Outdoor Reset Control questions

[Dutchie84]

Hi guys, I am having trouble wrapping my head around outdoor reset (ODR) control. I understand how ODR works, like cruise control for your car, only giving you the temperature that you require to heat you house depending on the outside temp, but this is where my confusion starts for my specific circumstances.

I am planning a house build this spring/summer. We want radiant in floor heat, but also want real hardwood floors, so I have been strongly considering Warmboard "S" for your subfloor as our only heating emitter on the upper 2 floors and pex in the concrete slab for the basement. I am thinking of a propane boiler to get us in the house and then in the next year or so a wood boiler with storage, and the propane will be our backup.

So I guess my question is how do you control an ODR system? Can I run the whole house on one water temperature? It will be pretty open concept, but I have come up with 20 btu/hr/sqft for the living room and like 10 btu/hr/sqft for the kitchen. So do I set my temp for the biggest load (living room) and then throttle back the valves for the kitchen, or does it not even matter since they will be so open to each other? What about smaller rooms like an office, powder room or bedrooms, wouldn't they over heat if they get the same temp as the living room? I would like to be able to change the temperature in the bedrooms depending who is using them. (if mom comes for a visit, or set them back until they are filled with kids)

When you change the temperature on ODR do you just have one controller and you shift the temperature line on the graph up or down so that the whole house moves together?? I was thinking of doing a manifold with actuators to control the zones, but if they are just going to be open all the time because the water temp is only just high enough to make up for the heat loss then they would just be a waste of money. Or would the water temperature be set for the biggest load (living room) and then use the same temp for the whole house and actuators with thermostats in the bedrooms, bathrooms and office to keep it from over heating.

That's enough for now I guess. So many questions it keeps me up at night . lol

 

[maple1]

I have no experience with ODR, but first thought is you would set the curve for your biggest zone/load, then the other zones would cycle off & on as needed via their separate thermostat & zone valve/actuator/TRV. Would likely require a bit of fiddling with, at first at least, as in-floor heat is slow to respond and prone to over shooting. (It keeps heating for a little while after the stat closes the zone - and also takes a little bit to get ramped up on a call for heat). You could also maybe add adjustable mixing valves to the lower demand zones to reduce their temps (manually if needed) further? Then once you get them dialed in/adjusted, they should be good.

Hopefully you'll get some 'real life' feedback...

 

[Bad LP]

In two places I am responsible for we have Tekmar ODR controls on multiple oil fired boilers. These boilers are feeding hot water to hydro air systems, 11 fan coil units in one location and 9 in another. With all the same type of heat throughout the buildings one temperature of water being adjusted works perfectly. If it is going to get bitter cold (sub zero days) I will adjust the ramp and sometimes shut off the priority to the domestic hot water.

In my own application I have LP feeding a boiler. This boiler is plumbed so that my current baseboard heat in the bedrooms is run directly off the boiler for the higher temp they require. One zone off my boiler feeds a Taco Radiant Floor Mixing Block. There is an outdoor sensor and a curve that is programed into the block as well as a maximum water loop temperature. I'm using air temp, not floor temp for the t'stats and it holds dead on whatever the temp is set at.

Soon I will be removing the baseboard heat and installing panel radiators to take more of an advantage of the lower water temps provided by the wood boiler. I'm not sure yet what I'm going to do about backup boiler plan.

As for your LP boiler plans, be careful with a low mass boiler. I'm using a System 2000 and the short cycling was driving me out of my mind on the RFH zone. I installed a 40 gallon buffer tank and the annoyance was drastically cut. In hindsight I should have used a cast iron boiler for the mass it provides.

 

[Dutchie84]

Well that was going to be a future question. If I have say just the master bath calling for heat then I'm sure the boiler would short cycle. However I can't seem to find a clear answer on how long a short cycle is. 5-10-15 mins???? But this also leads into what would be the best way to plumb it so that the Lp boiler will be back up in a year or two. I'm thinking maybe Lp feeds buffer tank then in the future wood boiler/storage would also feed the buffer tank.

 

[Bad LP]

My short cycle was firing 10+ times an hour.

