Help spec boiler and heat

Mike T., Swampeast MO

I didn't answer your question about a "constant call for heat" completely. Sorry.

Because TRVs are proportional and modulating devices they demand the constant circulation of water <B>(and constant supply of heat)</B> whenever space heating is required. This allows them to do their job in the most efficient and comfortable way as they'll regulate the flow through the radiator so that it is <I>just providing</I> the minimum amount of heat to maintain the conditions you desire.

If the temperature available to any given valve is higher than needed to maintain your desired room temperature setting, the valves automatically close <I>to whatever degree is needed</I> to just maintain your setting. This is called proportional flow control.

Systems using traditional wall thermostats cannot do this as they cannot regulate flow through (and thus the output of) each radiator. Sometimes the thermostats stop both the flow of water and the heat from the boiler. Other times they only stop heat from the boiler. In a multi-zoned system where <I>other</I> thermostats can also call for heat, it is difficult to stop heat without also stopping flow to a zone that no longer needs heat--thus what you've heard about "true" constant circulation "complicating" the system. "True" constant circulation is inherent to TRVd systems--it's only approximated with others.

To understand how the boiler works with a TRVd system, it's far easier to think in terms of outdoor reset. A reset "curve" is established based on slope (how much one value changes with another) and shift (how much one value is always moved). The boiler measures the outside temperature, applies the slope and shift values and produces a "target". This "target" is the temperature that the boiler trys to produce by modulating its burner. It attempts to produce this temperature as accurately and consistently as possible until outdoor temperature changes thus requiring a change in the target. The TRVs will modulate to achieve their setpoint as long as this "target" is somewhat higher than actually required.

This is where the extreme efficiency of such a system can be maximized. If the target is just barely adequate to maintain conditions, the temperatures throughout the system will be at their lowest possible values. This means highest possible efficiency. It also means that the sensors that operate the TRVs are best able to respond to other sources of heat--the sun, appliances, lots of people, etc. The cost of this extreme efficiency however comes in response time if you want to raise temperature. If such is needed on a regular (daily) basis, you add shift to your reset curve thus making higher temperatures available at all times. Some boilers have a front-panel dial for adjusting shift--with others you have to enter programming of the boiler. My personal opinion and experience is that daily temperature changes are completely unnecessary (from either a comfort or efficiency standpoint). Regardless, my real point is that the user is in control of both their comfort and efficiency. If you accustom yourself to <I>maintaining</I> the lowest possible temperature that you deem comfortable and adjust the curve to just barely maintain this you will achieve amazing fuel savings.

For the basement, the mixing valve will introduce some cooler return water into the supply stream thus reducing its temperature. If this is done on a separate heating curve, it is quite likely that you can achieve constant circulation there as well--there will be a thermostat but if you set the thermostat too high and then adjust the curve downwards to best maintain your actual desire you will likely find that the thermostat is virtually unnecessary. This will be especially true in a basement with tube-in-slab as the entire system already "wants" to maintain itself due to its own mass and minimal exposure to weather.

What I have described here is a very simple but also very effective way to control a system of the sort you describe--provided of course that you use TRVs. It <I>is</I> different than typical U.S. custom however. This particularly true with how the USER is put in full control of both comfort <I>and</I> efficiency. I use a similar system and it has taken me a couple of years to fully appreciate just how much efficiency is possible. If you <I>really</I> want to save energy and sacrifice some comfort such is certainly possible. With my previous conventional boiler any sacrifices in comfort did not pay back well with regards to fuel consumption. The opposite is now true! Regardless, you can find your own balance between comfort, versatility and efficiency.

I will say that the boiler control philosophy I've described is that used by the Vitodens by Viessmann. It also provides convenient and simple access to the heating curves, but it does take some observation over time to adjust for highest efficiency (e.g. barely adequate heating curves.) TRVs are required by law in Germany (where it comes from) and also much of Europe.

The Buderus GB uses a different control system. There is an indoor "temperature controller" (I can't bring myself to call it a thermostat) that automatically adjusts the heating curve.

While <B>all</B> condensing/modulating boilers have their roots in Europe (and many of their major components come from their as well), many, including the Weil-McLain Ultra have somewhat "Americanized" control systems. The Ultra and others offer a "boost" in target. This "boost" occurs in (usually adjustable and usually repeating) increments after a (usually adjustable) period of time if the heat call (from a traditional thermostat) is not satisfied. In other words, the longer a heat call lasts, the higher the temperature the boiler produces above target. This is completely opposite to the control philosophy of a TRVd system where you want a <I>constant</I> call for heat. This mode of operation may or may not operate by default, but all that I've seen that offer this mode also allow it to be turned off--then the boiler will attempt to most accurately produce the original target for as long as the heat call continues.

I sincerely believe systems similar to this are the direction of American hydronics. Such allows a simple and uncluttered boiler room, unparalleled comfort and exceptional efficiency.

TRVs have been around for many decades and enjoy an enviable record of reliability (think "indefinite").

