No Boiler Fill Valve/Loop for a Combi-Boiler? No Delta-P Thermostat?

gotgas

We are installing a hydronic system that uses Pensotti panel radiator and is powered by a Rheem RCBH199DVLP that handles both heat and domestic HW. We have an MEP engineer that has created a plan for this. Everything seems pretty standard, but two non-standard parts of this plan stand out to me and I thought I would ask here.

1. The engineer did not include a fill valve or loop. It makes sense to not have one, because the boiler is connected to the cold water supply, but it still seems odd to me. Does this boiler have an internal fill valve to keep the hydronic side up to pressure?

2. This is a thermostat free system. Each radiator has a Pensotti A40040A thermostat that opens to allow flow. When it does, the pressure drop signals the Taco 0018e3-2f2 pump, which is running on delta-p mode, to start circulating and that kicks on the boiler in heat mode to heat the circulating water. Basically, each radiator is its own zone and calls for heat by opening and allowing flow.

Does that make sense? It all seems to, and this was designed by an engineer, but it still concerns me a little that there is no central thermostat. Am I heading for a world of hurt, here?

I appreciate any advice you can give. Last time I posted here you helped to keep me from doing something very stupid, which I appreciated. I thought I would ask again about this issue.

EdTheHeaterMan

Sounds interesting, Looking at the I/O manual there is an electrically operated "Auto Feed" valve in the wiring diagram, so there should be no need for the conventional PRV type "Auto Feed" when the piping is done correctly.

As far as no thermostat. There appears to be a way to connect a "Sensor" or a "Dry Contact thermostat" to the control board. based on the engineer's specifications, you may have some sensor at some location that will enable the burner, the Boiler pump to operate and the System Pump to operate based on some outdoor reset algorithm built into the microprocessor. See what is supposed top be connected to the "Call for Heat" terminals 1 and 2 on the X13 terminal strip.

If there is nothing connected to 1 & 2, then be prepared to add a thermostat somewhere in the building.

mattmia2

this reminds me of the building my office is in that was built in 1980 where they thought if they just put radiation around the perimeter to replace the heat loss they didn't need heat anywhere else.

gotgas

EdTheHeaterMan said:

See what is supposed to be connected to the "Call for Heat" terminals 1 and 2 on the X13 terminal strip.

Great info, thank you. The boiler has separate inlets for dhw and heat return. The dhw fires because of flow. I believe the heat loop fires the same way. The pressure drop activates the circulator which starts moving water through the open loop. The boiler senses the flow and fires, heating water in the loop. 

At least that is how it was explained to me. I am confident that is how the tankless dhw side runs. Wanting to confirm the heat side is the same.

Jamie Hall

Consider the logic of the heating side. If all the radiation is controlled by a local thermostat (as you note, think of each radiator as its own zone, with its own zone valve), then there is no point to having any other thermostat. Now there would be a point to outdoor reset, if that boiler modulates, to control the water temperature and the modulation. With modulating the circulating water temperature on outdoor reset, I can see that it could be a very effective way to control.

mattmia2

Is the rheem a rebranded noritz? You need to be careful about microzoning here, can it modulate down to handle a single emitter calling? You have to be careful that the sensor on or near the radiator gets a good representation of the room temp if it is the only thing controlling the system. TRVs work better to keep rooms with variable loads from overheating than to control the overall system, it should be basically balanced and have some sort of overall system control, the goal would be to make the system keep the rooms comfortable without the TRVs needing to shut off.

gotgas

mattmia2 said:

You need to be careful about microzoning here, can it modulate down to handle a single emitter calling?

A lot of good questions to think about, thank you. Let me answer this one because I have this info. The boiler can modulate from 18k to 120k.

Also, I believe the sensors will get a good representation because of how they are placed.

I agree on TRV, that is how I have used them in the past. However, I spoke with my MEP PE and with an engineer from Granby (Pensotti). The Pensotti engineer was excited about this configuration and said, "This is how we want people to use our products!" They both seem certain this will work. I should probably believe them, but before I put this into two houses, I want to be sure.

gotgas

Jamie Hall said:

If all the radiation is controlled by a local thermostat (as you note, think of each radiator as its own zone, with its own zone valve), then there is no point to having any other thermostat.

Thanks. This is what my MEP engineer is saying as well.

mattmia2

gotgas said:

A lot of good questions to think about, thank you. Let me answer this one because I have this info. The boiler can modulate from 18k to 120k.

Unless you gang several emitters on one TRV your smallest zone is probably more like 5KBTU/hr, less if you are using outdoor reset.

hot_rod

About the lowest turndown I have seen is 8,000 btu/hr
See how that matches the smallest load

This formula shows how a buffer can get some longer run times if you have micro loads

gotgas

mattmia2 said:

Unless you gang several emitters on one TRV your smallest zone is probably more like 5KBTU/hr, less if you are using outdoor reset.

hot_rod said:

About the lowest turndown I have seen is 8,000 btu/hr
See how that matches the smallest load

This formula shows how a buffer can get some longer run times if you have micro loads

Thank you both.

As I understand the controls, the boiler will fire until the water in the loop is 180, no matter how many are loops are open. Once it is 180, it will continue to circulate without firing until the water temp drops. The whole system is about 1000' of 1/2" PexA and a 4.5 gallon expansion tank. With the water in the 3/4" supply and return lines, the manifolds, and the boiler, it is less than 20 gallons in circulation. On any single loop, only perhaps 25% of that is in play.

I think how it works is the circ pump keep positive pressure in the lines while nothing is open. When a thermostat opens, the pump senses the drop in pressure and begins running. The boiler intermittently fires until the water circulating is 180. Then the system fires and circulates, fires and circulates, until the thermostats close the line and the pressure in the lines lets the pump know it needs to stop.