 

[JohnDolz]

Well that was going to be a future question. If I have say just the master bath calling for heat then I'm sure the boiler would short cycle. However I can't seem to find a clear answer on how long a short cycle is. 5-10-15 mins???? But this also leads into what would be the best way to plumb it so that the Lp boiler will be back up in a year or two. I'm thinking maybe Lp feeds buffer tank then in the future wood boiler/storage would also feed the buffer tank.

I hardly ever use my propane boiler but I dealt with the short cycle by adjusting the minimum time required between firings. As an FYI I have a Triangle Tube propane boiler. It was designed in Belgium (I believe) and therefore set up to support the way they run in Europe (generally 1 or 2 zones in a house plus DHW). Of course it works in our US multi-zoned houses by leveraging additional controls (I have a Taco box that controls my 5 zones). For me this set up works very well as it is exactly the way my wood boiler is designed. It has ODR which controls mixing valves for 3 zones - 2 for heat and one for DHW. This allows you to pick the Curve/slope for each zone plus the target Temp for your DHW Cylinder. Because I have 5 zones in my house (plus DHW) I set my boiler to use ODR to control one mixing valve. I then use the thermostats to do the minor tweaking. 3 Zones are my primary living area and have baseboard. I choose a slope/curve that gives me very low water temp and target to have those pumps running constantly. The other 2 zones I move the thermostats up and down as needed (i.e. one is a home office that I keep at 55F and turn the heat up when I am going to work from home). The DHW runs off of a different temp setting and mixing valve. Going back to the short cycling the folks at Triangle Tube suggested I use the propane boiler, firing at a very low modulation to heat my 1000 gal of storage and then run off of that until the cycle needed to start over again. For how little I use the propane boiler it wasn't worth the effort but if I were starting from scratch I might think about how to best leverage a modulating boiler and storage - this way if you ever get tired of wood you have the most efficient propane system available. I know there is energy loss form storage but since it gets lost into the house I don't consider it a loss. Hope this helps a bit.

 

[__dan]

I would be looking for several things.

If I was firing a propane or gas boiler, it would have to be modulating fire, condensing rated, with the ODR built into on the boiler controls. If the boiler does not have ODR, OAT reset, built in, it is added cost and complexity to add it externally.

The boiler will have a minimum flow rate through the heat exchanger. If you cannot hit this flow rate through your minimum zones or DHW, you may have to have a primary secondary piping arrangement (likely).

I would use the DHW priority on the boiler with a DHW indirect tank and then let it reset for the radiant zones, one or the other. When the radiant loop distribution temp is reset directly by the boiler firing rate, all you would need is to zone the heating loads, circs or zone valves.

Choosing the right boiler is key. I would go with one of the standard top of the line units Weil McClain, Triangle Tube ... condensing rated, modulating fire, ODR OAT reset on the boiler controls. I would stay away from the off brand instant hot water types. Must be condensing type.

I would separately zone the living spaces with different heat requirements. One of the biggest problems with radiant is overheating the spaces which OAT reset and zone controls should cure. Basement slab most likely will not be fussy about the water temp as long as it is mixed down, same as the other floor heat.

Once the reset schedule is set you should not have to touch it.

Short cycling may be due to the low mass of the radiant distribution rather than the boiler. Boiler should turn down to match the load.

There is enough complexity that poor design could give very unsatisfactory performance. A good design with the excellent materials and equipment available today should give effortless and troublefree heat.

 

[Bad LP]

Just remember with complexity comes cost and potential problems and service visits. All of the add ons from the factory can have problems. The Tekmar controls that we added on have been 100% rock solid as well as the TACO equipment but this is all 20 year old stuff. I also want to say that I'm a believer that a fine line exists between too complicated to fix at midnight without the proper parts in the truck with a -10 air temp and the wind blowing like hell outside.
Not every boiler company runs around with $20K of spare parts in the van and because you chose to build something not normally available locally could have a very cold end result.
Buy normal gear of good quality and supported by local companies. You should have spares of odd ball stuff not normally available if you go that route.

