Buffer tank control for efficiency

rick in Alaska

I have a customer who is sending a heating package out to the interior, and I need to put this together to be able to send. I have figured the usual things, but am stuck on the best way to utilize a buffer tank for overall best efficiency. I am thinking of using a Tekmar 256 control to keep the tank at temperature, and a mixing valve on the outlet to go to the radiant system. My brain is really tired right now and I am having trouble with this.
The boiler is a lower mass oil boiler, but rated at 140,000 btu input. The boiler has to be this one. The radiant is 4 zones, and one can be as low as 2000 btu's, so the need for the buffer. I am concerned about having the tank at 140 degrees or so in order to keep the boiler from condensing, but I don't want it to have to be at that temperature when the outside temperature rises. I would love to be able to just keep the tank at whatever temperature the heat needs, but I don't see how I can do it without the boiler condensing.
Any thoughts are much appreciated at this time.
Rick

hot_rod

That is a challenge when you have a boiler with a return temperature protection need.

To best use of the tank capacity you would run it to say 180F, and draw down to the lowest useable temperature via the tekmar control. Under some conditions you may see a 100° delta!

Or run the tank with the boiler on it's ODR control, boiler and tank are always the same temperature. You lose some of the buffer capacity, but store temperature in the buffer at less standby loss. So the tank runs at the highest SWT on design, modulates down. This will cause more boiler cycling.

If the boiler has a min temperature requirement, add a return mixing valve, that way the boiler is always protected, regardless of what the load on it is.

What type of tank? If you can use a two pipe connection.

rick in Alaska

The tank is just a 30 gallon electric water heater tank. I had planned on using the side taps for the input, and taking the heat off the top fittings, but can go any direction if needed.
I need to get hold of the manufacturer of the tank and find out how low it can go for temperature.. It is a small stainless tank, and I know the cousin of it can go down to 110 output, but am not sure how it would stop from condensing. There are no condensate drain provisions for it.
Rick

Jamie Hall

Electric? Condensation in or on the water heater isn't a worry. It's fuel burning equipment that one has to worry about condensation with. The way to avoid that is to make sure that when the boiler fires to bring the tank up to temperature it gets hot enough to dry the boiler and the stack out -- and 180 should do that.

rick in Alaska

I just talked to the rep and he says that unit can run at 120 degrees. I also just talked to the homeowner/builder and he says the radiant is going to be a staple up/plate system. Information keeps changing on me. I still can't get a heat loss, so am going to have to wing it. I am thinking I can most likely run the buffer tank at 130 with a delta at 20 degrees and make it work, but some more info from him will let me know, I think.
Rick

rick in Alaska

Jamie Hall said:

Electric? Condensation in or on the water heater isn't a worry. It's fuel burning equipment that one has to worry about condensation with. The way to avoid that is to make sure that when the boiler fires to bring the tank up to temperature it gets hot enough to dry the boiler and the stack out -- and 180 should do that.

The buffer tank is the electric one. The boiler is feeding in to it, and my goal is to be able to keep the boiler, and the buffer tank, as low a temperature as possible. With the new information I got on the way the tubing is going in, and the fact the boiler can go down as low as 120 degrees, I am now thinking to just keep the tank around 130 degrees and not even use a mixing valve on the outlet of the buffer. But I might be missing something here.
Rick

hot_rod

Does the tank have 1" element on the side/ two of them?

Here is a 20 gallon electric tank I have used for a buffer. It has 3/4" side taps. So to increase to 1-1/4 I use a 3/4 male x 1-1/4" copper adapter to jump the size up.

Or a threaded version like the bottom with a 3/4 close nipple then bell up.

once you get to 1/14" connect both the boiler and load into that 1-1/4. this really maximizes the tank use.

the boiler can go directly to the load, or a portion tom the load and rets to the tank, or 100% to the tank when loads are satisfies.

Basically you are building a mini hydraulic separator in that increased pipe connection.

The brass nipple is a swage nipple, came out of an old Munchkin. that is how they increase the 1" header on the larger BTU models

hot_rod

here is what the piping looks like. Use the top connection on the tank for a good air vent, maybe a temperature pressure gauge in the other hole.

rick in Alaska

Thanks Hot Rod. The tank has two heating elements in it, so are piped with 1" threads, so I was going to use 1 1/4 x 1 reducers going to a 1" mip in to the tank.
Rick

HVACNUT

P/S off the boiler with injection into the buffer tank?

Or how about a plate HX?

Edit: Would that cause short cycling of the boiler? Why low mass on the boiler?

A 180° limit on the boiler with a 45° differential, low firing rate if the manufacturer allows.
I'm no designer, just throwing darts.

rick in Alaska

I don't think the boiler stat can be changed, but I can check in to it also.
The boiler holds something like 4 or 5 gallons, I think.
Rick

HVACNUT

> @rick in Alaska said:
> I don't think the boiler stat can be changed, but I can check in to it also.
> The boiler holds something like 4 or 5 gallons, I think.
> Rick


Didn't realize you were dealing with an existing boiler.