Gravity conversion ????

Mark J Strawcutter

Don't remove the large pipe unless you have to.

Pipe the boiler as a primary loop, cross-over bridge to a secondary manifold and connect the existing distribution piping there.

Control the secondary circ with an outdoor reset. Honeywell AQ475A is a basic, cost-effective unit with provision for indirect DHW control. Consider placing the "supply" sensor on the secondary return instead. 475 tells boiler aquastat when it needs heat. Boiler aquastat controls burner/primary circ.

Consider TRVs on all radiators, run constant circ on secondary, and install differential pressure bypass valves at the end of each supply/return loop. That way all that large piping can act as a buffer tank to help with short cycling.

Mark

David Sutton_2

What would be the best way to pipe this in....

Hello, I have a job that i am looking at , its a gravity system,i did the heat loss and the btu load is 225,000.It has 2-3" surplys and 2- 4" returns, but i can remove some pipe back to the 2 1/2". Would this be better piped in as a primary-secondary loop system??? and what would be the best why to control it??. It has no hot water yet, but maybe added in the near future. I have looked at the q+a section and seem to be leading to the preimary-secondary type system

Robert O'Connor_4

Could you

post a diagram?
This is a method I am trying to get a handle on .

Thanks

Robert

Mark J Strawcutter

I'll try

to throw something together in the next day or two. Won't be anything as fancy as some of the diagrams I've seen posted here.

Mark

Robert O'Connor_4

Anything rough will do and will be greatly appreciated.

Robert

Cliff Brady

I am considering something along those lines with my single pipe gravity HW heat. I have a Peerless Pinnacle 140, 40 gallon priority indirect DHW controlled by a Honeywell AQ474A reset control circulating with no P/S through about a 45 foot rectangle of 3 inch main in the basement. Radiator supply comes out of the top of the main and the returns come back in the side.

Since 3/4 of the basement is still rough I want to convert the south half of the first floor (ugly poorly placed radiators in the living room, sitting room, and kitchen/powder room) to joist mounted radiant under hardwood floors. Using your idea I want to put the 3 inch loop as a secondary in constant circulation and run the radiant as P/S off the return of the 3 inch loop with a 3-way mixing valve to ride the outdoor reset curve. I would put the remaining radiators on 1 1/4 inch TRV's (ouch roughly $1000).

Drawbacks:

1. Possibly overheating the basement with radiation from the constantly circulating 3 inch loop.

2. Cost of TRVs.

3. Loss of night time setback.

4. The AQ475A would shut down the constant circ at a fixed 70 degrees outside temp which is too high for our taste. Is there a way tool fool the unit to shutdown at closer to 60 degrees. Otherwise a manual strategy or ather sensor/ relays would be necessary.

The alternative to the constant circ plan would be using P/S and two zones on seperate thermostats with the radiators on one and the infloor radiant on the other.

Any comments on these options?

Mark J Strawcutter

some comments

I don't have any experience with single-pipe gravity systems and they may require a different strategy. Also, my suggested strategy was for leaving existing radiation in place. Replacing some with under-floor radiant is a different ballgame.

That said:

1. Possibly overheating the basement with radiation from the constantly circulating 3 inch loop.

Insulate the pipes

2. Cost of TRVs.

Only you can make that judgement. You get a separate zone for each room. TRVs also compensate for often over-radiated rooms.

3. Loss of night time setback.

Some higher end controls will do the setback and a corresponding "boost" of the reset curve in the morning. There are also TRVs available that can do setback with appliation of a control voltage (this does require wire to each TRV).

Another technique I've been thinking about is to use a conventional setback thermostat in the living space. Daytime program to as high a setpoint as it will go. Reset controller and TRVs effectively control things. At night, set back desired temp. TRVs will be wide open, reset controller will fire boiler whenever thermostat calls for heat since water will be well below curve target. Thermostat effectively controls system.

4. The AQ475A would shut down the constant circ at a fixed 70 degrees outside temp which is too high for our taste. Is there a way tool fool the unit to shutdown at closer to 60 degrees. Otherwise a manual strategy or ather sensor/ relays would be necessary.

This is probably the one feature I wish the 475A had. No way to fool it that I know of.

Mark

Cliff Brady

This sounds like a reasonable idea. I wonder what other wallies think?

.snip.
3.
Another technique I've been thinking about is to use a conventional setback thermostat in the living space. Daytime program to as high a setpoint as it will go. Reset controller and TRVs effectively control things. At night, set back desired temp. TRVs will be wide open, reset controller will fire boiler whenever thermostat calls for heat since water will be well below curve target. Thermostat effectively controls system.
.snip.

4. If resistance in the probe decreases with decreasing temperature then adding a resistor won't help. Maybe there is some other soultion.

Thanks for the comments.

Mark J Strawcutter

Here's a start

Mark J Strawcutter

Here's a try

Here is a diagram posted by Mark E a while ago that conveys the basic idea. Make the following changes:

Eliminate the flowchecks and dual circs on the "zones" and replace with a single circ on the secondary supply manifold.

Add balancing valve to the return for each "zone" right before the manifold.

Connect the end of the supply and return manifolds with a differential pressure bypass valve. Or better yet, install one at the far end of each original supply/return mains.

For DHW add a cross-over on the boiler loop from a point between the bypass and secondary supply T to a point between the secondary return T and the thermic valve. Then put closely spaced Ts on the cross-over for a secondary DHW loop.

Hope this helps.

Mark

Cliff Brady

I don't think I need a differential bypass valve because the 3 inch pipe is literally a "primary" loop with "secondary" pickups and returns going to the radiators. With that in mind the basement has always been colder than we would like especially since only about a quarter of the outside walls are finished. I plan to do another third of the walls eventually. Heat loss now is about 30,000 for the basement and I calculate about 15,000 btus/hr from the three inch pipe and exposed 1 1/4 inch radiator pickups at design temp. Add in the heat from near boiler piping and radiant over head and it should be much more comfortable down there.

The 66 feet three inch pipe alone supplies about a 25 gallon buffer.

The DHW is already on its own pump with priority over the heat. I prefer that to running two pumps in P/S for DHW.

Thanks for the diagram.

Mark J Strawcutter

single pipe gravity

I agree you don't need a differential bypass.

Your loop and radiators are directly coupled and are not primary and secondary. They're both part of the same loop.

Mark