What size buffer tank?

Mo9013

Hi to all. I have a Buderus GB145/24 boiler that has been running great with no problem till now. When I first installed it my house was baseboard heat on the first floor (18000 btu) and cast iron in the basement (12000 btu). 2 zones. As I am remodeling, I am switching over the first floor to radiant floor heat. I am having short cycling problems when the radiant floor is running caused by the low temps needed. At the moment I have a manual mixing valve to drop the water temp to 90 deg for the floor and the zone is controlled by a thermostat. I will be keeping the basement cast iron radiators when I'm done with the first floor. I feel the best way to fix the problem is to install a buffer tank but I have no idea what size I will need or the best way to run it.

My first thought was to run the buffer tank with a set point off 100 to 120 deg and use a mixing valve for the floor loops. and use the outdoor reset on the boiler to control the water output to the radiators between 70 outside temp at 120 to 30 temp to160 deg water. this way if my basement doesn't run then it only heating the buffer to 120 and if it does run its just going to make the buffer hotter and not have to turn on for a longer time.

Second is to set the boiler to 170 deg and put the buffer tank on an outdoor rest between 110 and 150 with the same mixing valve for the floor in the fist thought.

I'm not even positive if I want to run the floor on constant circulation with a outdoor reset mixing valve or keep it the way I have it now as thermostat controlled with a manual mixing valve.

I am not positive on what will be a more efficient way of solving my problem so any help with sizing the buffer tank or another way of fixing my problems would be a great help.

EBEBRATT-Ed

You will get longer cycles if you let the buffer tank run up to the boiler temp and take all the zones off the buffer tank mixing down for the radiant loop.

To size it you should get BTU rating of CI zone and BTU rating off radiant zone and BTU output of the boiler as well as the approximate flow rate for each zone. Then you can pick a buffer tank.

See the attached. If you google boiler buddy the have an online tank sizing calculator you can use along with piping diagrams etc.

Tom_133

Hey Mo9013, can you share a pic of how you piped the mixing valve and piping at your boiler? It may just be a simple fix

Mo9013

Thank you for the replies. I can get the pictures when I get home tonight. I know my boiler is running at 30000 btu from what I have it set at. I can lower it to 25000 btu. I have bumble bee circs and they run about 6 gpm and 4 gpm at the dt I have them set at for. I did all the calculations when I did the floor zones but I don't think I have them anymore. That's what I get for doing them on paper and not on my computer.

Mo9013

This is my piping set up. One mixing valve is for the bedrooms and bath the other is for the living space.

The cast iron loop is 12000 btu with a 6-7 gpm.
The floor is 18000 btu with .5 to 1.5 gpm per loop totaling around 8 to 9 gpm.

hot_rod

What is the lowest modulation rate on that boiler? You should not need much buffer. Too bad you need to run that hot, 170, on a condensing boiler?

Here is a super simple buffer idea I used on my home. Boiler connects on left side, tees up to high temperature baseboard zones also. Out the top is a manual 3 way mix valve.
So basically the boiler is controlled by the buffer tank. Buffer tank temperature is varying based on outdoor reset control on the Lochinvar boiler. the boiler is a 120 combi, kicks into DHW on a faucet draw.

This is a two pipe buffer piping, all the boiler output can go directly to the high temperature load, or both tank and load, or to just the tank when the high temperature zones satisfy.

With this configuration I dial in the 3 way mix valve with the boiler running at its highest SWT, around 150 for the fin tube, 110 for the radiant on design day. Then the SWT to the low temperature radiant tracks along with the boiler reset temperature via the manual 3 way mixer. An actuator and control could be added to that mix valve if you want even more reset control.

I also have an electric element in the tank as a backup if the boiler failed. Or I run out of LP

A 5500W element would get you 18,700 BTU/hr. so basically a hybrid system as far a fuel options.