Gene Davis_2

Boiler type and controls?

1650 sf house, 1200 sf footprint with half of mainfloor being one huge greatroom with a tall wall of window glass facing south. Large room has 550 sf of available floor area and 320 sf of glass on large wall, plus two other outside walls with some reasonable window area in each.

8255 heating degree days, 15 below zero winter design drybulb temp.

Want to heat the place with hot water. PEX in slab in unfinished basement, radiators all in above-basement living areas.

Likely to need as much as 32500 BTU/h in greatroom, other rooms all much smaller, with 8/0 ceiling heights and small windows.

Is there a type of boiler that can handle all this, and still vent out with 3" PVC?

How many different resets from outside thermometer will we need?

Mike T., Swampeast MO

Appropriate condensing/modulating boilers should be available from any manufacturer. Depending on your climate and the construction of the home, your real problem may be finding one small enough...

Different manufacturers of condensing/modulating boilers use different venting systems depending on the application--some are plain PVC, some are metal (usually stainless steel), some are proprietary. 3" PVC is rather common however...

As to "how many different resets":

There is only ONE reset. It has to provide temperature sufficient to supply the portion of the system with the HIGHEST heat requirement. Lower temperatures are achieved through mixing cooler return water into the supply stream to in turn produce cooler supply temperatures. Such does resemble reset, but it is mixing. The most sophisticated boilers and control systems allow this mixing to occur on a curve related to outside or inside temperature.

From what you stated you should need two temperatures--one for the radiators and one for the basement slab. The basement slab will be the one with the lower temperature requirement. The more closely you size the radiators to utilize temperatures similar to that of the slab, the more efficiency you will get from your condensing/modulating boiler. In other words the larger your radiators, the better. There is however CERTAINLY a point of diminishing returns. As long as the radiators are sized to provide design (e.g. worst case) output with 140F or lower supply temperature you're in the very high efficiency zone...

Gene Davis_2

OK, so let's say we use a small Weil McLain Ultra wall-hung, and Myson panel radiators with 140 degree F water input to the rads.

We'll need to mix down for the basement floor PEX tubing, but otherwise we'll have the same brew going to all the upstairs radiators.

Can you describe a system of control for this?

On the main floor I've got the greatroom with its own zone, a hall, laundry, and two baths on a second, then a bath on a third. Upstairs, I was thinking the bedroom would be a zone, and its bath another. Can radiator systems have multiple zones like this?

Mike T., Swampeast MO

I have only one recommendation for control in such a system: Thermostatic Radiator Valves (TRVs) for the radiators!!!! They provide an UNBEATABLE combination of economy (both in operation AND original cost), comfort and versatility!

Condensing/modulating boiler THRIVE when driving TRVd systems!

EACH AND EVERY ROOM becomes its own "zone" but they ALL COMBINE to produce a SINGLE zone! Even though YOU see each room as its own zone, the boiler only sees one! It only needs to provide the SUM of the heat required. This is because TRVs are neither "on" nor "off", they are in most all circumstances operating somewhere in between "on" and "off". In this way each room only presents its share of the load to the total.

TRVs require no wiring and in many cases are completely self-contained. Depending on the actual location of the radiator remote sensing and or remote operators may be required or highly desirable. These "remotes" are connected via tiny tubes--not wires.

Simple two-pipe direct return piping is by far best with TRVs. TRVs are very "forgiving" but such is not an excuse for sloppy engineering... Fortunately the required engineering is not difficult as it mainly entails designing a system that would provide reasonable balance were the TRVs not used. "Home run" systems to manifolds can certainly be used. In this case reverse return should be used at the manifold connections. Depending on the layout, remote manifolds can be cost effective.

With a boiler like the Weil McLain Ultra, you'll wind up with a boiler circuit (I call such the "primary") and two "secondary" circuits; one for the radiators, one for the basement floor. The circuit for the basement will have mixing valve to reduce its temperature. I'm not sure if the Ultra can inherently control such a valve. Depending on how sophisticated you want the control of the mixer, it might use a fixed adjustment or motorized (can be controlled a number of different ways). In any regard, a three-way valve should be used as four-way valves tend to increase return temperature which in return will decrease efficiency of the boiler.

The only "traditional" thermostat required for such a system will be for the basement. None is required for rest. Done this way the thermostat connections of the boiler will be "jumped". If I remember my Ultra literature correctly it would also be best to have a separate warm-weather shutdown control (simple and inexpensive) that "kills" the entire system in moderate weather--55°-60° outdoor temp is a common shutdown range. The basement thermostat will merely control the basement circuit (pump & mixer) itself. When it "kicks in" the boiler will simply have to modulate up to meet the load. This will happen automatically.

