Buffer tank for radiant system Steve M

Earthfire

We installed a Laars 500,000 btu heater in a concrete batch plant about 5 years ago and that little thing Makes between 1,000 to 1,200 gallons of hot water per hour. 50 deg F wellwater to 120 deg F storage temp.in 14 hr. shifts. Uses a 1,000 gal. storage tank, a poolpump for a circulator( continuos).The storage tank is also the heat source for the 40 x 40 storage are that it sits in. temp stays around 50 on cold days with two mandoors open on opposite walls for most of the work day. If memory serves Laars installation manual specified a a 20 deg. temp. rise between in and out temps on the heater. Those things are capable of making a lot of hot water. But they need the gallons at the correct temp.rise to operate efficently.

Steve M

Buffering system

I need some help here, hopefully some of you eastern time zone folks are home and surfing already. I got a call from a homeowner complaining about his 6th power venter motor failure in 6 years. I went to his house to check out the situation. This is what I discovered.
6000 square foot house at 7800 ASL, natural gas for fuel,
stapled up Entran on upper floors,
Entran poured in concrete in basement,
extremely well built home, well insulated. Approx 6 years old.
It has 12 heating zones, plus domestic hot water, 80 gallon Amtrol unit with a
400,000 btuh Laars Mighty Therm boiler.
Field Controls power venter vented out the side of boiler room. The boiler (and power venter) short cycle so bad on this system that I lost count how many times it fired while I was there. It appears that the contractor zoned every room and some of the zones are very short. The power venter cycling on and off is obviously the motor replacement problem. It's also the least of his problems. So, I have two suggestions for this poor guy. I need help with one of them. One consideration would be to install a buffering tank on this monster to give it something to heat. I've heard about doing this, I've read about it in "Hydronic Radiant Heating" using an Ergomax. The Amtrol is brand new, the first one leaked under warranty, so I'd like to keep it in there. Here's what I need help with:
Tank sizing.
Piping options.
Additional controls.
The zones are controlled by Honeywell V8043E1012 zone valves and T822D t-stats. This homeowner is at his wits end and is looking to fix this dog once and for-all. Any input is greatly appreciated.

heatboy

Forget the buffer tank and........

install a properly sized boiler. Then think about a buffer tank.

Warm Regards,

hb

climatecadvanced.com

"Expert in Silent Warmth" ™

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Jack_23

It wasn't right at the start!

What is the building heat loss. At 6,000sq ft, very well built, zoned up and down. 400,000btu ???
My assumption is that the boiler is multiples oversized or they have a heck of a hot water load. Going to a buffer tank can help with run times but system inefficieny is still huge. No matter what you try to do, you are still putting money into something that wasn't quite there at the start.
So here's what I'd do (and yes, I know I'm spending some of your customers dough). Guys back here (N.east) are having a ball with the munchkins. Sealed combustion, modulating, so the run times are good, hi eff. Don't know about hi elevation firing on them though.
Group your zones based upon common space, use pattern, etc., again to help run times.

On the DHW side, you can keep your Amtrol, but (this is self serving because I rep them here...but I believe in them)I'd size the boiler to the heat loss, ditch the indirect and put in Rinnai Continuums to match hot water load/demand.

When I was in the contracting business and I tried to "save" a system that was someone elses problem, my save attempt just let me inherit their problem.

Tom Meyer

Agreed

Recompute the heat loss. That's a pretty big pig to have it's mouth open every time a short zone goes into demand. And it short cycles because it doesn't take long for it to reach temp. (Like using a flame thrower to light a cigar.)

As long as we're dreaming with the owner's money, why not put in a multiple boiler system with small loadings for each boiler? You can use a DHW priority.

Tom Meyer
Senior Designer/Trainer
Precision Hydronics
www.precisionhydronics.com

Mark Hunt

Is there


any snow melt on this system?

400,000 BTU's?!?!?!?!?!?

If that is the case, put a smaller boiler in for the heating system and let the snow melt have the 400,000 btu's to itself.

Mark H

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Steve M

Then I'm on the right track

Because my first suggestion to this nice person was to sawzall the monster out and stage a couple of smaller boilers, do a true primary/secondary piping arrangement. He seized up at the idea. (I haven't even found a contractor yet to price the whole mess. He just knows it's going to hurt.)The buffering tank idea came up to help with cost control. It's piped as per the Laars piping diagram that they publish in the IOM with a pump on the boiler and then a separate pump for the system. I took temps with two zones calling, water out was 160 water back was 155(!) The poor thing comes on, sighs, and then goes off again. The power venter is terminated outside this poor souls bedroom too. Planning SNAFU between the contractor and GC from day one. I'm not sure where the heatloss came from, the homeowner doesn't recall either. One bright spot I guess is that the house is very comfy with 70 degree room temps throughout. The day I was there the outside temp was 15 degrees with a bracing Colorado breeze.