JakeCK

hot_rod said:

What is the lowest modulation rate on that boiler? You should not need much buffer. Too bad you need to run that hot, 170, on a condensing boiler? Here is a super simple buffer idea I used on my home. Boiler connects on left side, tees up to high temperature baseboard zones also. Out the top is a manual 3 way mix valve. So basically the boiler is controlled by the buffer tank. Buffer tank temperature is varying based on outdoor reset control on the Lochinvar boiler. the boiler is a 120 combi, kicks into DHW on a faucet draw. This is a two pipe buffer piping, all the boiler output can go directly to the high temperature load, or both tank and load, or to just the tank when the high temperature zones satisfy. With this configuration I dial in the 3 way mix valve with the boiler running at its highest SWT, around 150 for the fin tube, 110 for the radiant on design day. Then the SWT to the low temperature radiant tracks along with the boiler reset temperature via the manual 3 way mixer. An actuator and control could be added to that mix valve if you want even more reset control. I also have an electric element in the tank as a backup if the boiler failed. Or I run out of LP A 5500W element would get you 18,700 BTU/hr. so basically a hybrid system as far a fuel options.

Do you have a plumbing diagram of your setup. I drew up something that I wanted to do for my boiler a few years back but never found the time to do(kids) and sounds similar. Actually you might have been one of those who gave me a few ideas. I like the idea of the electric element as a backup. Thermo 2000 has a combination reverse buffer tank with electric elements that is used for HP setups. but its serious $$$. 

Someone with a combi boiler wouldn't need a reverse indirect but for those with a standard ci boiler...

hcpatel78

I followed @hot_rod advice when I did set up Radiant with Buffer. Please have a look there, I have pictures and diagrams for my setup. By the way, it works perfectly... No short cycling of my boiler at all at any temperature.
https://forum.heatinghelp.com/discussion/179216/need-advise-confirmation-for-underfloor-between-joist-radiant-installation-northern-nj-zone-5/p2

hot_rod

Here is the formula for sizing a buffer for a modulating boiler. The modulation help reduce the tank size compared to the standard formulas or tables out there.

hot_rod

JakeCK said:

hot_rod said:

What is the lowest modulation rate on that boiler? You should not need much buffer. Too bad you need to run that hot, 170, on a condensing boiler?

Here is a super simple buffer idea I used on my home. Boiler connects on left side, tees up to high temperature baseboard zones also. Out the top is a manual 3 way mix valve.
So basically the boiler is controlled by the buffer tank. Buffer tank temperature is varying based on outdoor reset control on the Lochinvar boiler. the boiler is a 120 combi, kicks into DHW on a faucet draw.

This is a two pipe buffer piping, all the boiler output can go directly to the high temperature load, or both tank and load, or to just the tank when the high temperature zones satisfy.

With this configuration I dial in the 3 way mix valve with the boiler running at its highest SWT, around 150 for the fin tube, 110 for the radiant on design day. Then the SWT to the low temperature radiant tracks along with the boiler reset temperature via the manual 3 way mixer. An actuator and control could be added to that mix valve if you want even more reset control.

I also have an electric element in the tank as a backup if the boiler failed. Or I run out of LP

A 5500W element would get you 18,700 BTU/hr. so basically a hybrid system as far a fuel options.

Do you have a plumbing diagram of your setup. I drew up something that I wanted to do for my boiler a few years back but never found the time to do(kids) and sounds similar. Actually you might have been one of those who gave me a few ideas. I like the idea of the electric element as a backup. Thermo 2000 has a combination reverse buffer tank with electric elements that is used for HP setups. but its serious $$$. 

Someone with a combi boiler wouldn't need a reverse indirect but for those with a standard ci boiler...

here are three buffer piping options. I like 2 and 3 best as it has the least interaction with the tank until it is needed.

Heat pump designers like 3 pipe as the lowest possible return goes to the HP (in heating mode). same for mod cons. You need a tank with a large top connection.

4 pipe tanks are harder to source $.

Electric tanks come in many sizes, are fairly well insulated and cost effective. Plus you get a listed element included. With increased 3/4 connections 8 gpm is reasonable. Commercial electric tanks will have larger connections.