Jumping out a "normal boiler" to run off protection devices is easy. A complicated factory boiler might not allow that simplicity.

 

[JohnDolz]

I would be looking for several things.

If I was firing a propane or gas boiler, it would have to be modulating fire, condensing rated, with the ODR built into on the boiler controls. If the boiler does not have ODR, OAT reset, built in, it is added cost and complexity to add it externally.

The boiler will have a minimum flow rate through the heat exchanger. If you cannot hit this flow rate through your minimum zones or DHW, you may have to have a primary secondary piping arrangement (likely).

I would use the DHW priority on the boiler with a DHW indirect tank and then let it reset for the radiant zones, one or the other. When the radiant loop distribution temp is reset directly by the boiler firing rate, all you would need is to zone the heating loads, circs or zone valves.

Choosing the right boiler is key. I would go with one of the standard top of the line units Weil McClain, Triangle Tube ... condensing rated, modulating fire, ODR OAT reset on the boiler controls. I would stay away from the off brand instant hot water types. Must be condensing type.

I would separately zone the living spaces with different heat requirements. One of the biggest problems with radiant is overheating the spaces which OAT reset and zone controls should cure. Basement slab most likely will not be fussy about the water temp as long as it is mixed down, same as the other floor heat.

Once the reset schedule is set you should not have to touch it.

Short cycling may be due to the low mass of the radiant distribution rather than the boiler. Boiler should turn down to match the load.

There is enough complexity that poor design could give very unsatisfactory performance. A good design with the excellent materials and equipment available today should give effortless and troublefree heat.

Dan - I am no expert but based on what I have learned over the past 2-3 years I completely agree. You never know what you don't know until it is too late. There are certainly a few minor things I would do differently if I had to do it over again but probably the most important lesson would be to hire a hydronic engineer that truly understands what you want to do with the wood and propane boiler, have it expertly designed from the beginning and consider it money well spent. In fact I am still looking for someone with this skill set local to CT - open to recommendations. I probably would not rip and replace but wouldn't mind bouncing a few things around and having some predictable outcomes on potential modifications. The unpredictable ones have been amusing but I think I have had enough of them for now.

 

[__dan]

Just remember with complexity comes cost and potential problems and service visits. All of the add ons from the factory can have problems. The Tekmar controls that we added on have been 100% rock solid as well as the TACO equipment but this is all 20 year old stuff. I also want to say that I'm a believer that a fine line exists between too complicated to fix at midnight without the proper parts in the truck with a -10 air temp and the wind blowing like hell outside.
Not every boiler company runs around with $20K of spare parts in the van and because you chose to build something not normally available locally could have a very cold end result.
Buy normal gear of good quality and supported by local companies. You should have spares of odd ball stuff not normally available if you go that route.

Jumping out a "normal boiler" to run off protection devices is easy. A complicated factory boiler might not allow that simplicity.

I will qualify this by saying electrical is my professional specialty, not heat. I do a lot of heat, but not as a contractor.

Radiant requires the lower water temp, below the condensing temp. It also requires, or has a design philosophy of, always or mostly on with lower constant heat input closely matching the load.

To do this, the boiler of choice has a modulating fire and can turn down the firing rate in the range of 3 to 1. The boiler will be operating in the condensing range so has to be condensate rated. Boilers like this on the market, great quality state of the art, have ODR OAT reset built in, essentially for free.

A standard non condensing boiler with an on / off at 100% firing rate design, it does not match to the load. The boiler that matches the load reduces the complexity imo, and the standard boiler, could be made to work but that will certainly give you short cycling and greatly reduced efficiency.

It is necessary to select the best appropriate equipment, then design and install according to the manufacturer's instruction. The factory engineering, equipment, and support is where you want to spend the money wisely.

I would be looking at something like this or a similar competitor:

http://www.weil-mclain.com/products/wm97-ct-wall-mount-gas-boiler

 

[__dan]

open to recommendations. I probably would not rip and replace but wouldn't mind bouncing a few things around and having some predictable outcomes on potential modifications. The unpredictable ones have been amusing but I think I have had enough of them for now.

Probably best to start a new thread to discuss possible changes ...