds to temperature.

Gooch

Zaman, would you recommend doing primary direct return then? Just seems like a lot of head and gpm to try to push it all through the 300ft loops and the boiler. I've slowed the infloor in the basement and got a wider delta. I much prefer ed the way the floor operated with the lower delta but that means more gpm and head

john walsh_2

Zman said:

john walsh said:

The reason for primary/ secondary is to protect the boiler from cold returning water and to prevent flue gases from condensing causing premature corrosion. You can do what you need using manual or automatic mixing valves for desired supply water temp on the secondary branches with what ever flow rates they require using the correct corresponding circulators on them.

He has a condensing boiler.
I see no need for primary/secondary based on info provided.

I missed the fact that his boiler is condensing. But I was just explaining what I understood as the main reason for primary/ secondary piping. Hot rod made a very interesting point.

Zman

Fire tube boilers have about the same resistance as a bath tub.
If the boiler had high resistance, I would feel differently.

Gooch

Was not aware, I was under the impression that Mod/Con boilers had a large resistance to flow. You don't think it will be an issue pushing possible 15 GPM across the boiler? Thanks for the Advice. I've tried getting local contractors but when I talk to them they seem to not be interested in or the one I was able to look at it wanted to put 160* water in the joist track and put 5 zones in 1500sq ft. So I'm left with teaching myself. I'm an Electrican by trade so I've got the mechanical apt to figure it out, just need to do the research and teach myself.

Thanks for all your help.

Gordy

Pay close attention to what Zman said about the boiler HX design. A firetube design you can pump direct. Other designs with small passage ways in the HX developed high head conditions so P/s piping is needed.

Tell us why you think you need 15 gpm? Because I'm 100% certain you dont.

At 15 gpm

10 delta moves 75k
15 delta moves 112.5 k
20 delta moves 150k

What is your heat loss this determines everything?

Gordy

The boiler you select comes with a I/O manual which most can be obtained online. In the manual there will be a chart as to exceptable flow rates at various deltas the manufactors like to see run through their boiler.

Have you used find a contractor on the main page here?

Gordy

We can nail things down better with a heatload calc.

Gooch

Heat load calc was just over 30k btu upstairs and just under 25kbtu down stairs. I'm going to be using the staple up radiant for hard surface only. The fan coil will take care of the carpeted rooms. While the btu load doesn't require 15gpm. It is entirely possible that all 3 zones will have a heat call and be running at the same time. That's how I came up with the 15 gpm number.

At a minimum the two floor systems will run about 11 gpm to supply 55k btu at a 10 degree delta

Or am I looking at it wrong?

hot_rod

A hydraulic separator is another way to pipe the system. It provided air, dirt, magnetic and separation in one component.

If you have small "micro" loads a buffer tank can help smooth out short cycling and provide dirt and air removal.

Gooch

Should be fine without a buffer. A hydraulic. Separator would be nice but just can't justify the cost since I already have air and dirt separation components

Gordy

11 gpm is fine. Now what is the load of the smallest zone?

Gordy

You are using an 80 k boiler to supply 55k load. Even at low end of 7.5 k low end at design it will cycle on the warmer days, and with zones less than 7.5 k requirement. Something to think about when zoning small loads.

Zman

If you where doing a new system, I would see a slight benefit to setting it up primary/secondary, Based on your other post, you already have a functioning system that you are just adding to. I think it will work just fine as is. Your flow rates will drop slightly when all zones are trying to pull through the boiler at the same time but you will never notice the difference.

Gooch

Smallest zone will end up being the fan coil. About 10k btu. But for control I'm not gonna allow it to run unless the staple up is already running. So would the whole upstairs count as one zone then? If so the basement at 25k btu is smallest

Gordy

The uft flow requirements

Gooch

Yup. She one in my manual which is what led me to believe pumping 15gpm through wouldn't be a good idea