Taco RMB-1

MikeBos

I have an RMB-1 I'm using for my newly installed radiant infloor heat. I also have a new boiler. [1] Boiler is sized to match IDHW. My radiant is way overpowered w/ the new Buderus 95k BTU/Hr boiler. JOY!! I've been working w/ the installer and distributor to get the short cycling under control. Last move we made was to put the RMB-1 into boiler control mode and add a sensor to the boiler supply. I like the results w/ regards to now cycling the boiler at a hi/lo that matches my radiant supply temp better. Boiler is doing the 150-180f diff when it's Hydrostat is under control. When the RMB is under control the boiler temp will be depleted down to the 120's. Rather warm outside now. Have not seen below 30f for a week or so. When the RMB calls for heat down around 120's the boiler will only burn for 1 minute or less generally and only get temps up to 140-150. In the RMB manual [2] at the very bottom of page 7 section a 'Differential' is described under 'boiler control'. Could someone reiterate that 'differential' paragraph so I can understand. What supply temp are they talking about? For the RMB I have the outdoor sensor and am in ORS mode. I have a sensor on the manifold supply and now the sensor on the boiler supply. Looks like the differential is automatically derived. Btw my design day temp is 135f. Why the 1 minute burns then ? Before going to 'boiler control' I would see swings only 150-180f and 3-4 min burns w/ only radiant calling.



[1] http://imgur.com/a/BnkwF
[2] https://www.taco-hvac.com/uploads/FileLibrary/Radiant-Mixing-Block-Instructions_102-151_Rev-Jul 30-2015.pdf

Joe Mattiello

When controlling boiler, the RMB control calculates boiler differential to achieve an appropriate balance between temperature swings, boiler efficiency, and adapting to changing loads and conditions. However, there are other considerations to minimize short cycling.
Boiler sized correctly? If it's a low mass boiler, the boiler could be reaching set point prior to reaching peak efficiency which can take a few minutes to achieve. Installing a buffer tank can help in this situation.
The thermostat with a 4 degree differential can exasperate short cycling too. Are you satisfying thermostat set point temperature?
When operating in outdoor reset mode, the RMB will modulate boiler so boiler output satisfies temperature curve generated by control algorithm, and differential is set automatically, which is explained at bottom of page 7 of RMB instruction sheet.

MikeBos

Thanks Joe. The boiler is high mass I believe. It is a Buderus 115/4. It was sized for the DHW tank not the heat loss. Heat loss, I'm told, is 45,000 on the 1st floor and 1st floor is 3 zones of in floor. The boiler is 95,000. I'm trying to find the best setup to maximize efficiency and maximize burn times. I know it is oversized for the 1st floor and not surprised it is short cycling. Aside from replacing the boiler w/ a smaller unit how would you improve the burn times ?

Harvey Ramer

The easiest way to do it in your situation is to add a couple relays that will energize the DHW pump whenever there is a call for heat from the RMB. I've done it before and it works great. It adds a good bit of mass to the boiler.

You have to make sure your piping configuration is conducive to the operation and you also have to make absolutely sure you have a mixing valve on the DHW side of the indirect.

MikeBos

Harvey,
That's interesting as far as I can understand. <-- this means I'm not sure understand. The call from the RMB would run the boiler from 150-180. @ 180 boiler shuts off. At this time the IDHW has a temp in between 150-180 thus the mixing valve to not scald anyone at a sink/shower. Am I close? THANKS !

MikeBos

Fundamental followup questions about my setup here: http://imgur.com/a/BnkwF . The big loop shown in the picture is the primary loop, correct? The RMB in the middle is 1 of the secondary loops? The T's shown connecting the primary to the RMB are 'closely spaced T's' ? Top right grundfos pump is the primary boiler pump? I this all accurate? These are just some fundamentals I want to be sure I understand correctly.

I'm not clear if when a secondary circ is running is it a given that the primary boiler circ also be running ? I am suspicious that my system w/ the RMB in 'boiler enable' mode will not call to the Buderus for heat. When an infloor call for heat the RMB pumps will start to circulate the manifold at this time the primary boiler circ is not running. Then the order of events is that the 'purge' mode in the Hydrostat will show eventually on the screen and then primary boiler circ will run. Then after a few more minutes the Hydrostat must sense a need for fire and does so. Is this expected? Is it what 'boiler enable' is?

Thanks,
Mike

Solid_Fuel_Man

That's a nice looking setup.

Solid_Fuel_Man

Boiler enable is essentially just allowing the burner to run. Known as TT, the aquastat on the boiler shuts the burner off on high limit and back on again when the boiler temp drops a predetermined amount, usually 20 degrees. The primary pump should run anytime there is a call for heat from any of the secondary loops. The RMB is just one of the secondaries.

Looks like the manifold actuators are operated by the Taco ZVC and that end switch should place a demand on the RMB, which then enables the boiler to operate, as well as turns on the primary pump.

MikeBos

Solid_Fuel_Man, Thanks for the reply. Yes, thermostats call the ZVC and it calls the RMB. That is as far as the signal goes when in 'boiler enable' mode on the RMB. System pump only will go on when the boiler goes to purge mode. It does not seem correct that the RMB circ will run wo/ the primary system pump. How does it get new hot water for the injection pump. Are you saying this setup is wrong? I like the idea that Harvey had earlier to use the IDHW tank for some load to reduce short cycling but wo/ the system pump running I just don't get how it would work.
Mike