Modcon with IHW Tweaking

[David1]

Lo Outdoor Temp 0 degrees
Hi Outdoor Temp 70 degrees
Lo Boiler Temp 110 degrees
Min Boiler Temp 150 degrees
Boost Time 1200 secs
Max Outdoor Temp 180 degrees

[Dana]

If you put a buffer tank on the boiler loop of the P/S system it'll contribute some standby loss when operating in water-heating-only mode. It's best to put it in series with either the supply or return manifold of the heating loops only. But in this climate the heating season is 8-9 months long, so the small efficiency hit on water-heating-only would only relevant in the summer. If it would shorten the plumbing to both the indirect & buffer by having the buffer in the boiler loop it's at worst a wash- the standby losses from the less-insulated plumbing can easily exceed standby losses through the walls of an insulated electric HW heater used as a buffer.

The "ideal" spot for the buffer would be to have the closely spaced Tees for the zone supplies right at the boiler-output connection to the tank, and the zone return tees connected close to the boiler return, essentially making the tank the point of hydraulic seperation, something close to this:



(You may find this a useful read: http://www.radiantandhydronics.com/A...00000000597932 and this http://www.radiantandhydronics.com/Articles/Glitch_and_Fix/BNP_GUID_9-5-2006_A_10000000000000201092)

But if it's just in series with the boiler output (or return) and the manifold to the zones the it'll still stop the short-cycling.

With fin-tube 110F would usually be too low an output for consistent results at low heat loads, but see how it does. It might be OK until dust-kittens build up in the convectors. Boiler output of 180F @ 0F is probably more than necessary for the actual (as opposed to calculated) loads with 125' of fin tube, but about right if your load is truly 70-75K as opposed to the built-25% margin of many calculations. If the 0F heat load it's actually closer to 55K (likely) you'd be able to keep up with a curve that crosses 160F or 165F @ 0F outdoor temp, and have more condensing time. Even when operating above condensing temps, you reap ~2-3% fuel savings for every 10F you can peel off the water temp. With a condensing burner those savings are something like 8% when dropping from 140F down to 130F though, and you get another hefty chunk dropping the output down to 120F, so setting the curve as low as it can be and still keep up at design conditions counts. Using the boiler to measure the heat load is far more accurate than any Manual-J or IBR type calculation.

[Tom Sawyer]

All of the delta T problems can be solved with a VFD circulator, say a Wilo

[mage182]

Those Wilo units look pretty cool. I have an HVAC guy coming over this week to tweak my system. I'd rather lay out the cash for these than try to put in a buffer tank. There really isn't room on the return side for me to pipe anything like that. If the VFDs will fix the deltaT/condensing problems, hopefully they will be a direct swap for the Tacos I installed.

[David1]

Those Wilo units look pretty cool. I have an HVAC guy coming over this week to tweak my system. I'd rather lay out the cash for these than try to put in a buffer tank. There really isn't room on the return side for me to pipe anything like that. If the VFDs will fix the deltaT/condensing problems, hopefully they will be a direct swap for the Tacos I installed.

Mage182,

Please keep us posted on your freq pumps, it will be interesting to see how you make out. Thanks,

[Dana]

Those Wilo units look pretty cool. I have an HVAC guy coming over this week to tweak my system. I'd rather lay out the cash for these than try to put in a buffer tank. There really isn't room on the return side for me to pipe anything like that. If the VFDs will fix the deltaT/condensing problems, hopefully they will be a direct swap for the Tacos I installed.

No pump is going to add thermal mass to your system, but ECM drive pumps with programmable features can make it easier to tweak things toward min-modulation. Still...

