hydronic piping design using condensing boilers

Jim_88

Seems like a simple question: What is the most appropriate piping design when using a condensing boiler, without a domestic hot water loop (can't pay for that right now), and you want, say, 6-8 individually-controlled zones (using a lower supply water temp w/ radiant panel baseboard)? These boiler manufacturers' product liturature is not enough. Are primary-secondary loops necessary? Could it be as simple as an 8-zone manifold w/zone valves? If so, why do all these condensing boilers use things like 3-way valves, etc.? I was thinking of using the Viessmann but it seems it doesn't allow for this simple piping configuration.

Brad White_200

Some Basic Principles

Most but not all condensing boilers need primary/secondary piping for the simple reason of low water content.

If the boiler is not guaranteed a certain minimum constant flow, the water will flash into steam.

Now, a simple system, directly coupled to such a boiler can and work well, provided:

1) The residual circulator head available outside of the boiler is at or below the radiation system pressure drop. (If you are feeding an older, large-pipe system, chances are you will be fine, for example. If serving radiant floors with small tubing, you will need a secondary circuit.)

2) There are few if any control valves and no bypass. If too many valves close and the water flow is reduces, flashing to steam happens if not nuisance shut downs on high temperature.

3) Recognize the down-side of bypassed flow: The un-needed hot water flow goes back to the return, raising the temperature, dropping efficiency and short-cycling the boiler.

Now, micro-zoning is what I call what you are planning. Six to eight zones is a lot. If one zone calls, even on the coldest day, that may not be enough to keep the boiler operating for a minute. The boiler wants to see the full load for best efficiency.

You might consider a buffer tank in your scheme, to "Bank" excess capacity between longer cycles. Either that or you may consider simplifying your zoning strategy.

Most homes are more open in plan than you think and a zone per floor is more than enough. One large zone, using thermostatic radiator valves, is a very economical and effective way to get individual control and higher efficiency.

Your question RE: 3-way valves? They are used to mix-down a lower temperature when for example the boiler temperature is required to be higher to serve baseboard or coils, but lower temperature is needed for panel radiators or radiant floors. There are a lot of applications, that is but one.

My $0.02

Brad

Uni R_2

What Brad said...

"Most homes are more open in plan than you think and a zone per floor is more than enough. One large zone, using thermostatic radiator valves, is a very economical and effective way to get individual control and higher efficiency."

But before all that, what's your measured heatloss?

Mike T., Swampeast MO

First, you want to use only a two-pipe system for the radiation. One-pipe systems (very frequently used with baseboard) work on a diminishing supply temperature principle that will reduce efficiency with a condensing/modulating boiler.

Second, you want to have as much radiation and budget and space will allow. More radiation = lower supply temperature = higher efficiency. If you're using Manual-J to calculate heat requirement, selecting radiation based on 160F AWT will likely ensure that you won't need more than 140F AWT except in extreme circumstances. Just ensure that ALL radiation is sized similarly!

Third, avoid creating multiple-temperature systems. This usually means using the same form of radiator (sized to the same AWT at design of course) throughout. If using panel radiators avoid mixing "convection enhanced" models with others--even if from the same mfgr. If you must mix, carefully compare the output ratings at lower say 100F or so temps. Ideally the output of each radiator will drop off at the very same rate as AWT temperature falls and in general, the greater the proportion of convection of a radiator the more rapidly its output drops with falling temp. If you must mix and find a significant difference, I would suggest upsizing the convection enhanced models to get the low temp output of all as close as possible.

Fourth, give UTMOST consideration to proportional flow control as opposed to using on-off zones with valves or circulators. With proportional control you (in most cases) wind up with only a single "zone" as far as the boiler is concerned. USERS however will "think" that each an every room is independently zoned as you have quite versatile, independent temperature control for each radiator. Non-electric zone valves can definitely be used but be aware that the operator in each room connects to the control head with a capilliary tube (not a wire) so you have to plan ahead during construction to keep the pain of running them to a minimum. You can run them in electrical conduit for protection if desired and if bend or use only "long sweep" elbows you can actually suck them through a surprisingly complicated in-place system using a shop vac (think drive-in bank machines).