Chris Maderia

Boiler is Oversized BY

a ton. A 6,000 sqft house heated radiantly has a heat loss of somewhere between 135,000 and 150,000 btu's. There are 2 approaches to resolve this, it all depends on what price he's willing to pay.

Approach #1 would be to change out the boiler. Do a new "Radiant Heat Loss" you will find that you are not going to need 2 Boilers. I will be shocked if you do.

How is the radiant system mixed? What Water Temps are required for each zone? Sounds like the original heat loss was wrong from the get-go so I would tend to believe that after doing a "Radiant Heat Loss" you will find that you have to high water temps running through your loops. Can you post a piping diagram?

Approach #2 - Let's ask a question first. If I have a 135,000 Btu boiler based on my heat loss and the home is heated totally with radiant with system outdoor reset, wouldn't my boiler be oversized 95 percent of the time? How do we overcome this? It's all in the piping.

Steve Ebels

Where's the load

To keep that boiler running?? Is there a huge demand for domestic hot water? Like a 200 gallon jetted tub or one of those human car wash showers? 400 K btu's into 6000 sq ft gets you a heat loss of around 66 btu's per sq ft of heated area. That's nuts!!! The heat loss of that place likely averages 20 btu/sq ft or less.

Do him a favor and axe the boiler out of there. It's going to live a short life running like that anyhow. By not replacing it he's just postponing the inevitable. A Munchkin or better yet, A Vitodens, yeah that's the one, will solve his problem and be absolutely silent. He'll never know if it's running or not.

The buffer tank idea might work if done right. You would have to isolate the buffer from the system when it's producing domestic hot water so the indirect can ramp up to temp as fast as possible. An either/or type relay and a zone valve would be needed or something like that.

hot_rod

Short of

using a modulating boiler, or boilers, it's hard to match the heat source output to the building load on a continous basis. I agree the DHW load, when using an indirect, is often the biggest load. It is easy to priortize this load. Crown makes it easy with their "on board" on their sealed combustion CABO model.

What the buffer will do is be a good resevior for storing BTUs until they are needed. Much better than the boiler, due to the lacket and flue loss.

Here is a job with a 125,000 Crown. The heat load is 60,000, but I wanted the recovery for the indirect so I sized the boiler at 125,000. A 20 gallon buffer give a bit capacity for low load conditions. From a cold start @45 degrees, I had a 19 minute boiler run till the tank reached 180.

A setpoint control with a 40 degree delta t will run the boiler 140-180. The aquastat in the boiler will act as the high limit, not the control.

There are a number of ways to "help" that system. Is it possible the boiler may have been oversized for future loads? Hate to throw somebody "under the bus" without knowing all the facts

hot rod

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Steve M

Thanks for the replies

I guess the responses are what I expected. Remember though, we live in the mountains. A 400,000 btuh boiler NETS about 217,600 at 7,800 feet above sea level. It's one of the quirks we struggle with here. Our gas is cold too, some areas it's under 700 btu's per cubic foot. Mark Eatherton posted a de-ration formula here (back when the Wall was first built) and I use it for every job I design. So, given the altitude the house is about 36 btuh per square foot which is still way too much. I would guess the heat loss was done with a baseboard program or worse yet, square footed with the ol' 20% for good measure thrown in. As for the piping, I guess the contractor was going for a "primary/secondary" look. It's close but not true P/S. The staple up is running right off the hi-limit at 160, the concrete is piped through a mixing valve. Oh yeah, the pump on the mixing valve was wired to the boiler relay so when anything is calling for heat, it comes on. If the concrete zones are not calling, dead-head city. I will go back with the trusty lap-top, my Kitec program, and a reliable contractor (TD, you listening??) and design a proper system. As for my poor homeowner, it's only money, he'll make more.

hot_rod

Derate

First take the 400,000 input and divide by the boiler efficiency for an output of 348,000. That generally is on the boiler tag as output. Then take your altitude hit at 7800 altiyude take a .69 multiplier for an adjusted output of 240,120. Gas value in your area may be a bit different in Utah at 7800 Mountain fuel calls it 820 btu/ cu ft.

Your local gas supplier should have one of these handbooks available for you. Their servicemen and meter setters generally have them in the truck, ask em for one.

My experience with some homes in the mountains with large glass great rooms 36 btu/ sq. ft. is not uncommon.

Generally those large copper boilers had either step firing, mech modulation, or at least two stage gas valves.

Can't say about the piping, without seeing it but I'll bet you can improve this system in the boiler room.

I'd still keep an open mind about a buffer tank if you have some space. An 80 gallon insulated tank would do wonders for that short cycling and micro loading. Those issues won't go away with merely a piping fix, in my experience. That boiler will tend to misbehave with a 20,000 btu load calling

hot rod

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Steve M

Thanks, HR

Us mountain dwellers have some weird situations to put up with, for sure. I'll propose both options, talk to a contractor, price up the goodies, and let my homeowner know all the facts.