Multiple zones - Single Manifold: TRVs and Central Thermostat?

tocker

I'm renovating and replacing my baseboard and conventional boiler with Runtals and a Mod-Con boiler in a 1955 Ranch. Of course I would like to limit the pumps and manifolds. Will the system behave properly if I use one manifold and a variable speed, pressure regulated pump to service individual bedroom and bath radiators with panel rads with TRVs (5) along with actuators controlled by a central thermostat for panel radiators (3) in a common Living and Dining Room? Does this make sense? I have two floors with the same issue, rooms that I would like to control individually as 'zones' on TRVs and a common area with multiple radiators that I would like to control as a single zone.

Jamie Hall

Presumably the grouped radiators in each area will be controlled by a zone valve for the group? That will work, yes.

After a fashion.

To avoid losing most of the efficiency you are paying for with that mod/con, though, try like mad to get the system so well balanced that neither the TRVs nor those zone valves ever close, with the system modulating off its outdoor reset.

Otherwise you will be much better served -- at much less money and maintenance cost -- with a good solid conventional boiler, although you may need a buffer tank as well to control short cycling when only one TRV is open.

Speaking of which, make sure that the pump you use will turn on when the system goes from a no flow state to having only one TRV open...

tocker

Hi Jamie,

Thanks for responding. See the attached sketch. Please let me know if I am off-base. I just realized that I should have included some information clarifying that this is a Primary/Secondary system with home runs to each radiator.

I do plan on using an outdoor reset and a variable speed pump to avoid using balancing valves on the return for the TRVs.

On the main floor I am thinking two manifolds and 2 pumps. For the main floor common area, the plan is to centrally control the common area zone (living room and dining room) with actuators for each of 3 radiator loops on a central thermostat. The heat output should range from a planned 11K BTU to 16K BTUs.

The main floor bedrooms would be on another manifold and fixed speed pump(?) each having radiators @ 3,800-6,500 BTU output each on TRVs depending on the outside temp.

The basement will be on one manifold, but the initial idea was to use 2 thermostats. Do you think that I can effectively use one variable speed pump and manifold to feed both 'zones' and 2 TRVs in the basement?

Jamie Hall

Well, as I said, what you are proposing will work. As I also said, it is very much at cross purposes with the whole concept of a modulating boiler operating on outdoor reset.

If you really have your heart set on using TRVs and the two grouped zones -- a very normal arrangement -- ski9p to the mod/con boiler and the outdoor reset. The two modes of control will fight each other, and no one will be happy -- either you, your wallet, or the boiler.

Or use balancing valves on the home runs and a mod/con with outdoor reset and skip the TRVs. Once you get it balanced -- which, granted, takes some patience -- you will need a thermostat only for trim for odd days, if at all.

In other words, one approach or the other.

tocker

Good morning. I realize that ideally, it would be best to have one or the other method on a single manifold and across the entire 'system' for that matter. BTW, I'm not arguing...I'm debating in order to gain a better understanding. I'm not tracking how TRVs and outdoor reset are working at cross-purposes. The outdoor reset affects how the boiler provides piping across the primary loop. The variable speed pump itself adjusts to provide balancing among the emitters, correct? Also, in going through the Siegenthaler, Modern Hydronics book and watching his presentations, he sings the praises of ECM variable speed/pressure regulated pumps to TRVs from mod/con and using outdoor reset, (even while having another zone of emitters controlled differently). Having said all that.. my head hurts and I'm a mile wide and an inch deep, so I am all for simplifying this. Do you think I should go to only one pump and manifold for each floor by using thermostats and balancing valves? I don't think I need one per group do you? The thing I'm wrestling with until I got into this discussion is finding a more efficient way to run piping from the boiler to the bedrooms at the other end of the house. I'd like to avoid running 10 PEX lines for the bedrooms and baths, but not sure how/where it is best to do that.

Jamie Hall

Yeah. Outdoor reset and primary secondary and variable speed pumps and lions and tigers and bears oh my...

Let's see if we can help here.

First off, outdoor reset (with or without modulating boilers). The objective of that exercise is to ensure that the water temperature being fed to the radiation is exactly what is needed to let the radiation provide the heat needed for the space. That way, the circulation never stops in the secondary loop. Mind you, it's never perfect, and so outdoor reset controls provide for an input from a space thermostat which either shuts the pump off if the water temperature (or sometimes drops the water temperature slightly) is a little too high, or tweaks the water temperature up a bit if the space is too cold.

