Indirect WH as buffer tank for radiant floor, does this diagram look about right?

ThatF_inGuyInNY

https://imgur.com/a/oa2rUKY



The house is currently heated with cast iron and an oil fired, NC boiler. Ive been adding radiant floor tubing to small sections of the house (mud room/kitchen, 1st floor bathroom/entry, 2nd floor bathroom) during some remodel work but have not yet hooked it up. The boiler needs to be replaced and my plan is to rework all of the mech room black iron pipe to add low temp manifolds and home runs for each radiator.

My original plan was to use mixing valves but after reading up a bit on the concern over short cycling for small radiant zones, Im reconsidering adding another indirect (I have one installed for my DHW) to act as a buffer tank.

Isolation valves, unions, hyvents and drains not shown.

Does this look about right? There are two PRV/BP in the diagram, but do I need a second PRV for the radiant loop or can I tie in after the boiler PRV and add a second backflow preventer or check valve?

All comments welcome.

Hot_water_fan

Is there a reason why you aren’t using a buffer tank instead? Mixing valves don’t cause short cycling. Follow the Idronics recommendations and you’ll be just fine. 

EdTheHeaterMan

Hot_water_fan said:

Is there a reason why you aren’t using a buffer tank instead? Mixing valves don’t cause short cycling. Follow the Idronics recommendations and you’ll be just fine. 

This is an interesting concept. Try typing in BUFFER TANK on SupplyHouse.com and all you get are Indirect water heaters. So that would be one reason. When I google "buffer tank" all the results are commercial ASME tanks that have 2" flange connections and go up from there. The concept of
"just use a buffer tank" makes sense for large applications but where do you get a 20 or 30 gallon insulated tank with 4 tappings to use as a buffer at a reasonable cost.

With an indirect if you need 115° water temperature for the radiant loops, then set the indirect tank temperature to 115° and let the boiler heat the tank to that temperature. No mixing valve, no motorized 3 or 4 way valve, or injection pump needed, with all the electronic devices and sensors to operate them. This is basic and simple. I Like it!

As far as 2 PRV and 2 BFP, and 2 expansion tanks. If that is how you want to do it, then you must treat the tank of the buffer as a separate boiler source that is not connected to the main boiler. The main boiler is heating the coil side of the buffer tank. That tank side of the buffer indirect tank is considered a separate system that has the possibility for water to expand and cause a hydraulic overpressure condition as you would have on any solar, fired or electric boiler system. And since it is isolated from your fired boiler system, you did it right with separate accessories and accouterments.

hot_rod

EdTheHeaterMan said:

Is there a reason why you aren’t using a buffer tank instead? Mixing valves don’t cause short cycling. Follow the Idronics recommendations and you’ll be just fine. 

This is an interesting concept. Try typing in BUFFER TANK on SupplyHouse.com and all you get are Indirect water heaters. So that would be one reason. When I google "buffer tank" all the results are commercial ASME tanks that have 2" flange connections and go up from there. The concept of "just use a buffer tank" makes sense for large applications but where do you get a 20 or 30 gallon insulated tank with 4 tappings to use as a buffer at a reasonable cost. With an indirect if you need 115° water temperature for the radiant loops, then set the indirect tank temperature to 115° and let the boiler heat the tank to that temperature. No mixing valve, no motorized 3 or 4 way valve, or injection pump needed, with all the electronic devices and sensors to operate them. This is basic and simple. I Like it! As far as 2 PRV and 2 BFP, and 2 expansion tanks. If that is how you want to do it, then you must treat the tank of the buffer as a separate boiler source that is not connected to the main boiler. The main boiler is heating the coil side of the buffer tank. That tank side of the buffer indirect tank is considered a separate system that has the possibility for water to expand and cause a hydraulic overpressure condition as you would have on any solar, fired or electric boiler system. And since it is isolated from your fired boiler system, you did it right with separate accessories and accouterments.

Looks like Lochinvar 4 port buffer tanks on SupplyHouse, although some are backordered

https://www.supplyhouse.com/sh/control/search/~SEARCH_STRING=lochinvar%20buffering%20tanks

I thought the Boiler Buddys were available there also

ThatF_inGuyInNY

EdTheHeaterMan said:

Hot_water_fan said:

Is there a reason why you aren’t using a buffer tank instead? Mixing valves don’t cause short cycling. Follow the Idronics recommendations and you’ll be just fine. 

This is an interesting concept. Try typing in BUFFER TANK on SupplyHouse.com and all you get are Indirect water heaters. So that would be one reason. When I google "buffer tank" all the results are commercial ASME tanks that have 2" flange connections and go up from there. The concept of
"just use a buffer tank" makes sense for large applications but where do you get a 20 or 30 gallon insulated tank with 4 tappings to use as a buffer at a reasonable cost.

With an indirect if you need 115° water temperature for the radiant loops, then set the indirect tank temperature to 115° and let the boiler heat the tank to that temperature. No mixing valve, no motorized 3 or 4 way valve, or injection pump needed, with all the electronic devices and sensors to operate them. This is basic and simple. I Like it!

