Hi -
I need to replace my cracked NG gas boiler, a Slant/Fin rated at 90,000 BTU (IBR net) which has a listed water capacity of 597.3 sqft.

Ideally I would like to install a high efficiency tankless wall-hung direct-vent unit. I have heard that these are available that can provide hot water for the heating system and potable hot water at the same time - but I have not yet found a unit like this. Am I imagining this or do they really exist?

Some more questions -

* pros/cons of aluminum vs. stainless heat exchanger? Aluminum heats faster/more efficient and stainless is more durable?

* for a tankless solution, do I need to worry about water capacity? I've seen ratings of GPM (gallons/min) for temp ranges, but am not sure how I could convert that to match my current system.

* if I direct-vent from my basement, I am concerned about discharge outside in people-space - should I be? Should the vent go above head-level?

* for a direct-vent unit with a 3" PVC exhaust, can I vent into my existing chimney duct, which I believe to be 4"? Think I've heard draw might be an issue..

* general opinions on direct venting?

* can you recommend brands?

I know it's a lot of questions - thanks for any info!

Remember to get hot water you have to move X amount of gpm of water through the system to get hot water. If you don't open the bath room sink full open to get hot water when you shave its going to be cold, the same with the kitchen sink. Did you check out the Marathon water heaters?

The first question you need to answer is what temperature you run your water at on your current boiler. The answer to this question will tell weather you can successfully use a high efficiency condensing boiler to start with.

So the question is what is the current setting for the water temperature of your old boiler?

Unfortunately, I do not recall the operating temperature, although the operating pressure was about 22-25psi. The unit manual says this is the lower pressure option, and it mentions the starting temperature as 180F in a procedure or two. That does not appear to be the max or steady state temperature though.

There is (strangely?) no mention of the operating temperature in the product's literature or on the unit itself.

Clarification: I located the aquastat and it is set to 185F.

185 degrees is a hot water temperature for home heating. This temperature is usually used with fan coil systems.

If you are looking for a dual purpose system most high efficiency boilers are used with a secondary tank to allow a lower temperature for domestic (shower,bath, and kitchen use). You never want to have 185 degree on tap for showers,bath and kitchen use.

Hight efficiency units like to run at 130 degrees or less so they will condense and give up the heat that is normally lost up the flue. If you purchase a expensive condensing high efficiency boiler then run a higher temperature than 130 then you loose the condensing capability and are wasting your money that you spent for the high efficiency unit.

These are just the facts. In this area I have done a few inspections for different manufactures of high efficiency units. This were triggered by complaints to the factorys about high gas bills when the units should have lowered the gas bill due to there high efficiency as compared to the original units. After field testing each unit with my Kane-May combustion tester it was discovered that these units were all running way over 130 degrees with most of them running 160 to 175 degrees and the units were not condensing so there was no gas savings.

The temperature was raised on the units because of customer complaints that there home heating would not keep up when the temp dropped outside or the heat itself felt colder that the heat from there old unit.

The old systems were designed for a water temp of 180 degrees to heat the house and with the new system set at 130 (so they would perform correctly and condense) just would not heat the home correctly so the selling company (heating contractor) just raised the temperature to make the customer happy.

None of this worked since the gas bills went up when the temps were raised and the customers were very unhappy. I will not go into detail of what the outcome was but there is a court fight in process over this with one homeowner right now.

One brand designed in Germany I like. There are others also but Monitor has the best design.

Monitor Products » Gas Fired Condensing Heating Systems


There are many other brands available and Monitor is probably one of the most expensive. In this case you get what you pay for and Monitor only sells to there qualified dealers so as to prevent all the problems stated above.

No matter what unit you get it has to be installed to the manufactures specs. So you will need to see there websites to get your answers for your questions.

Happy hunting for your new unit.

As an update, I did a Manual-J calculation using the Slant-Fin HE2 tool and arrived at a total heat load of 35.6K for my house (using 0F as outside temp and doing a one-room calculation).

Then I calculated the heat output of my radiators using this Radiator Sizing Guide
And according to that, my radiators will put out 40K BTU with water temp of 140F.

I contacted Monitor to ask them about their efficiency drop-off for temps above 130F, and they responded that the units (MS25S and MZ40C) stay within their rated 92%-97% efficiency across their full output temp range.

I am not sure I will go with Monitor since there are many other factors also - it's certainly a nice product - but even if I have to run at temps over 130F I feel it should not be a deterrant. Someone else told me that they have fixed input rates - I don't understand the full implication there yet, I suppose it has to do with modulation rate.

I hope I am on the right track here, at least I feel like I'm a lot more educated. Thanks everyone!

At least you are working on it. Monitor is not a well advertised company since they deal only with the trade but they do make the best product or any boiler line and I have seen or tested most of them at the university. They are not cheap that is for sure.

You will find that other manufactures make nice units but they lose efficiency as the temperature of operation rises. Monitor is just about the only one that can maintain there efficiency in the entire operating range. Did some testing on Monitor 15 years ago and they were great machines. Very little has changed over the years except size and the standard parts venders. Grundfos pumps and Honeywell burner controls.

Good luck with your project.