Installing a combi system in an old house - confused

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Assad Jarrahian

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Hi there,
New to the forum. Live in an 1890 house in Denver Colorado and doing a bunch of demo work. Have a 40 year old cast iron boiler on its last leg and a 20 year old tank water heater. I wanted to replace it with a wall combo-unit (tankless). Am a bit confused as to where to look for equipment and good installers.

Some facts on the house:
Hot water baseboard heat over two heating zones - 41 feet of baseboard in zone one and 38 feet in zone 2
2 full baths.
1500 Sq feet.


Any ideas of where to start looking and what to expect (cost - problems etc) would be very much appreciated. Thanks much.

-Assad
 

Assad Jarrahian

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So I had a local firm come out and they think its much better and cheaper if we install
2 x rinnai ex22c (one on each floor)
1 x Rennai RL75i in the basement.

Any thoughts on whether direct vent furnaces are a good idea?

Thanks much.
assad
 

Tom Sawyer

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Your current radiation will put out about 45,000 btu's @180 degree water temperature.
 

Assad Jarrahian

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I am not following. You said:
". Putting anything in that's suppose to be high efficiency, condensing won't operate at anywhere near the high efficiency numbers because it will never condense. Putting three in is just stupid."


I am getting rid of the boiler and tanked water heater in its entirety.
The boiler is replaced by two different Direct Vent furnaces. Do you have experience with the direct vent furnace above? (ex22c)
Then I am moving to a tankless water heater (located in the basement) to heat the water. RL75i

Just making sure we are on the same page. Given the above - Do you still think its a stupid solution? Thanks much.
assad
 

Tom Sawyer

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It depends on the price. The two rinnai heating units are both sized pretty well for the load so they probably will operate in condensing mode as long as it's cold outside. The problem is, your baseboard is rated for water temperatures @ 180 degrees and when you run lower temperature water through them, they don't put out as much heat. The condensing boilers are looking for outlet temps of around 140 degrees with returns coming back at 120 or less for the boiler to condense. They only operate at max efficiency when condensing. So, again a lot depends on the price here. Also be advised that rinnai's don't have the bat reputation for longevity in the world so my concern is that you are going to spend a lot of cash that will never give you any return on your investment.
 

Assad Jarrahian

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Tom,
Thanks for responding. Much appreciated. With that said, I think there is still some confusion here. If I put the rinnai's in - all the baseboards around the house come out! It's a direct vent unit that is forced air. Do you have experience with these units? They are not tankless boilers.

Also, what makes you say that the rinnai's do not last long?
 

Assad Jarrahian

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btw, its 4.5k installed for both ex22c units.
the RL75i tankless water heater is 2.8k installed.

Thanks again for all your help!
 

Dana

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If either floor has rooms all all doored off it might be a code violation to heat each floor with a point-source heater like a wall furnace, but if the doored off rooms are fairly low-loss you can still be comfortable. (Building codes require that the heating system be capable of heating each room to a minimum of 68F at the 99% outside design temperature.)

In high-R houses way better than code-min with better-than code windows this is sometimes possible, but not very likely in an 1890s antique (unless you're adding 4" of foam insulation to the outside and adding triple-pane windows every where, etc.). I've seen the Rinnai direct-vent units work fine in ~600-800' open floor plan ski condos with sleeping lofts instead of doored off bedrooms, but those still needed supplemental heat in the bathroom to meet code.

It's unlikely that your actual heat load is the ~45,000 BTU/hr that the baseboards would deliver with 180F water. If the windows are at code-min and there is wall insulation it's only be half-that, which means you COULD operate at condensing efficiency, as long as the min-fire output of the condensing burner is low enough to match the output of the fin-tube baseboard at low temp. At an average water temp of 130F (say, 140F out, 120F return) the fin tube delivers about 250BTU/ft, so your ~40' zones would balance if the boiler/combi can run as low as 250BTU/ft x 40' = 10,000 BTU/hr or so. You can cheat that a bit, but not by much. If the min fire output is 15,000BTU/hr it's going to short cycle.

If the baseboard is cast iron you get a bit more wiggle room out of it due to the thermal mass of the system-water + cast-iron, but it's not nearly as forgiving as big old high-volume cast iron radiators. Water has a lot more thermal mass per lb than cast iron, and the water volume of cast iron baseboard is small relative to radiators.

The RL75i modulates down to 10,300 BTU/hr-in, so it WOULD cut it from that point of view. But is not a combi-heater, it's a tankless hot water heater, and using it for space heating would violate the warranty.
 