If you insist on a thermostat for the rest of the house, it should be centrally located. In general the best location for such is in an internal hallway without any source of heat. In "normal" operation you will set it HIGHER than needed anywhere in the house--this ensures a CONSTANT call for heat. You then adjust the TRVs for whatever comfort level you desire. Unoccupied spaces can be set back as deeply as you want. If you want to reduce temperature in the ENTIRE area without changing all of the TRVs, you then lower the traditional thermostat setting. Note however that this will "starve" the TRVs and they will open fully. The resulting temperature balance in the house will be only as good as allowed by the original engineering of the piping and sizing of the radiators.

A system as I'm describing here will provide exceptional comfort and efficiency. The TRVs aren't free of course, but they eliminate zone valves, thermostats, pumps and wiring. Depending on layout they can significantly reduce piping as well. In a system as you seem to be describing, the net effect on cost should be quite minor.

If budget allows, I would consider alternative boilers. The Viessmann Vitodens is truly amazing and will perform wonderfully in this system. While a Viessmann mixing valve and operator will be required, the boiler itself has inherent mixing control ability (one mixing device only) to a level of efficiency that would cost significant $$$ to achieve with other systems.

The Buderus GB comes with a very nice control system that, in effect, automatically adjusts the heating curve to achieve the highest possible efficiency. It uses indoor temperature as opposed to outdoor temperature to achieve this. It may not however provide such "sweet" mixing control as the Vitodens and I would NOT use the GB if you use "American-type" zoning without TRVs. (A new control system for the GB using outdoor temp is in the works, but IMHO such is a compromise to American-style multi-zoned systems.)

If you don't want TRVs and instead prefer somewhat traditional wall thermostats, Tekmar makes some extremely sophisticated control systems that attempt to approximate the action of a TRVd system. If you investigate this option I suspect you will find that TRVs are very cost effective...

D107

great explanations Mike

1. by producing a 'constant call for heat' you are saying constant circulation, yes? I have heard of a lesser degree of constant circulation, something like 'only when system is calling' but isn't there always circulation when system is calling? I have seen some contractors wince when they talk of true constant circulation--with the three-way mixing valves, etc. as if that complicates the system too much. Some have said that they can work it so that in effect it will be in constant circulation most of the time, but without the three-way valves, etc.

2. Location of indoor thermostate/sensor: Must this always be in a room at the end run of a zone to ensure heat will reach that space? Sometimes end run may be near an entrance hall which could be influenced by door opening/closing etc.

thanks,

David

Mike T., Swampeast MO

Again I speak from the perspective of providing the best comfort with the highest efficiency via the most cost-effective method while still providing versatility as allowed by the design of the structure. "Yea, sure!" you say...

The "constant call for heat" only means that heat and circulation are provided whenever you need heat in the house. Don't forget that 55°-60° outdoor temperature during the heating season is a very reasonable range for saying, "I don't need heat."

The TRVs demand constant ciculation to give the best degree of comfort. The modulating function of the boiler allows it to most accurately supply just the amount of heat needed by the system as modified by the TRVs. The TRVs inherently modify the need based on desire, SUN, occupancy, etc.

For the system you describe you WILL need a mixing valve for the basement. Such is a necessary "complication" because the load (generally low in a basement) and heat transfer method (tube-in-slab) both demand very low supply temperatures compared to above grade radiators of any reasonable size.

Again, there is no need for an indoor thermostat for the above grade space!!!

With a two-pipe TRVd system there is no "end run of a zone. There is no real need for any indoor thermostat. If such is provided merely to allow whole-house setback just put it in a centrally-located space. A fully interior hallway without a source of heat is generally ideal.

Mike T., Swampeast MO

Panel Sizing

Gene:

With such a low design temperature the panels might get impractically large and/or expensive--particularly in the great room--with a design supply temperature of 140°F.

Sizing the panels for 140° return temperature with a design delta-t (temp drop across the panel) assumption of 20°F will likely reduce the panel size significantly with little impact on efficiency. I would avoid using higher design temperatures if at all possible.

Multiple panels in the same room (like the great room) are no problem whatsoever. Each should have its' own TRV and they should NOT be piped in series.

As long as you have decent insulation and your windows are multi-glazed and have good infiltration control, radiators need not be installed underneath windows. In fact, at least one study shows that radiators sized as I've mentioned actually work better when installed on interior walls in full view of windows. Most of the radiators in my house are installed this way and I've NEVER had a draft problem even though the house was built in 1903 and has the original windows with storms. A large south-facing, floor-to-ceiling wall of glass can however be a problem--both with heat loss (particularly at night) and heat gain during the day. The architect in me doesn't particularly care for window coverings, but some sort of insulated window covering can be highly desirable in such an application. Such are ideal candidates for automation using timers and/or "smart" sun sensors. To lessen the chance of overheating in the summer, such a window really should have a quite deep eave. If the window follows a gable (is triangular at top) the best eave design will project farther at the ridge on top than at the bottom "base" of the triangle. This minimizes heat gain in the summer while maximizing it in the winter.

There is no problem with slight oversizing of panels in spaces where you either desire a slightly higher "normal" temperature or desire comparatively enhanced ability to raise the heat level.