Converting Gravity System to Circulated Water System

Timco

You live & you learn...I ended pulling the guts out of the TV and useing a thermostatic 2-wy valve w/sensor to control the bypass...works like a champ. When new valve bypass opens, near full flow to bypass. Heats return water well, and allows system to heat.

Tim

Jeff_90

I have been called in to be the "expert" on a system that had been converted from a gravity HW system to one that uses a pump. The old gravity boiler was replaced with a cast iron boiler, no bypass was installed to keep the return water up. There were also 3 additional zones added with fin tube radiation, all have their own t-stat. The main t-stat is on the 1st floor. There are multiple problems. The boiler has deteriorated from condensing and needs replacement. The customer can not heat the 1st floor without over heating the 2nd and 3rd floors. The areas with the fin tube seem to heat OK, but they are small areas.

I sure would like help on this one. I would like to give the customer some 'value added' energy savings if at all possible.

Mike T., Swampeast MO

Nothing particularly unusual about a converted gravity system overheating on upper floors--particularly if insulation and weatherization are improved.

A few questions:

1) On what floor(s) are the fin tube zones located?

2) Are these fin tube zones heating a significant area of the home?

3) How much space heated by each fin tube zone and is the size (by space heated) among them consistent or very different?

4) Does water flow through the original (gravity) portion when fin tube zone(s) call but the "main" t-stat is NOT calling? If so, what stops the flow in the main (original gravity) portion?

Al Letellier_9

gracvity problems

I do a lot of this kind of stuff, Jeff. Where are you? Can try to help if you can send a piping schematic or some pics.

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Jeff_90

There are 3 fin tube zones, 1 in the basement, and 2 on the first floor.

The 2 on the first floor heat small areas (kitchen and small bath) and the basement zone is about 1/4 of the basement.

The size per zone is not consistant with the basement zone being the largest. All fin tube zones equal 35' of standard 3/4" fin tube. That compares to a total of 912 square feet of radiation with the cast iron radiators.

Water does NOT flow through the original gravity portion, it is controlled by a zone valve.

Jeff_90

I'm in east central wisconsin...sorry, I don't have any pics of this job yet, and the piping is pretty straight forward. Imagine a typical gravity system with 3 new supply and returns added for the fin tube zones, all with zone valves to include a zone valve for the original gravity system.

Mike T., Swampeast MO

So all four zones (the three b/b) and the original gravity (vast majority of the house) have zone valves and there is a single circulator?

The boiler is most likely destroying itself because it gets nice and hot when the b/b zone(s) call, but that nice hot boiler is flooded with a huge amount of cold water when the gravity zone calls.

Also, without bypass, flow through the boiler is going to vary wildly depending on which and how many zones are calling.

If you're going to replace with a conventional boiler and don't have the budget for extensive changes to the system, a supply mounted ESBE type TV (thermic) bypass valve will at least keep the boiler reasonably happy. It will provide nearly full system bypass until the supply temp reaches 160°F at which point it starts allowing flow to the system. At supply temp increases towards 180°F less and less flow is diverted back to the boiler.

When the gravity zone calls, it will still return a lot of likely cool water to the boiler but the bypass valve be forced to open since the boiler won't be able to maintain 160°F supply.

While in all likelihood you need a true two-temperature system, you might be able to slide by without a mixing valve if you take off the fin baseboard zones before the gravity zone (but everything after the ESBE type TV bypass of course).

Allowing a single tiny zone (like a bathroom) to fire the boiler is not a very good idea unless you're using a rather large buffer.

The "upstairs too hot" is not uncommon in converted gravity systems--particularly when insulation has been added as the upper rads were sized before the insulation and are now relatively oversized compared to the lower rads. TRVs (thermostatic radiator valves) are [probably] the least expensive and most reliable way to control this problem. In this case you would use them only on the upper rads in rooms that tend to overheat. They'll slow or stop flow as the room meets or exceeds setpoint thus allowing more flow through rads in rooms that have not yet reached setpoint.

If you have sufficient budget and the homeowners want a truly fine and efficient system, then:

1) Use a condensing/modulating boiler installed according to mfgr. instructions.

2) Use TRVs on ALL of the old iron rads.

3) CAREFULLY assess the fin-tube zones--if they need a significantly higher supply temp than the iron rads (VERY likely), either change their type (standing iron rads, panel rads, iron baseboard, etc.) or increase their size. Your overall system efficiency will be affected most significantly by the highest temp needed any zone.

4) A two-temperature system may well be required.

Jeff_90

At least I know I'm on the right track... Thanks for the info. I believe I can definately give them some value added changes and plenty of options depending on their budget.

