Combining reheat coils and radiation on the same heating loop 1

[canadeng]

When designing new hydronic systems, we generally have separate heating loops for radiation units (i.e. wall fin, unit heaters, etc.) and reheat coils. I have seen many older buildings where the reheat and radiation are on the same heating loop (i.e. share the same supply and return piping and pumps).
Usually, we see outdoor air temperature reset used for the building radiation units, so the temperature in the piping to the radiation units fluctuates. I could see this being a problem on a combined system, since reheat coils usually operate at a constant water temperature.
By having separate loops you can shut down the reheat at night and just operate the radiation units. Also, with two separate loops there is some redundancy built in, in that if there is a problem with one of the loops, you still have the other one operational. There seems to be several advantages for keeping the reheat coils and radiation units on separate loops.
Are there any advantages to combining the reheat and radiation units off of the same water loop?
Thanks.

 

[EnergyProfessional]

I've never seen a system where the heating supply for the VAV reehat coils and the perimeter heat are separate. They always are in the same loop with same temp etc. This is just my experience and don't claim to know everything.

And why would reheat coil require cosntant water temp? Whne it is cold, you typically need more reheat, so the warmer water temp helps.

The valves shut off each branch if no heat is needed in that zone (and with less flow the Pumps run slower due to VFD and static pressure reset). The redundancy argument exists, but for the pumps you have lead/lag anyway. so your redundancy would be when a pipe breaks. How likley is that?
and do you really run 2 boiler plants and only have supply temp reset on one?

Control-wise if your zone have both perimeter heat and VAV reheat, you can control that the reheat coil and perimeter heat at the same time and VAV heating airflow gets increased gradually if the first step doesn't heat enough (fan energy is what you want to save). typically I would not design VAV temp to go above 85°F so you still avoid stratification. Actually I like VAV reheat to only provide neutral air, and the permiter heat the rest.

If you are smart you have separate AHU for core and perimeter zones and then you can reset AHU DAT and migth even be able to avoid reheat.

 

[317069]

How the radiant unit water temperature fluctuate?
what do you mean by separating loops, do you mean to connect the reheat loop to a separate raiser or separate boiler and system.

 

[Drazen]

there is scenario in spring time when supply water temperature can be reset to lowest values, while ventilation is increased or fixed and then supply temperature can be too low for air heater, this should be checked beforehand as wall as flow concept - mixed/variable, automatic balancing valves or three or two way valves.

than decision can be made whether it is acceptable or not.

 

[EnergyProfessional]

Drazen: why would ventilation increase in spring? You mean more economizer mode? Wouldn't that be taken care of with supply air temperature reset, that when there is not much cooling demand, cools less, hence reduces reheat. so the AHU delivers over 60°F. i find it hard to beleive you could recuse water temp so much to still deliver much under 70°F air. and even if, the space will have some load to take care of that.

We operate most our VAV systems with boilers disabled in summer, and also don't have humidity issues. One system has really bad design and dedicated ventilation (never do that in VAV system!) and that requires the boiler to be on. it is all about sophisticated control. And out climate is pretty humid in WI.

 

[canadeng]

317069, the energy savings control sequence is set up so that the hot water temperature is adjusted based on the outdoor air temperature. For instance the supply water temperature varies from 140F .. 180F as the outside temperature varies from 64F .. -30 F. The same heat source (i.e. boiler, heat exchanger) is used for both loops (i.e. there is a set of pumps and piping for radiation and a set of pumps and piping or reheat).

HeerKaleun, in some applications (i.e. hospitals for instance), the air is supplied at 55F and the reheats are sized to raise air temperature from 55F to 75F. Reheat is required in the interior zones, especially in the summer. If the two loops are combined and the water temperature fluctuates, as discussed above, you would have to size your reheat coils based on the lowest possible water temperature in the system (i.e. say at 140 F instead of 180 F). This is possible, but your coils could end up being vary large depending on how low the water temperature reset schedule is.

 

[317069]

if they are connected to the same boiler then what is the difference if they are in the same loop or separate loop as long as the boiler will give 140 F to all loops.
Can you describe your application in more details

 

[EnergyProfessional]

i think his concern is when it sets back to 90°F or so... however, i don't see that as an issue at all.
in addition you can implement supply water temp reset to take into account OAT and valve position. So if your have a heating demand for long and the valve stays open long, temp will be increased. It is all a matter of control.... some proper programming is cheaper than running dual piping system.

