Weil-McLain GV 4 Series two N. Gas Boiler Replacement

[Barbarrosa1]

I have a home in central Ohio with a Wirsbo hydronic floor heating system. The heating system water & the domestic hot water is heated by a Weil-McLain GV4 Series 2 boiler. This system was installed in Fall 1993. The domestic water was originally heated by a W-M Combo 30 indirect fired heater. This was replaced by a W-M 42 gal Aqua Plus in December, 2016. My local plumber used the priority valve system utilized by the old combo 30 unit. We have been having problems with the 24 year old boiler hammering a lot & short cycling for the last several months. I think this is due to sludge build up in the boiler & debated trying to clean & flush the boiler. My wife & I are fast approaching our "Golden" years, 80s-90s & are planning to build a condo unit at a local Senior complex. So plan to sell our current house late this year. Rather than spend much on the old boiler I am thinking of replacing with a smaller high effiecency boiler. The home is an Amos Winter Panel hybrid 1.5 story with a timberframe family room 32'w x 30'l x 22'h. The entire first floor has gyp-crete poured over the Wirsbo tubing with ceramic tile on 3/4 of it. We have a full basement that has not been finished. The heating system has 3 zones, 2 with tubing, & 1 with baseboard upstairs plus a Modine style heater in our 2 car garage. And the indirect water heater would make a 4th zone. I had Vent-aire Systems do a design heat loss study prior to building. Their result 37,037 BTU/Hr at 15.56C. The plumbing supply firm recommended the W-M GV4 Series 2 105,000 BTU/Hr in 90,000BTU/Hr out. I did a calculation using this past Jan. 2018 to Feb. 2018 gas usage & come up with about 50,000 BTU/Hr output at a 1.7 ratio. I am looking at at the Weil-McLain HE boilers such as the WM97+ 70 CT Series 2 57,000BTU output , the ECO70 57,000BTU output, or the EVG 70. Any comment or suggestions?

 

[Dana]

With low-mass boilers and low mass radiation like baseboard have a very low minimum-fire is important, to keep it from short cycling, since unlike your cast iron boiler there is very little thermal mass to work with. If there isn't sufficient zone radiation to emit the full min-fire output at condensing water temps it's going to short-cycle itself into lower efficiency and a short lifespan.

The ECO 70 only has a 5:1 turn down and a 14,000 BTU/hr minimum fire input, and over 13,000 BTU/hr minimum output at condensing temperatures. The gypcrete zones probably have enough water mass in the tubing and the thermal mass of the gypcrete to work with, but the baseboard zone could be a real issue, since it would need to be 65-70' of baseboard to emit the full min-fire output, and at 40' or less could be a severe short cycling problem.

Since this is likely to be the Achilles' heel zone for the low-mass boiler, how much total baseboard length is on that zone? Multiply the feet of length by 200, and that's about the most it can emit at condensing water temperatures. Ideally the boiler would have a minimum fire output that's at least comparable to or lower than that number.

A pretty good easy to retrofit modulating condensing boiler that works without a lot of fuss is HTP's UFT-080W. It comes pre-plumbed with a second output port to support an indirect water heater. At high fire it takes in 80,000 BTU/hr, but it has a 10:1 turn-down ratio that can throttle it back to 8000 BTU/hr in, 7,600 BTU/hr out. At that firing rate as little as 35-40' of fin-tube baseboard can emit the entire amount at an average water temp of about 120F, which is where it needs to be to hit 95% efficiency. Even 25' of fin-tube baseboard won't usually be a severe short-cycling problem (even though it would be cycling on/off when that's the only zone calling for heat.)

A more expensive mod-con series that would cover you would be Lochinvar's KH-055 (55,000 BTU/hr max input) and KH-085 (85,000 BTU/hr max in), either of which can throttle back to about 8000 BTU/hr out. (Or the wall hung versions of the same boiler, the WH-056 & WH-086, same specs).

There are others.

There is no need to go with a 1.7x oversize factor- that just happens to be the oversize factor used for AFUE testing. ASHRAE recommends a 1.4x oversize factor, which is also a bit overkill, more than sufficient to cover the worst Polar Vortex low temps with margin. With modulating condensing boilers operated under outdoor reset control even a 1.1x oversize factor is fine.

See this bit of bloggery covering the napkin-math for sizing modulating condensing boilers, and this for variations on fuel-use load calculations.

 

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[Barbarrosa1]

With low-mass boilers and low mass radiation like baseboard have a very low minimum-fire is important, to keep it from short cycling, since unlike your cast iron boiler there is very little thermal mass to work with. If there isn't sufficient zone radiation to emit the full min-fire output at condensing water temps it's going to short-cycle itself into lower efficiency and a short lifespan.