"In most buildings, according to hydronic heating expert Henry Gifford, the level of savings we will achieve with the high-tech (expensive) variable-speed pump can be more cheaply achieved simply by right-sizing a constant-speed circulator. The reality, however, is that most heating contractors don't do actual calculations for pumps (most probably don't even know how to do those calculations), preferring to cover themselves by significantly oversizing the pumps. For this reason, this new generation of advanced smart pump makes a lot of sense."

http://www.buildinggreen.com/live/index.cfm/2010/8/12/Grundfos-and-Wilo-VariableSpeed-ECM-Circulator-Pumps-for-Hydronic-Heating

A ball valve approach achieves a similar end by addding head to the loop, lowering the flow rate. Variable rate ECM pumps will save a on electricity over a ball-valve approach, and may be cost-effective in constant flow applications, but not necessarily with an oversized boiler's intermittent cycling, since the duty cycle is already low. The goal here is to stop the short cycling while running it in condensing mode as much as possible. $300 smart-pumps will only get you part of the way there- it won't solve the more fundament design issue which is the imbalance between radation & boiler output that occurs at temps below the crossover where the radation output is equal to min-output of the boiler. It can't help the boiler make longer burns when operating in outdoor reset mode when the boiler output temp is below that critical crossover point, but a buffer tank will. It'll do the right thing during intermittent zone calls on the boiler loop of a primary/secondary though, as long as the radiation can deliver the boiler output at the temps delivered. The real problem remains: we're over-boilered/under-radiated for low temp operation and for the size of the load. If there's too low a delta-T at low temp operation isn't likely from being over-pumped as much as from being under-radiated. It might be the right thing to do if pumping direct (not primary/secondary) using zone valves and 1 pump rather than the 3-pump P/S system David1 has set up. (You'd have to pay close attention to the min-flow rates specs on the boiler to be sure- this isn't a no-brainer to design, but it's not rocket science either.)

[jadnashua]

How about adding a loop or two to keep the snow/ice off the sidewalk/driveway? If you consider the cost of a snow blower and the aggravation of slipping and sliding around, it may be a useful addition.

[tk03]

The Alpine requires p/s piping. The idea of most mod/cons is to keep the flow rate up when the boiler modulates back. If the flow in the boiler slows it will kinda defeat the purpose. When modulating keep the flow rate up and reduce the delta-T which extends the run times.
Variable speed pumps on the system will help widen the delta-T so cooler water comes back to the boiler. But you will still heat it too fast. When boilers are over sized the boiler flow rate is more then the system flow rate, so you will have hot supply water reverse flowing between the tees and preheating the water returning back to the boiler.
Over sized boilers are definitely over sized boilers. As stated earlier the ratio of input to water volume/connected load causes the short cycling.
I hate to sound like a broken record but keep turning it down and change the modulation rate to #5 or add a buffer tank.

[Dana]

Broken records are good, when they're on the right track. (Thanks for the clarification of why all Alpines need to be set up P/S. I've seen several Contenders and Solo designs set up to pump direct thought, but they were DESIGNED, and not oversized for the radiation.)

Reducing the modulation to the minimum it would still take 165-170F water to not cycle on a single 60-65foot fin-tube zone like David1s to run with the radiation perfectly balanced to the boiler output. At 140F output (the beginning of condensing) a single zone can only deliver ~2/3 of the min-mod output. Whether there's enough hysteresis in the boiler's controls to make those cycles sufficiently long is still an open question-more mass may still be the ultimate answer. Alternatively , baseboard is cheap, and if there is sufficient wall length in each room to install enough additional radiation/baseboard that it can deliver design-condition heat at ~140F boiler output you should be able to tweak it to always run in condensing mode, without cycling issues.

[mage182]

Another thought. Would consolidating the 3 zones I have into 2 make things better? I have the two bedrooms on the first floor on one zone, The living room, kitchen, bathroom on another, and the upstairs on a third. Would combining the two zones on the first floor into one help to deliver more heat? It's all piped with pex in the basement so making a few cuts to combine the two zones isn't a big deal. The only loss there is the money for one circ pump that won't be used.

[jadnashua]

Probably won't make much of any difference...odds are that those two zones have some current overlap, and forcing them to both come on at the same time (you could do this by controlling both circulators from the same control without cutting or repiping anything) wouldn't make much of any change in the heat load. And, you'd lose the flexibility of separate temps in the individual zones. If you can't demodulate the burner low enough, your best bet is a buffer or increase the total load.

[mage182]

I'm back to this topic after taking care of some other things. I'm looking into buffer tanks and getting one installed before the heating season in the fall starts.