Fifth, do you best to ensure that secondary (emitter) flow does not exceed primary (boiler) flow by more than 20%. With a properly sized boiler and well designed emitters this won't be a problem, but verify! Significantly higher secondary flows reduce the efficiency of the condensing/modulating boiler by requiring it to produce higher temperatures just to get the "lower" temp for the secondary. This problem is at its worst when hydro-air is used and instead of increasing the size of the HX, you just increase the flow rate...

Sixth, use a low-loss header (or "hydro separator") instead of primary-secondary piping. Such costs a bit more, but low-loss headers utterly guarantee that return temperature is as low as possible. Primary-secondary is really about keeping return temperature up--the exact opposite of what you want to do with a condensing/modulating boiler.

Seventh, if you want to avoid the use of any form of primary/secondary you must design VERY carefully. While rarely mentioned in U.S. mod-con literature, primary/secondary is not a true requirement. You will NOT however be able to do this with on-off zone control and personally I would consider TRVs or other form of proportional flow control to be a must if you want to avoid. In general you'll need to keep head losses VERY low. If this is new, well-insulated construction in a not-too-harsh climate with reasonably generous radiator sizing, it's not that hard. Unless runs are unusually long, 1/2 branch piping [should] be able to supply most rooms with almost no head loss (again using proportional flow control). The Vitodens 200 (especially the 6-24) model is well suited to such a system and will give the utmost in efficiency. With its much lower than average head loss in the HX, the Triange Prestige is also well-suited.

Eighth, use TRUE constant circulation. Again, this is only really practical if proportional flow control is used. True constant circulation means that anytime the outside temperature is low enough to require heat in the house you are circulating water heated to the lowest possible temperature to maintain the desired comfort level. In other words, you want a CONTINUOUS call for heat. The control brains built into mod-cons vary considerably in this ability--the LAST thing you want is an non-defeatable "ramp up" that continues to increase supply temperature the longer the duration of a call for heat.

Ninth, use setback intelligently and in moderation. Set back unusued spaces as deeply as you want. Use no more than one setback event in a single day and even then keep such to a minimum. In general, the better your insulation and weatherization the less setback. Some here (including me) have found that daily setback with a mod-con and reasonable insulation does not save fuel and actually decreases comfort. Even though my TRVs have a built-in setback ability, I prefer not to use on a daily basis. I find it more comfortable and economical to set a reset curve that allows me to barely maintain the highest desired toom temp in the house and then adjust TRVs DOWN to a level most suited to the general activity in the room. Warmest in baths; next warmest in public rooms and my daytime home office; cooler in sleeping rooms; and "cold" in unused spaces.

Tenth, AND MOST IMPORTANTLY, DO NOT OVERSIZE A CONDENSING/MODULATING BOILER! "Standard" (like Manual-J) heat loss calculations produce VERY safe--even overstated--numbers. My own boiler (a Vitodens 200 6-24) is technically quite undersized (input rating about 10mbh below Manual-J "loss") for my seemingly interminable home renovation. Even with the ground floor in bare studs and no weatherization of the ground floor windows (not even the stop molding) I was able to RAISE temp to 72F from an average of about 62F at design weather conditions in about 5 hours. In "typical" nighttime conditions I have calculated system efficiency above 99%

tim smith

Mike, well written

I usually size to >160 on my radiators, in Seattle that keeps my operating temps below condensing almost 90+% of the time. I have not gotten into thermostatic valves much yet in our installs but we do alot of work on them in service. I like the idea but the looks of the heads would be a little put offish to the designers on our projects. I am seriously looking at going back to zone valves and eco pumps which we use to do many years ago. I just hope I can find a zone valve I am happy with now, past types were a bit problematic for service. Tim

tim smith

double post dangit