This can be done with primary/secondary piping in two ways: either a controlled mixing valve on the secondary loop which recycles some of the returning, cooler water to temper the water that's circulating which reduces the amount of hot water taken from the primary loop, or it can be done by reducing the temperature in the primary loop from the boiler. The latter, however, only works with a modulating boiler If you are using a non-modulating boiler with an aquastat control, but the boiler. instead of modulating down, simply turns on and off to provide warm enough water to meet the demand.

Now if you are using a modulating boiler, it will modulate down based on the outdoor temperature -- and this is where the gain in efficiency comes from. If the boiler return water is below 140, then some or all of the water vapour in the exhaust is condensed in the boiler and you get the benefit of that extra heat.

Now note. If you are using an outdoor reset with either a modulating boiler or a mixing valve, the idea is that the circulating is constant and the space thermostat only modulates the water temperature.

A completely different approach is the older one, using a space thermostat (or thermostats, or TRVs which are essentially single radiator thermostats) to turn the circulation on and off as required, with the water temperature being controlled by an aquastat set so that the water is hot enough to heat the space on the colder days. Here, while it isn't absolutely required, primary/secondary piping is very very desirable, as it ensures that there is enough flow through the boiler at all times to keep the boiler from overheating, even though there may be only one or two radiators calling at that moment and thus very low flow (this is where the constant delta T or delta P pumps are used in the secondary). If the system is set up with what are called microzones -- ;single zones which are very much smaller than the boiler capacity, such as with TRVs on radiators -- it may and often is desirable to have a buffer tank on the primary loop, so that the boiler can have longer runs.

Now there are many ways to pipe a system out in the wilds, as it were. Home runs do have some real advantages, in that you can have a single delta P pump on the secondary system and easily balance the spaces with balancing valves on the manifolds (and, for that TRVs on individual radiators, if some spaces want to be independently cooler from time to time). However, as you note, that can create quite the pipe collection. You can, however, do much the same thing using reverse return piping. That's a system where there is one -- or perhaps more, if it's easier -- main supply line and the radiators take off individually from that. To ensure an even temperature supply and flow to each radiator, there is also a single (or more than one, again) matching the supply line -- but the most distant end of that picks up the radiator which is closest to the boiler, and so on, so that the total length of pipe feeding each radiator is the same Perhaps a very simple illustration. You have only two radiators. One is close to the boiler (call it A), and the other is at the other end of the building 50 feet away (call it . A reverse return system would have the supply come from the pump and the first takeoff would feed radiator A. Then it would run out the 50 feet and feed radiator B. The return pipe would start at radiator A and run that 50 feet and pick up B, and then return to the boiler.

Now in that example there's an extra 50 feet of pipe in there -- that return line coming from B back to the boiler -- but in most structures with a little thought there is very little extra pipe needed.

What you mustn't do -- in any system -- is go from the boiler to a radiator, then out of that radiator and into the next, and so on. Yes, it cuts down on the amount of pipe. But it also pretty well eliminates any hope of individual radiator balancing and control.

I hope this helps a little...

tocker

Jamie, kind sir, that is a ton of information and helps me enormously. I'm certainly leaning toward a buffer tank and will do some more homework on the Reverse return. I live in a ranch and the boiler is at one end of the house. Just FYI..my old 'system' has one large 1.5" return coming back down the middle of the house to the boiler with two 3/4" supply loops one in the front and one in the back of the house, which meets the supply at the opposite end of the house. It was a bit bizarre, and cycled quite a lot. We have very few people around here who know these systems, which is why I'm here. You're a true gentleman! I'll be back, but it may take a day or so.

EzzyT

I would suggest you have the entire 1st panel rads on 1 manifold and the basement on another manifold. I would just install trvs on all the panel rads so there won’t be a need for any manifold acuators.
Most likely you can get away with one circulating pump depending on the system curve in relation to the pump curve.
You can go with a single master thermostat to turn on the circulating pump on or you can use a outdoor reset control which in then will do constant circulation. 
Might also want go with a high mass modcon boiler or buffer tank with a low mass modcon boiler to prevent short cycling.

tocker

Ezzy, Thank you. I am leaning toward a buffer tank. I like the simplicity of your suggestion and will probably go with two smaller circulators. Where can I investigate the high mass mod-con? Are you talking about a big boiler with stainless steel?

EzzyT

Both HTP and Viessmann have high mass boilers.