 

[ChasBean1]

canadeng,

Funny you mention that new designs have separate loops and “older designs” combine the two.

The smart design is to have two loops. I’ve seen the opposite, where “value engineering” has most new installations combined into a single loop, like you mentioned. If there are two loops, the radiation can be enabled on OAT (e.g., less than 50-55°F OAT) while the reheat would operate year-round.

For reheat, in summer, you might need to cool the supply air to design (50-55°F) to dehumidify. Reheat coils can then tend to have their highest load to reheat spaces to a comfort level.

To answer your question briefly, the only advantage in combining the two loops is installation cost. You need more piping and controls for the two separate loops. The best installation will be to have separate loops…

 

[cry22]

I see this quiet often about people relying on reheat to get comfort
this is not possible for two reasons:

1. ASHRAE 90.1 prohibits simultaneous heating and cooling.
2. ASHRAE 90.1 requires outdoor temperature reset on boiler plants and boiler plant automatic shut-down - i.e when outdoor temperature is 60F or higher, the boiler plant is OFF, thus no hot water available for your comfort sequence of operation. AND most (if not all) boiler plants are shut down in summer completely (From April to September).

Hospitals are exceptions of course.

 

[EnergyProfessional]

I'm not familiar with hospitals, but don't see why they require 55°F AHU supply air. You dehumidify in the DOAS.... and in winter you don't need to dehumidfy.

And isn't running 2 loops more expensive than just slightly larger coil + control, commissioning etc?

 

[ChasBean1]

HerrKaLeun, sorry - I haven't been on for a while. You said "isn't running 2 loops more expensive than just slightly larger coil + control, commissioning etc." No, running 2 loops is not more expensive. The added expense is in the installation. If there are separate reheat and radiation loops, the radiation (serving baseboard radiators, radiant ceiling panels, etc.,) needs to only run while the outdoor air is below about 50°F. The reheat would run year-round...

 

[EnergyProfessional]

well, two loops twice the material, twice the labor, twice heat losses, twice maintenance, twice potential for leaks, twice replacement cost at end of life.

 

[ChasBean1]

Material and labor at a higher cost, yes.

“Twice heat losses:” No. Loops that are fed from a combined reheat/radiation system will be hot year-round so all of the capillary piping (to fin tubes or radiant ceiling panels) is hot year-round where it would not be with the separated loops. The radiation loop would only be hot and have heat losses when OA temps are less than ~50°F.

“Twice the maintenance.” It depends, but I don’t think so. There is a separate pump and heat exchanger. A radiation loop controlled by OAT is likely to keep spaces more comfortable so maintenance hot/cool complaints should be less. Hot and cold complaints need to be factored in as maintenance costs because on the person-hour scale, they might consume even more than routine maintenance (I could be wrong, but ask someone who does this for a living).

“Twice the potential for leaks.” Yes, more piping equals more potential for leaks. BUT, unless there is a bad batch of solder or piping, installation standards are pretty good these days. It is unlikely that there would be much more maintenance/repair related to leaks just because there is more piping.

“Twice the replacement cost at end of life.” Not sure about that. There could be, for example a single 30 HP pump set and HX delivering 500 gpm year-round to building reheat and radiation loops. If this were instead a 10 HP 250 gpm reheat pump that runs year-round and a 10 HP 250 gpm radiation loop pump that runs for half the year, would there really be higher replacement cost when a pump motor requires replacement? The motor sizes would be less for each system and the radiation loop would have half of the run time…

 

[cry22]

the advantage of running a baseboard loop is to obtain night set-back without running the fan systems at night/week-end.
I don't see a special need for a separate loop, as the reheat coil reacts to space temperature with dead band in controls with the baseboard loop. Baseboard being the primary source of heat in the space.
The same pump can be used for both loops without pumping through the reheat coils. You would need VSD's on HW pump.

The only separate loop making sense is for separate south-side with high glass with dedicated pump for south zone baseboard - too much radiation on the south side in winter requiring cooling for the south facing side and heating for other building sides.

 

[ChasBean1]

"the advantage of running a baseboard loop is to obtain night set-back without running the fan systems at night/week-end."

Concur with a nuance: The other (and bigger) advantage is to shut off the baseboard loop during warmer outdoor air temperatures. This is not the case with reheat.