On paper, it all seems to make sense. However, since we are spray foaming both homes and there will be no wall voids, I am concerned that this really works as advertised before the insulation crew shows up. Adding thermostat wires after would be a real hassle. Maybe I will run some anyway, and leave them in the wall.

Do I need a much larger expansion tank so their is more volume? I hate to do that because it would make the system less efficient.

mattmia2

gotgas said:

On paper, it all seems to make sense. However, since we are spray foaming both homes and there will be no wall voids, I am concerned that this really works as advertised before the insulation crew shows up. Adding thermostat wires after would be a real hassle. Maybe I will run some anyway, and leave them in the wall.

I would put a box with conduit or smurf pipe to some space you can get back to the mechanical room from. If you are putting in remote manifolds I would run the same to those spaces.

gotgas said:

Do I need a much larger expansion tank so their is more volume? I hate to do that because it would make the system less efficient.

That isn't how and expansion tank works, it is just absorbing and releasing a small amount of water as the water in the system expands and contracts. You may however need a buffer tank. A buffer tank is an insulated tank that is part of the circulation loop of the system. There are formulas to calculate the needed size based on boiler firing rate, emitter output, and desired cycle time.


I would also recommend a more controllable boiler and a separate indirect or HX for hot water if you are going to invest this much complexity in to the rest of the system.

What will happen if the minimum turndown of the boiler it say 18k and the only load calling is 5k the water in the circulating zone will reach 180 in couple minutes or less*. Once the boiler is shut off the emitter will cool that water the 10 degree or whatever differential the boiler is set to, it will turn back on, since it is about 4x the output if the emitter it will be back up to 180 in a minute or 2 of firing, the boiler will shut off again. Then the cycle repeats again until the thermostat is satisfied. You can mitigate it somewhat by either setting a minimum off cycle time on the boiler or a larger differential but that may leave the emitter unable to produce enough output to heat the space.


*(if you are running outdoor reset the setpoint will vary based on the outdoor temp, if you are using a mod con boiler you will want to size your emitters to keep your return water temp under 130 or so to keep it condensing, so you probably size your emitters for a supply of about 140)

The circulator control can't work exactly how you describe, the ciruclator has to be running for there to be a pressure differential. You could use a delta p ciruclator and a small bypass or something similar to maintain flow while no zone is calling, the pump will maintain a fixed pressure so it will produce more flow as zones open. The boiler would probably be controlled on supply water temp so it would basically be always on but only would fire as loads consumed heat or heat was lost from the system.

JohnNY

I've stayed in many homes and hotels in Italy, Spain, and to lesser extent France and every one of them heated with hydronic systems was designed exactly as your engineer has laid out for you. TRVs on each radiator and a call for heat activated by integral ODR logic. Some have one small radiator, usually in a bathroom (towel warmer), near the end of the loop that receives flow all the time. That's their alternative to using a pressure differential bypass valve.

hot_rod

The boiler manual will typically show the minimum flow required. So a single radiator needs to be able to flow or the boiler may heat quickly and trip off and short cycle. So a buffer tank may be needed. See what the manual indicates

Some of the newer delta P circulators may have a switching function that could fire the boiler

Or a flow  switch could start the boiler anytime the pump runs

gotgas

JohnNY said:

I've stayed in many homes and hotels in Italy, Spain, and to lesser extent France and every one of them heated with hydronic systems was designed exactly as your engineer has laid out for you. TRVs on each radiator and a call for heat activated by integral ODR logic. Some have one small radiator, usually in a bathroom (towel warmer), near the end of the loop that receives flow all the time. That's their alternative to using a pressure differential bypass valve.

Great idea and great information. Thank you.

gotgas

mattmia2 said:

since it is about 4x the output if the emitter it will be back up to 180 in a minute or 2 of firing, the boiler will shut off again.

Can I increase the range to mitigate? For example, on at 150, off at 180? Also, is short cycling a problem with these units? The DHW part continually runs for just a few moments and then shuts down. All day long it runs for 10-15 seconds and shuts down. Will those short cycles harm the heat side?

mattmia2 said:

The circulator control can't work exactly how you describe, the ciruclator has to be running for there to be a pressure differential. You could use a delta p ciruclator and a small bypass or something similar to maintain flow while no zone is calling, the pump will maintain a fixed pressure so it will produce more flow as zones open. The boiler would probably be controlled on supply water temp so it would basically be always on but only would fire as loads consumed heat or heat was lost from the system.

Does this mean the circulator runs continuously, 24/7? Is this what they are designed to do? Sorry to sound ignorant, but I am about this.

hot_rod

If you use a delta p circulator, it will always be spinning. It needs to have  flow going through it do it can sense when a zone opens.As zones open and close the pump will speed up and down
You want to keep that type of pump powered alway as they have exercise functions if that is useful.

Only about 7W when it us in that standby mode

Like any motor, the less you stop and start them, the better they like it🥴

Same with the boiler, the life expectancy is based on the on off cycles

I think they engineer around a 15- 20 year life span for pretty much any appliance these days.

gotgas

hot_rod said:

If you use a delta p circulator, it will always be spinning. It needs to have  flow going through it do it can sense when a zone opens.As zones open and close the pump will speed up and down
You want to keep that type of pump powered alway as they have exercise functions if that is useful.

Only about 7W when it us in that standby mode

Like any motor, the less you stop and start them, the better they like it🥴

Same with the boiler, the life expectancy is based on the on off cycles

I think they engineer around a 15- 20 year life span for pretty much any appliance these days.

I love this forum! Great info. Thanks again. You folks have really helped.