Jeff_90

One thing I just noticed...

> So all four zones (the three b/b) and the

> original gravity (vast majority of the house)

> have zone valves and there is a single

> circulator?

>

> The boiler is most likely

> destroying itself because it gets nice and hot

> when the b/b zone(s) call, but that nice hot

> boiler is flooded with a huge amount of cold

> water when the gravity zone calls.

>

> Also,

> without bypass, flow through the boiler is going

> to vary wildly depending on which and how many

> zones are calling.

>

> If you're going to replace

> with a conventional boiler and don't have the

> budget for extensive changes to the system, a

> _I_supply mounted_/I_ ESBE type TV (thermic)

> bypass valve will at least keep the boiler

> reasonably happy. It will provide nearly full

> system bypass until the supply temp reaches 160°F

> at which point it starts allowing flow to the

> system. At supply temp increases towards 180°F

> less and less flow is diverted back to the

> boiler.

>

> When the gravity zone calls, it will

> still return a lot of likely cool water to the

> boiler but the bypass valve be forced to open

> since the boiler won't be able to maintain 160°F

> supply.

>

> While in all likelihood you need a

> true two-temperature system, you might be able to

> slide by without a mixing valve if you take off

> the fin baseboard zones _I_before_/I_ the gravity

> zone (but everything after the ESBE type TV

> bypass of course).

>

> Allowing a single tiny zone

> (like a bathroom) to fire the boiler is not a

> very good idea unless you're using a rather large

> buffer.

>

> The "upstairs too hot" is not uncommon

> in converted gravity systems--particularly when

> insulation has been added as the upper rads were

> sized before the insulation and are now

> relatively oversized compared to the lower rads.

> TRVs (thermostatic radiator valves) are

> [probably] the least expensive and most reliable

> way to control this problem. In this case you

> would use them only on the upper rads in rooms

> that tend to overheat. They'll slow or stop flow

> as the room meets or exceeds setpoint thus

> allowing more flow through rads in rooms that

> have not yet reached setpoint.

>

> If you have

> sufficient budget and the homeowners want a truly

> fine and efficient system, then:

>

> 1) Use a

> condensing/modulating boiler installed according

> to mfgr. instructions.

>

> 2) Use TRVs on ALL of

> the old iron rads.

>

> 3) CAREFULLY assess the

> fin-tube zones--if they need a significantly

> higher supply temp than the iron rads (VERY

> likely), either change their type (standing iron

> rads, panel rads, iron baseboard, etc.) or

> increase their size. Your overall system

> efficiency will be affected most significantly by

> the _I_highest_/I_ temp needed any zone.

>

> 4) A

> two-temperature system may well be required.



You mentioned a SUPPLY MOUNTED bypass valve. In the past we have been mounting them on the return. What's the difference, or is it a different valve?

Jeff_90

One thing I just noticed...

You mentioned a SUPPLY MOUNTED bypass valve. In the past we have been mounting them on the return. What's the difference, or is it a different bypass valve?

Mike T., Swampeast MO

This is a Danfoss "ESBE Thermic Valve TV". There are other mfgrs. and they use similar nomenclature.

Avaiable in both supply and return mounted varieties.

I suggested supply mounting in this case to ensure a high supply temperature for the baseboard zones. When the original gravity zone kicks in, there will still be bypass as such will be required to maintain at least 160° supply temp. Flow through the gravity zone will decrease significantly (compared to present) in most conditions and the reduced flow might help somewhat with temperature balance with a greater portion of the flow going through the lower floor rads. (If reduced too far however, you might not have enough flow ability for the upper floor rads--VERY hard to determine beforehand due to the backwards sizing of gravity piping with low and close rads having the least pressure restriction and high rads having the most.)

Were this strictly a gravity conversion with conventional boiler, I'd suggest a return mounted ESBE type TV to ensure 140° return temp to the boiler.

This suggestion was the simplest and least expensive thing I could think of to at least keep a conventional boiler reasonably happy and owners reasonably comfortable when driving three zones of fin b/b and one zone of original gravity via a single circulator. There are MUCH better options, but cost will certainly increase.

While all-too-common to add "zones" of fin baseboard to gravity systems (usually because of a downstairs kitchen remodel/expansion), such is among the worst possible match of emitters.

Timco

I used a TV on a p/s system with ODR, and found that when the ODR set the water back to 150* or so, the TV would not open. Took FOREVER to get HW to the secondaries.

Tim

Mike T., Swampeast MO

Tim: Please don't tell me you tried to reset the boiler itself back to 150F or so with the ESBE type TV installed.