Continuous circulation

GMcD

One major sub-set to the continuous circulation basics is to also match the heating control and circulation to the right heating terminals. The original article posted above references wall fin and panel radiators, but a thermally massive system like a topping slab or full-on concrete core conditioning system (thick radiant slabs) can also provide additional energy reductions, as well as steady-state comfort levels, by duty-cycling the radiant heating system pump(s)during a lot of the off-peak heating season using slab temperature sensors to maintain the slab at a relatively constant temperature. A small dead-band of a couple of degrees F maximum is all that's needed and pump energy use can also be decreased quite a bit over the year.

This works as long as the room/building envelope doesn't create fast-acting thermal load variations in the room with the high mass radiant system in the first place.

Roland_4

Continuous circulation

Hi All,
Could someone give me a breif primer on the advantages of continuous circulation coupled with ODR? Thanks,Roland

Derheatmeister

Better because

That's all we did and still are doing in Germany. the system ( when dialed in the right way) is very stable and constant in temp. Therefor the overall system efficiency is very high! It has only one main system circ(less power robbing) and Thermostatic valves that only give you proportionally the amount of heat that is needed or if you Adjust everything Correctly they act as a "Room temperature Limiter. Also i found that (if you use the right parts) they require less maintenance i.e. i have worked on systems in Germany that had therm.Valves that had failed after approx 25years. is that not Great not having to change Zonevalves every year (To bad for the embodied energy that it takes to make these "Throw away Valves. Just my opinion.Richard from Heatmeister.

Paul Pollets

Feidrich's paper

Here's the link to the origanl thesis, from Rich Trethewey's website:

http://www.comfortableheat.net/pdfs/continuous.pdf

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ALH_4

Analog Control

Outdoor reset allows control of the temperature of the heat emitters proportional to the outdoor temperature. The heat loss of the structure is proportional to the outdoor temperature, so the temperature of the emitters is controlled indirectly in proportion to the heat loss of the structure.

This works very well, but outdoor reset alone is unable to respond to disparities in heat loss or heat gain from room to room. To control this, indoor feedback must be used to provide a high limit on the temperature of each room or zone. This can be an on/off control, such as a thermostat, or this can be a proportional control such as a thermostatic valve(TRV or FHV). Thermostats provide a simple high limit to keep the zone from overheating. They essentially chop off each zone's outdoor reset curve once the setpoint is reached. Conversely, thermostatic valves are able to provide flow control proportional to the ambient temperature. They essentially change the slope of each zone's outdoor reset curve based on the indoor temperature by reducing the flow to that zone as the temperature nears the setpoint.

Constant circulation is necessary with thermostatic valves because they have no way to provide a heat demand to the boiler. With thermostatic valves the outdoor temperature sensor provides the heat demand when the outdoor temperature drops below the desired indoor temperature (exact operation depends on the control). Theoretically, any time the outdoor temperature is below the desired indoor temperature, there is a heat load.

With thermostats, constant circulation is not necessary, because the thermostat calls for heat and turns on the circulator.

Modulating condensing boilers are particularly well suited to constant circulation because of their low-temperature sealed-combustion operation. They can take advantage of the low temperatures allowed by the use of outdoor reset to provide efficiencies above 90%.

Which is better, thermostats that call the boiler or thermostatic valves with constant circulation? In my opinion, thermostatic valves with constant circulation will provide the highest degree of comfort in the most elegant way. The way I look at it, the heat load is analog so why would we heat with digital heating systems?

Roland_9

Continuous Circulation

WOW! I'm always amazed at the level of scholarship on this site. It's going to take me some time to digest all this information. That said, all else being equal, how does the constant circulation scheme stack up against other hydronic systems (primary/secondary etc.) in efficiency? Also, it seems that indirect DHW would not fit well in such a system as DHW,on occasion, would demand a higher temperature than that which is circulating to maintain a set room temp. Nes pas?

ALH_4

P/S & DHW

Primary/Secondary piping can, and usually is, used with constant circulation. The choice whether to use P/S piping is based on the hydraulic restriction throught the boiler.