As far as 2 PRV and 2 BFP, and 2 expansion tanks. If that is how you want to do it, then you must treat the tank of the buffer as a separate boiler source that is not connected to the main boiler. The main boiler is heating the coil side of the buffer tank. That tank side of the buffer indirect tank is considered a separate system that has the possibility for water to expand and cause a hydraulic overpressure condition as you would have on any solar, fired or electric boiler system. And since it is isolated from your fired boiler system, you did it right with separate accessories and accouterments.

You nailed it in the first paragraph. I looked for a buffer tank, found a bunch of indirects. My plan was to us an amtrol CH-41z (or WM aquaplus) to match my existing indirect. Part of the reason is because they are so ubiquitous. Last thing I want is to pipe for some obscure buffer tank only to have it be discontinued if I need to replace it.

I am still on the fence about if this is truly the way to go. The Boilermate indirect I installed 7 years ago was much cheaper, they are like $1400 now. I would save a good bit of fittings and mixing valves (no boiler protection needed either in this setup), but no where near the cost of the indirect. I do think the buffer tank would allow a lot more wiggle room in terms of hooking up and dialing in zones as well as future expansion. But it also adds a pretty big point of failure. Not to mention that the warranty on the indirect might not be honored if used in a heating application. On the other hand, my current system uses 85yo black iron pipe and cast iron so the water ends up filthy. Even with a mag separator, this could be an issue with the mixing valve cartridge's over time. The indirect allows me to create a completely clean radiant loop and segregate the much dirtier water.

ThatF_inGuyInNY

Hot_water_fan said:

Is there a reason why you aren’t using a buffer tank instead? Mixing valves don’t cause short cycling. Follow the Idronics recommendations and you’ll be just fine. 

Ed nailed it in his comment. There are an incredibly limited number of small residential buffer tanks available. I have to consider future availability as well as current in case I ever need a replacement. I also like the idea of segregating the dirty cast iron/black pipe water from the clean radiant water. Even with a mag separator, Im concerned that the debris will be tough on the mixing valves and flow control valves on the manifold.

ThatF_inGuyInNY

Another question; In the configuration shown in the diagram, do I actually need primary/secondary piping? If using a dP pump on the boiler, with priority on the DHW, I wonder what P/S accomplishes. Alternatively, I could use a standard pump on the boiler loop and keep the P/S.

Any thoughts?

ThatF_inGuyInNY

Peakedtoosoon said:

"With an indirect if you need 115° water temperature for the radiant loops, then set the indirect tank temperature to 115° and let the boiler heat the tank to that temperature. No mixing valve, no motorized 3 or 4 way valve, or injection pump needed, with all the electronic devices and sensors to operate them. This is basic and simple. I Like it!"

It's not very efficient IMO. Now you've got two indirect hot water tanks, with associated standing heat loss, when there is no demand. If you don't want the two circuits to blend, install a plate heat exchanger. Also seems strange that you can't get a small buffer tank.

Well one indirect is for DHW, so thats a fixed cost. I am considering the heat exchanger as well. I have not looked into it much. Do you know how tunable they are? Can I use a balance valve to dial in the flow (and thus output temperature) and avoid a mixing valve altogether?

Also, would I need a tertiary loop (with its own pump and separation) so that flow can be consistent through the HX despite zones opening and closing?

hot_rod

Hmmm, in addition to SH.com, every major wholesaler in America shows buffer tanks from 20- 120G on their offerings. At least 6 manufacturers that I know of?

If you go to the trouble and expense of a buffer it would be nice to pull your load off the tank via ODR. That way you use every last ounce of the stored energy.

ThatF_inGuyInNY

hot_rod said:

Hmmm, in addition to SH.com, every major wholesaler in America shows buffer tanks from 20- 120G on their offerings. At least 6 manufacturers that I know of?

If you go to the trouble and expense of a buffer it would be nice to pull your load off the tank via ODR. That way you use every last ounce of the stored energy.

Ill check again, SH search sometimes doesnt find what I am looking for and I find it later through a google search link to SH

hot_rod

ThatF_inGuyInNY said:

hot_rod said:

Hmmm, in addition to SH.com, every major wholesaler in America shows buffer tanks from 20- 120G on their offerings. At least 6 manufacturers that I know of?

If you go to the trouble and expense of a buffer it would be nice to pull your load off the tank via ODR. That way you use every last ounce of the stored energy.

Ill check again, SH search sometimes doesn't find what I am looking for and I find it later through a google search link to SH

.https://www.supplyhouse.com/sh/control/search/~SEARCH_STRING=lochinvar buffering tanks

I posted the link above. I did a google search, FW Webb shows Lochinvar, Boiler Buddy, SpacePak and HTP

Looks like the SpacePak has some resistance elements also. Nice for when they take your gas away

ThatF_inGuyInNY

hot_rod said:

ThatF_inGuyInNY said:

hot_rod said:

Hmmm, in addition to SH.com, every major wholesaler in America shows buffer tanks from 20- 120G on their offerings. At least 6 manufacturers that I know of?

