Taco RMB-1short cycle

MickCorp · November 2018

Hello all, first I want to say I've used this site for a long time before becoming a member. Some very sharp people on here willing to help and I appreciate it even though you didn't realize you did it!

Anyway, I have a system in my place that I have installed myself and it is quite simple. I've always believed in keeping these systems simple, so i have one zone upstairs that is underfloor held off the subfloor 1/2"insulated with foil and 3" of Roxul, one zone downstairs that is in-slab insulated with 2" rigid foam.

Using closely spaced tees, a manifold is fed after a 3-way mixing valve for the in-slab.

Also Using closely spaced tees, the upstairs manifold is fed using a Taco RMB-1.

For a couple winters I was using both outdoor resets on the Taco and the Boiler (Trinity NTI 150 Combi) and I had the Taco set to control the boiler. It unfortunately struggled to find the happy medium it promised between short cycling the boiler and using only what it needed for Outdoor reset. As a result I feel like my propane costs could be better.

So instead of having both Outdoor resets trying to work in the same parameters I've gone to using the Taco as a setpoint control enabling the boiler and I've moved the sensors accordingly. I've set the set point higher than it can achieve with the water temperature the boiler will supply (calculated by boiler's reset) so that it maxes out the injection pump in hopes it would return colder water to my boiler.

This situation has provided me with nice long boiler cycles, however 1. My Taco Mixing block is now an overpriced pump. 2. The loop that circulates the in-floor has a much cooler return temperature that never gets back to the boiler.

Would it be wise to sell my fancy injection pump gizmo and just size a pump and put it in ? It's hard to bail on it, but I feel like getting that cooler temp directly back to the boiler is going to make a happier, more efficient condensing boiler.

On a cool day like today -6 F this morning, my boiler was producing 123 Deg supply water, and 107 was returning. I don't have a gauge on my in-floor return on RMB but it feels at least 10 more degrees cooler to the touch.

Thoughts

hot_rod · November 2018

Can you just let the boiler run at the temperature for the upstairs and use a mix valve for the lower slab temperature? Why two mixing devices?

A manual mix valve on the slab would modulate along with the reset curve.

MickCorp · November 2018

hmmm, not sure if I understand the question, the in-slab is currently using a 3-way mising valve, but I think your vote is ditch the RMB ? I've attached a crude sketch

hot_rod · November 2018

Do you know the required SWT for the two different zones?
Let's assume 130° for the upper, 100° for the lower.

Run the boiler to 130° on design day, let it modulate around ODR.

Use the thermostatic, or a manual 3 way for the infloor. A manual would "float" SWT based on ODR, a thermostatic would try and maintain 100° or whatever you need.

It's nice to modulate slabs to prevent overshoot.

Or is it possible to run both at the same temperature? If they are with 15° of one another on required SWT, I'd run them at the same temperature, but on ODR.

MickCorp · November 2018

I hear what you're saying, ditch the RMB. I'd rather keep the 3-way mix valve on the slab it runs fine as is. I was just asking if I'm crazy to think I should just bail on the mixing block. Curious if anyone had the same situation with it not finding the sweet spot it claims it can for short cycling, and ultimately protecting your boiler from becoming more efficient.

hot_rod · November 2018

I don't see the purpose of the RMB if you do not need temperature mixing? I think you are correct the multiple ODR functions are not getting along, and probably overkill anyways?

The only concern with the 3 way thermostatic is it really likes to see a 25° delta between hot in and mix out. So if you run the loop at 120° and try to mix down to 100 or 110 the valve will "hunt" a bit, probably not noticeable in a slab system.

That is what a manually set mix there would modulate along with the loop or boiler ODR. But since it is already in there, go with it.

Temperature instability is noticeable in dhw mixing with thermostatic valves that have too low of a ∆.

MickCorp · November 2018

I'm always up for improving though, whats your manual mix valve of choice ? Its currently a Taco 5120

hot_rod · November 2018

Are there individual zones within the upstairs and down stairs? Or just one thermostat one circulator on each?
Proportional reset control doesn't work so well if multiple zones open and close changing return temperatures. If you have one warm and one cooler temperature requirement it will work.

Also you mentioned short cycling with the 150,000, is this an issue? Is that what you tried to lessen with the mix block?

Maybe you have two issues you are trying to fix.

Did you mention the building load?

MickCorp · November 2018

Short cycling was actually the only issue, and only on upstairs underfloor, 1 zone up, only circulator is the Taco RMB, 1 zone downstairs, Grundfos circ pulling off the Taco 3 way.

If you're familiar with the RMB-1 when set to boiler control, it uses temperature sensors to supposedly to lessen boiler short cycling. My key issue is that I don't feel it is returning cold enough water back to the boiler because the injection side has a lot smaller delta T, than the zone side of the RMB does.

The basement zone has nice long boiler cycles, it modulates down, all is well, now the boiler as it stands has nice long boiler cycles with upstairs, but I was looking for a larger delta T. I hope this is all clear. Heat load total is a little over 60k btu, I realize the boiler is large, but it modulates fairly well.

MickCorp · November 2018

just to add, it actually will modulate down to 25k btu/hr

hot_rod · November 2018

Keep in mind those heat loads are estimated, design day. In milder days your load upstairs could 10, 15 K maybe, so the boiler will still cycle, maybe more than you like. Many areas are at design a small % of the year, maybe 10% these days.

For radiant floor comfort you want a tight delta 10 maybe 15, to keep consistent floor temperature between loops and from end to end.

However you want wider ∆ on the boiler maybe 20, 30 or 40 if allowed. Getting the two components heat generator and heat emitter, on the same page is a challenge.

That is why I'm a proponent of buffer tanks, it allows two deltas to work. Cycle the tank input on a wide ∆, trying of course to keep condensing mode which requires low return.

Pull all loads from the buffer, flow designed for tight delta, and on ODR.

Best way to leverage a buffer to lessen cycling is to charge it to 180°, pull down to 100° or so, but not ideal for mod con efficiency. So, some compromises to work through.

Taco RMB-1short cycle

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