The ECO 70 only has a 5:1 turn down and a 14,000 BTU/hr minimum fire input, and over 13,000 BTU/hr minimum output at condensing temperatures. The gypcrete zones probably have enough water mass in the tubing and the thermal mass of the gypcrete to work with, but the baseboard zone could be a real issue, since it would need to be 65-70' of baseboard to emit the full min-fire output, and at 40' or less could be a severe short cycling problem.

Since this is likely to be the Achilles' heel zone for the low-mass boiler, how much total baseboard length is on that zone? Multiply the feet of length by 200, and that's about the most it can emit at condensing water temperatures. Ideally the boiler would have a minimum fire output that's at least comparable to or lower than that number.

A pretty good easy to retrofit modulating condensing boiler that works without a lot of fuss is HTP's UFT-080W. It comes pre-plumbed with a second output port to support an indirect water heater. At high fire it takes in 80,000 BTU/hr, but it has a 10:1 turn-down ratio that can throttle it back to 8000 BTU/hr in, 7,600 BTU/hr out. At that firing rate as little as 35-40' of fin-tube baseboard can emit the entire amount at an average water temp of about 120F, which is where it needs to be to hit 95% efficiency. Even 25' of fin-tube baseboard won't usually be a severe short-cycling problem (even though it would be cycling on/off when that's the only zone calling for heat.)

A more expensive mod-con series that would cover you would be Lochinvar's KH-055 (55,000 BTU/hr max input) and KH-085 (85,000 BTU/hr max in), either of which can throttle back to about 8000 BTU/hr out. (Or the wall hung versions of the same boiler, the WH-056 & WH-086, same specs).

There are others.

There is no need to go with a 1.7x oversize factor- that just happens to be the oversize factor used for AFUE testing. ASHRAE recommends a 1.4x oversize factor, which is also a bit overkill, more than sufficient to cover the worst Polar Vortex low temps with margin. With modulating condensing boilers operated under outdoor reset control even a 1.1x oversize factor is fine.

See this bit of bloggery covering the napkin-math for sizing modulating condensing boilers, and this for variations on fuel-use load calculations.

 

[Barbarrosa1]

Thanks for the input. My calculations using 294 ccf (gas used from Jan. thrughFeb., 2018 readings) Springfield, OH info (we are 25 mile North), the 87.3% efficiency of the W_M GV4 Series 2 boiler, 47 degree differential yeilded 30,357.5 BTU/Hr x 104 = 42,500 BTU/Hr needed. As for the baseboard it is never on because the area consists of a 25'x8' loft w/ bathroom open to the timberframe area & a 25'x12' bedroom. Receives majority of it's heat from first floor. So that zone has about 35-36' of baseboard plus the "Modine" style heater hanging in the two car garage. The garage thermstat is usually set at 50 degrees. A new owner may have more heat requirements than we as we never turn up the thermostat in the bedroom, do keep the master bath & hallway at 70 degrees. 16'x16' sunroom with lots of glass 3 sides & the timberframe room 32'x 30' at 70 degrees. Any additional comments appreciated. Thanks. PS: From your suggestion 200 x 36' would yeild 7,200 BTU/Hr.

 

[Dana]

As mentioned previously, a modulating condensing boiler operating under outdoor reset control doesn't "need" the ASHRAE 1.4x oversizing factor. The ASHRAE oversizing factor is at least partly about recovery ramps from overnight setbacks. With modulating condensing boilers overnight setback strategies backfire, since a rapid recovery requires a higher water temperature, resulting in lower combustion efficiency, which erases most or all of the fuel savings from having the house at a lower temperature.

With your high thermal mass gypcrete radiant floors the best fuel economy will result from dialing in the outdoor reset curve on the boiler for the absolute lowest water temperature (=highest combustion efficiency), taking a "set and forget" approach with thermostat settings.

So, with a ~35' baseboard zone ideally any boiler you installed would have a minimum firing output of no more than 9,500 BTU, so that the baseboard could emit at least 3/4 of the heat, and less than 2400 BTU/hr (40 BTU/minute) of extra heat going into the zone. Assuming you have at least 15lbs of water (a couple gallons) in the zone plumbing + boiler, 40BTU/minute of excess the temperature of the water would be rising about 2.5 degrees/minute, and with a temperature swing of 10 F you'd get minimum burn times on the order of 4 minutes. The UFT-080w would be fine, since it's min-fire output would be about 10 BTU/minute of excess heat. That would slew 15lbs of water less 1 F per minute, with minimum burn times north of 10 minutes for a 10F swing. There will be some temperature limit and reset curve tweaking to do, but there's no doubt that a boiler with a sub-8000 BTU/hr minimum firing range can run a 35' fin-tube zone at condensing temperatures without short-cycling.

The fan coil in the garage is probably ridiculously oversized for the actual load to allow rapid heat-up when needed, and would likely to be able to run at condensing temperatures without inducing short cycling. That is assuming the fan-coil isn't saddled with a non-adjustable aquastat control that won't let the fan turn on with low-temp water.