Based on the article above I have the following questions

Article:
http://www.radiantandhydronics.com/A...00000000201092

On my boiler, P1, P2, and the ODR are controlled by the boiler. The zones are controlled by the Taco box. How does the boiler know to turn on P3 and not P2, vice verse, or both when a zone calls for heat? Currently I only have P2 for heating, so that turns on whenever there is a call for heat. With the buffer tank, something will have to make a decision on what combination of P2 and P3 needs to be turned on. The schematic is excellent, it's the wiring/control side of it that I'm trying to understand.

[BadgerBoilerMN]

I like it better when you talk about insulation .

There is not enough information here to respond decisively, it is why they make field technicians.

Over sizing any boiler (either for the total design load or for the available insulation) will assure bad performance, raising operating and maintenance costs while lowering potential comfort.

When you oversize a boiler (on purpose or from shear ignorance) it must cycle more often with less efficiency (think city vs. highway miles). When you oversize a condensing boiler that modulates input, "Over sizing isn't usually as big an issue with mod-cons" you may not only suffer short cycling as a result of not being able to transfer the heat through available radiation, but the boiler may also bump off the bottom. Since it is oversized for the maximum load it is often oversized for the smallest load as well.

If may be that the boiler is over-fired. I wonder if the output has been checked.

While your at it a combustion analysis would be in order as it is the only way to determine if you have the boiler "tweaked" correctly. Burnham calls in "fine-tuning".

When you heat domestic hot water, using a ModCon and indirect-fired water heater, the load changes with the potable water temperature and the boiler should modulate downward as the boiler and WH temperature rise. If the controls are not set up correctly, this may not happen. If the boiler is oversized, this may not happen. If the indirect pump is undersized, this may not happen. If the pipe between boiler and indirect water heater is undersized...well you get the picture.

As to DHW priority; it is standard and will not affect the heating mode or even be perceptible unless setup incorrectly. The maximum DHW run time is adjustable on most ModCon boilers.

It is possible to de-rate some ModCon boilers - including the new SAGE controlled Alpine - but the minimum output will stay the same. Ideally a heating boiler should be size to operate between the smallest and largest load conditions, which in turn depend on construction, location and zone loads.

The best time to find a competent hydronic contractor is before you start. It is accomplished by calling the local contractors and interviewing them specifically on the subject of high efficiency (condensing) boilers and more specifically on the computer program they use to determine the output of the boiler you require.

The experienced designer has data for all of North America is able to read and follow the manufacturer's recommendations. The local installer however, must consider his local support. When I specify a boiler, I first look to the local support system for the boiler I want to use.

The specification, installation and maintenance of ModCon boilers is not a DIY project.

[mage182]

After firing the first two contractors I hired to install my ModCon (the first because he started and then wanted me to wait weeks for him to come finish the job, the second because I didn't like the quality of work he did or his attention to detail), I finished it myself. I know the unit it oversized because I plan on building a large extension in the next few years adding two more heating zones and another bathroom.

My further efforts to locate another contractor that has specific knowledge on how to tune and tweak a Burnham Alpine so that I know it is running at optimal efficiency and provide guidance on whether or not I need a buffer tank to achieve this in the interim have come up empty.

So I have come here to query information from whoever may have some words of wisdom. Your comments on when to find a contractor, design, and implementation methods are all valid, but this isn't the case. I have an installed and fully functioning boiler that I suspect is oversized and a buffer tank has so far been the best route to remedy that. However, All I have so far is the diagram posted above. So I need to know how to properly size the tank, and how to wire it correctly as the article posted above doesn't cover that aspect. Any information needed I would be happy to provide.

[BadgerBoilerMN]

From the information provided, the buffer tank would have a marginal affect. I would live with your 6 minute cycles (not short for a ModCon) and hurry with the rest of the system, remembering that the more radiation you have the more your boiler will condense. Naturally the radiation has to be balanced to the room and the load. Small rooms with large loads e.g. sun room, present special challenges that may be resolved by the use of a buffer tank, but can usually be addressed by much less expensive control strategies.

A good designer can head off many of the headaches you are suffering.

Every good hydronic system starts with a proper heat load analysis performed by an experienced designer on dedicated software.