Indirect DHW works fine with outdoor reset and/or constant circulation. Typically, the indirect tank is put on priority so that the heating circ stops for the time that it takes to satisfy the tank. With a high mass non-condensing boiler you typically have a mixing valve or injection control that controls the temperature sent to the emitters. This will prevent the hot boiler from sending 180°F water to the emitters.

Modulating/condensing boilers are typically low mass, so even if a mixing valve is not used, there is only a small slug of hot water sent out to the emitters when switching back to space heating mode from DHW mode.

The only efficiency drawback to constant circulation with a mod/con I can see is that the circulator will run more than if the thermostats called the boiler. This will use more electricity, but with a single circulator system this can be a relatively small amount and with new variable speed circulators it will be even less. My preference is for constant circulation and no primary/secondary. This is not always possible for high heat loads and certain boiler configurations. Each system must be evaluated individually to determine the best piping and control scheme.

Jed_2

You are

assuming, maybe, MOD/CON boiler, low/medium temp system. Trinity/Smith GT for one prioritizes to hi-fire for DHW. Constant circ works well with hi-temp systems also, so no DHW issues. There are ways around it. Proper reset control and piping design can provide the happy medium.

Constant circ vs. P/S is not a either/or decision, but, a decision based on many factors relevant to the project as a whole. They both have their place. I don't believe it's black and white.

Jed

mtfallsmikey

And, while we are on this subject

I'm doing HWBB in my house, constant circ. w/TRV's, and using a diff.pressure bypass valve for low/no flow situations so I don't blow my pump...Is there a elctrically-operated valve made, and if so, is it a better way to go other than the manual valve?

D107

just to clarify

So TRVs obviate the need for zone valves. The true single circ system sounds like a dream. And this is possible with, let's say. a direct or reverse return multizone system? Are pressure differential valves needed here or just flow balancing valves for each zone? (until the smart circs are available.)

And in choosing the circ size, as I understand it, the head of longest run is calculated with the flow being the sum of the gpms of all the zones. (?) Seems to me that very few of the install photos posted on the wall use this single circ system.

Thanks,

David

ALH_4

Single Circ

TRV's do take the place of zone valves or zone circs. Circ sizing is a somewhat difficult question for an existing system if the head loss varies significantly from zone to zone. The sizing method you mention will probably err on the safe side, and if you use a 3-speed or VS circulator you can always slow it down. For a new system it is best to design the zones to have nearly equal head loss and then you simply add the required flows for each zone at that head.

Wilo has variable speed circulators available today. Otherwise a differential pressure bypass valve is necessary because many of the TRV's could potentially be closed at any time. I have not yet used one of these VS circs but I believe primary/secondary piping, or another method of guaranteeing flow through the boiler, is necessary if you are using a variable speed circ.

Single circ systems should be used more. It's been a slow transition from traditional zoning with circulators. Zoning with circulators is wasteful of electricity and provides far more flow to each zone than is typically necessary for most residential installations.

D107

I'd guess

it's a little harder to do a one-circ system if it's not all the same kind of emitter and the required flow rate (and as you said pump head)and supply temp varies. Probably many homes have a mix of radiant, CI and baseboard.....

Thanks for the input,

David

ALH_4

Temps

Sorry, I was making some pretty broad generalizations there. Each temp obviously needs its own circulator, but if the system is designed for low temperature operation it isnt very often that you need more than two temperatures for a new system. My comments were aimed more toward the use of circulators for zoning.

Abro

Shouldn't it be easier?

I would think that as long as you are in the ball park sizing the emitters for a somewhat given water temp the TRVs will even out the indoor temps while the constant circ will even out the effect of the mass of the emitters since they aren't doing warm up cool down cycles at different rates.

Abro

A no DPV option...

I'd love to have TRVs throughout, but I think that I'd just leave TRVs out of one major room that doesn't get much in the way of internal gains (solar or otherwise - in my case a family room).

Put the t-stat in that room and pipe the rads in that room so that they can handle full flow without excessive velocity (but maybe a bit more than 4 fps). This might mean 3 small emitters instead of one large one. Then even if all the TRVs around the house are closed, no DPV is needed.