If you go to the trouble and expense of a buffer it would be nice to pull your load off the tank via ODR. That way you use every last ounce of the stored energy.

Ill check again, SH search sometimes doesn't find what I am looking for and I find it later through a google search link to SH

.https://www.supplyhouse.com/sh/control/search/~SEARCH_STRING=lochinvar buffering tanks

I posted the link above. I did a google search, FW Webb shows Lochinvar, Boiler Buddy, SpacePak and HTP

Looks like the SpacePak has some resistance elements also. Nice for when they take your gas away

Thank you

I started from square one and dug in to manufacturers instead of retail stores. There are some good options out there that I hadnt seen before.

I think some of the folks in this thread were correct that using an indirect might be overly complicated. By using the buffer, I can scrap the Hydro separator, one ZV and one of the pumps.

Heres the updated diagram

https://imgur.com/a/Dq5znff

This should solve the problem of allowing me to tinker with zoning and zone sizing as I please.

hot_rod

Maybe consider a two pipe, direct to load buffer? What is the radiator load compared to boiler output size? If it is 20-25%, I would not worry about return temperature protection.

The indirect should be before the buffer, parallel to the boiler for fastest recovery, highest SWT

ThatF_inGuyInNY

hot_rod said:

Maybe consider a two pipe, direct to load buffer? What is the radiator load compared to boiler output size? If it is 20-25%, I would not worry about return temperature protection.

The indirect should be before the buffer, parallel to the boiler for fastest recovery, highest SWT

Yea, I understand running the indirect off the primary loop, that was my oversight. Im trying to wrap my head around if a 2 pipe would promote higher or lower return water temps. Feels like I would get lower return temps but im honestly not sure.

hot_rod

Well if you want the lowest possible return, direct to the boiler a 3 pipe give you that, plus...

The subtle difference from a 2 pipe, 3 pipe has flow always going across the bottom, so you always engage the thermal mass. This is critical for HP buffering, but if one goal is the use of the tank for micro loads, this is also a good option.

No need for a valve to prevent return passing through an "off" heat source, required with 2 port..

It does require a 3 port tank, or a 4 port with one port capped. So it eliminates using an electric water heater tank as a buffer.
It is a nice compromise between 2 and 4 port buffer tanks.

4 pipe is best suited for bio systems, wood boilers, it is generally the over-heat mechanism, so the tank is always hot. No delay to a heat call as a result.

2 and 3 are direct to load, so if the tank temperature has dropped, a heat call will have all the boiler output going to the load. The tank recovers as the loads start dropping off. They are also best if you do setback and need quick recovery temperature.

ThatF_inGuyInNY

hot_rod said:

Well if you want the lowest possible return, direct to the boiler a 3 pipe give you that, plus...

The subtle difference from a 2 pipe, 3 pipe has flow always going across the bottom, so you always engage the thermal mass. This is critical for HP buffering, but if one goal is the use of the tank for micro loads, this is also a good option.

No need for a valve to prevent return passing through an "off" heat source, required with 2 port..

It does require a 3 port tank, or a 4 port with one port capped. So it eliminates using an electric water heater tank as a buffer.
It is a nice compromise between 2 and 4 port buffer tanks.

4 pipe is best suited for bio systems, wood boilers, it is generally the over-heat mechanism, so the tank is always hot. No delay to a heat call as a result.

2 and 3 are direct to load, so if the tank temperature has dropped, a heat call will have all the boiler output going to the load. The tank recovers as the loads start dropping off. They are also best if you do setback and need quick recovery temperature.

Followup question;

With a buffer tank, if I am running a taco 3 zone control, does it make sense to not tie the end switch to TT on the boiler? If the buffer tank is capable of satisfying micro zones, would it make sense to just allow the aquastat to cycle the boiler based on boiler temperature alone and not based on calls for heat? This also assumes the primary loop pump is run off the zone control.

Alternatively, should TT be tied to a temp sensor in the bulbwell of the buffer tank?

hot_rod

The tank calls on the boiler. Either by it's own aquastat, or if your boiler has an optional sensor to put in the tank. If you need to supply a tank control, get one of the electronic set point controllers, they have more adjustability, digital display, etc. Ranco, Johnson controls, Penn, Honeywell, they all make them.

The next choice is whether to run the tank to a fixed setpoint temperature, pull the loads off via ODR.

OR just let the tank be maintained at ODR temperature.

With wood or biomass systems I let the tank run hot 180F. That give me them most drawdown, keeps the boilers running long hot cycles.

With a mod con, and direct pipe 2 or 3 port I think running the tank off the boiler, which is on ODR, is the most efficient way. So the tank is always at the correct temperature to go to the load. Not quite as much draw down as a 180° tank with the loads being pulled off via a valve on ODR, but more condensing operation.