Assad Jarrahian

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Hey Dana,
Thanks for your reply.
First - to clarify - RL75i is NOT being used to heat the house. It's a replacement for the tanked hot water heater from 20 years ago.

The direct vent furnaces (1 on each floor) are going to replace the furnace/Ducting and all the baseboards. They would all get yanked out.

Second -
" I've seen the Rinnai direct-vent units work fine in ~600-800' open floor plan ski condos with sleeping lofts instead of doored off bedrooms, but those still needed supplemental heat in the bathroom to meet code."

I believe here in Denver - these units meet code for whole floor heating - including bathrooms. The kicker is your doors have to have 1.5 inch gap from the floors.

The baseboards are copper piping.


Your further advice on the topic would be very much appreciated. As i am still lost and not sure if I should go down that route.

Thanks much.
-Assad





If either floor has rooms all all doored off it might be a code violation to heat each floor with a point-source heater like a wall furnace, but if the doored off rooms are fairly low-loss you can still be comfortable. (Building codes require that the heating system be capable of heating each room to a minimum of 68F at the 99% outside design temperature.)

In high-R houses way better than code-min with better-than code windows this is sometimes possible, but not very likely in an 1890s antique (unless you're adding 4" of foam insulation to the outside and adding triple-pane windows every where, etc.). I've seen the Rinnai direct-vent units work fine in ~600-800' open floor plan ski condos with sleeping lofts instead of doored off bedrooms, but those still needed supplemental heat in the bathroom to meet code.

It's unlikely that your actual heat load is the ~45,000 BTU/hr that the baseboards would deliver with 180F water. If the windows are at code-min and there is wall insulation it's only be half-that, which means you COULD operate at condensing efficiency, as long as the min-fire output of the condensing burner is low enough to match the output of the fin-tube baseboard at low temp. At an average water temp of 130F (say, 140F out, 120F return) the fin tube delivers about 250BTU/ft, so your ~40' zones would balance if the boiler/combi can run as low as 250BTU/ft x 40' = 10,000 BTU/hr or so. You can cheat that a bit, but not by much. If the min fire output is 15,000BTU/hr it's going to short cycle.

If the baseboard is cast iron you get a bit more wiggle room out of it due to the thermal mass of the system-water + cast-iron, but it's not nearly as forgiving as big old high-volume cast iron radiators. Water has a lot more thermal mass per lb than cast iron, and the water volume of cast iron baseboard is small relative to radiators.

The RL75i modulates down to 10,300 BTU/hr-in, so it WOULD cut it from that point of view. But is not a combi-heater, it's a tankless hot water heater, and using it for space heating would violate the warranty.
 

Dana

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You won't get any heat to move into the doored off areas with just a 1.5" door cut. Leave the doors wide open and you'll have a shot at staying within 5-10F of the common space with the heaters.

A small ~85% efficiency cast-iron boiler with purge control, and an indirect fired tank for the domestic hot water may cost about the same as your proposed solutions, and will run more efficiently. Something like the Burnham ES2-3 (smallest in the series) if atmospheric drafted chimney-vented, or better yet the ESC-3 , which is power vented with ducted-in combustion air (no chance of backdrafting issues, no matter how tight you make the house.)

At the full DOE output of ~60,000 BTU/hr into 80' of baseboard you're at 750 BTU/ft- it'll hit ~200F on long calls for heat, with these boilers but that's OK. When only a single zone is calling there will be some cycling on/off, but if you set up the controls for a maximum differential between high/low limits it won't cut much into operating efficiency due to the heat-purge features of the controls, drawing the heat out of the boiler to minimize standby losses, and utilizing the thermal mass on the system to keep the total burn cycle numbers minimized.

If the indirect-fired hot water heater tank is set up as the "priority zone" there is about enough output to take a 24/365 shower at 2 gpm. You'll still want to size the tank to be able to fill your biggest tub though. An indirect tank should be good for 20 years or so, and has a fraction the standby loss of a standalone gas-fired tank.

The average efficiency of this setup would likely beat the ~80% efficiency wall furnaces + EF82 RL75i solution too.

Different markets, different costs, but I've had comparable systems installed in MA for ~$7-8K, to replace failing inefficient steam systems and that included installing the baseboards (that you already have), and disposing of the asbestos hazards (the old steam boilers & asbestos insulated steam piping.) The boilers themselves run around $2K (at internet pricing), figure another $1K for the indirect and related controls, then it's a matter of the hydronic-design/labor/overhead costs.
 

Tom Sawyer

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Reminds me of a few years back when everyone was putting monitor heater in and trying hard to convince themselves that they were comfortable. Lol
 
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