How to combine Pellet stove, buffer tank, underfloor heating

Deering

Hi all,

I am looking for confirmation or helpfull tips on the diagram below. I'm not a native speaker, so pardon my french

I have run a pellet stove in my gas heated system of radiators for some time now. This summer, I am installing underfloor heating so I will need a buffer to store the excess heat of the pellet stove. After reading many posts and watching videos I cooked up this diagram but are not sure if it is correct. I did not include many details such as valves, expansion tank and bypass to make sure I understand the principle.

About the diagram:

On the left side is my pellet stove which feeds the radiators, underfloor heating and buffer.

In the middle (marked orange) is the original gas heated system with 2 radiators. Valve A prevents the gas heater from using the pellet stove as a radiator, as well as it charging the buffer. Valve B prevents the pellet stove from using the gas heater as a radiator. The radiators do not have a dedicated pump as they will be operated by the pellet stove or gas heater, both of which have a circulation pump.

To the right is my underfloor heater, pump P1 can feed it from the buffer when the buffer is charged.

Please advise, all help is welcome.

Zman

Your plan could use a bit of help.
I would suggest reading this https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf
Around page 52 there are some good examples.

hot_rod

What are the heat exchangers for in that middle unit?

Deering

Thanks for replying, I really appreciate it.

@Zman : thanks, will do. I watched (and thougt I understood ) the 'coffee with caleffi' episode on Idronics 17. I will study it and post a revised plan here.

@hot_rod : They are supposed to represent radiators. Did I use a wrong icon?

Meanwhile, my 'cv-guy' advised to solve it like this (attached pdf). I trust his judgement, but he has no experience connecting a pellet stove. Any thougts on this diagram?

Zman

What are the required temperatures of the existing radiators? What temperature does the radiant require?

I like the "CV Guy's" layout and how it uses the buffer as a hydro separator. I don't understand how he is managing the radiators.

Deering

The radiators are designed to run at 60deg Celcius (140F). We almost never use them at this temperature (more than one year since last use like this) and are actually just frost protection and run on 45deg Celcius (113F) when used like this. However, if they are fully functional that would be a bonus.

One radiator (1.2KW) is fitted with a manual valve, one (2.4KW) has a thermostatic valve.

The underfloor heating (ground floor) will be controlled by a thermostat, switching on the underfloor heating pump, using it's own thermostatic valve to control the temperature in the subcircuit. I need to figure out how to control the main circulation pump (with said thermostatic valve or by pressure delta).

The radiators could maybe be controlled by a thermostat in their room, switching the main circulation pump.

I am diving into the Idronics issue tomorrow, curious to find new insights.

hot_rod

Mine was mostly homemade. A 500 gallon tank (1900L)with two 3-1/2' pipes welded into it.

Solar thermal drainback input 160 sq ft array (15m2)
EKO 40 KW wood boiler input
Lochinvar 120 LP (35 Kw) boiler input to just very top of tank
DHW via plate HX
High temperature radiators 180F (82C)
Low temperature radiant via 3 way thermostatic 110F (43C)
about 160 gallon (600L) expansion space at top, also the drainback air bubble
approx 1 bar pressure

The large pipes into the tank serve as the hydraulic separator, any or all pumps could run without any interference between them.

This two pipe concept was based on the design that I see the Austrian pellet boiler manufacturers use, very similar to what you have shown, but mine all connect into the tank at two large pipes.

The LP boiler would only heat the upper portion of the tank if the solar and wood were not adding heat, it was a backup. No need for it to heat the entire tank.

hot_rod

Here is another option for a two pipe buffer. Biomass boiler pipes with return protection valve into top and bottom of buffer. It "sees" the entire tank as its load.

If the Cv Kesel boiler is a condensor, you want the lowest possible return temperature, and you may not need it to heat the entire buffer tank, so the biomass has more load.

The low temperature loads piped from a mix valve, on outdoor reset, perhaps.

The main difference is the drawing you show has the condensing boiler as a secondary loop. It may not see the lowest return temperature. Either connect its return mid tank, if possible. Or at the bottom of the tank so it sees the coldest return temperature to maximize efficiency.

The big advantage to two pipe is direct to load. All output goes to loads first, then charges buffer as loads drop off. You do not need to "involve" the buffer tank on heat loads, necessarily.

When all loads satisfies, the the tank gets heated to setpoint.
On next heat call, energy comes only from tank, until it drops in temperature and calls on one of the boilers.

Resol makes a nice control that could run all this. The BX model. Use the solar function as the biomass boiler.

Deering

that is some impressive installation @hot_rod

@Zman : I realised just now that 'cv-guy' does not translate in english, I meant 'gas fueled heating systems-guy'.

Here is my next try. Will the 'two pipe' buffer serve as a hydraulic separator like this?

Deering

the supply and return on the open divider need to be swapped obviously

hot_rod

I'm assuming the gas boiler only runs when the pellet drops too low? Do they ever both run together?

There may be a reason, but I'm not sure why they tie the gas boiler into the system with close tees? There will be temperature blending there. Depending on boiler flow and distribution flow, you might blend the return to the gas boiler up and lower its efficiency?

Here is an example how blending occurs with tees like that. There is a mixed temperature formula that could predict that temperature at any placed in those two tee connections. You need to know flow rates and temperatures.

Bringing the colder return from the heat zones directly to the boiler/tank would get you the best use of a condensing boiler. Your underfloor zones could return 80F back to the boiler to get good condensing, if piped direct.

Deering

I started again from my 1st sketch. It may prove I learnt either something or nothing. Let me know Thanks for your patience!

The pellet stove uses the buffer for hydraulic separation (video: Coffee with caleffi 09:10).

The gas heater uses a hydraulic separator so it will not interfere with the mixer pump. I am not sure where to put the separator though; close to the heater or close to the rest of the system (it seems pointless to pump around water when not needed, so I placed it near the heater).

I do not want the gas heater to supply the buffer as it can modulate and is only used as a backup. It will not run simultaneously with the pellet stove. I think adding two backflow valves near A and B will solve this.

Any thoughts on this?
Thanks!

hot_rod

With a 2 pipe buffer tank, the large diameter pipe at the tank IS the hydraulic separator. So no need to add another. In your drawing you would need another pump on that separator you show, one for the boiler side, one to push into your heating loop.

The pipe connections at the tank need to be a larger diameter to accomplish separation, in US sizes 1-1/2 or 1-1/4 for the flow rates you are working with. Keep them short also, you want low velocity in those header pipes to prevent unwanted flow between boilers and heat loops. Idronics 17 goes into more detail regarding thermocline, exergy, etc of a buffer tank.

For the controls, put the operating sensor from the pellet boiler into the tank, near the bottom 1/3 of the tank. So the pellet boiler only sees the tank as its load. Its only job it to maintain that tank temperature. Let it run to 80C or higher. The higher you store the temperature, the more useful that tank. Longer pellet runs, and longer drawdown before it fires again. Think of it as a flywheel.

There are a number of ways to control the gas boiler. Simply, its control sensor attaches to that pipe on the top of the buffer tank. If it sees the tank is too low, maybe below 35C? It will kick on and take over.

A better control, maybe even available from the Biomass boiler folks is that Resol I showed above.

It watches temperature in multiple places, called a differential control. Knowing 4 temperatures in one control it can better control when the boilers switch between one another. I assume you want the bio boiler doing most of the work?

It shows it as a solar control, just use that solar function as the biomass control, second function R-2 calls on the gas boiler, only when the tank is exhausted. But it also knows when the pellet is back on, hot enough to bump off the gas boiler.

It has a display to show all the temperatures, measures energy, can run the pumps on variable speed, has alarms, error codes to show if a sensor fails, you can manually run pumps to check temperatures, etc. Data loggers can be added and it can be viewed online if you connect to the Resol portal. They are designed for exactly what you are trying to accomplish.

A good control guy could build such a control from off the shelf relays, etc. But Resol has spent years perfecting all the options for you.

I tried to simplify the drawing to show the separation, the large diameter pipes at the upper and lower tank connections.
You would have two temperature mixing valves connected to the "header". Tank maintains at maybe 80C, one mixer set to 60C, the other to 43C or whatever the infloor needs.

Yet another option for even better temperature control would be to use motorized mixing valves that operate based on outdoor reset control ODR, that better matches the temperature you send to the heating zones. A thermostatic valve always send the same temperature to the zones.
It comes down to how much $$ you want to spend for how much control perfection

Deering

I like the control possibilities, but let me get the piping right first; is this a diagram I could work with?




My (intended) buffer has a 1" connection. Will that be okay or will that behave like a venturi and stir up my buffer too much?

hot_rod

not too much of a concern. Your highest flow will be when zones are off and one of the boilers is just recharging the tank. The 10Kw bio, 34,000 BTU/hr probably only flows 3- 4 gpm? 15l./min? so not much velocity.

The gas boiler 6 gpm, 22l?

If you use threaded pipe I have used these swage nipples, a nice smooth transition
1X 1-1/2"
Or copper to male adapt, we have 1 mip X 1-1/2" copper adapters over here.

Or custom braze one like this.

Deering

Up for review: here are two diagrams, the difference is the connection of the gas heater. It is a non-condensing heater, so I would not need the lowest return temperature.

In the first diagram, it is therefore connected parallel to the buffer.

In the second diagram, it is connected with close t's; the reason behind this would be that it will only provide heat to radiators and underfloor when needed, not heating the buffer (apart from excess return temperature).

Any thoughts, advice, or preferences?
Much appreciated!

hot_rod

The headers at the buffer are the hydraulic separation, oversized to accomplish this. Both pump stations have 3 way mixers included. It will work fine as drawn.