Suggestions for boiler replacement! Condensing?

Mike T., Swampeast MO

Even the standard TRV actuators from Danfoss can be easily adjusted for a maximum setting. Easy to set, but not so easy to defeat unless you know what you're doing.

Tamperproof actuators are also available.

TRVs on the iron rads combined with a condensing/modulating boiler will give exceptional efficiency and comfort.

Brad Notter

Help.

I have been a lurker on the wall for years. I met Dan at his seminar in Brighton Michigan a few weeks ago. He asked why I never post, and I did not have an answer for that question. So now I will be an occasional poster on the wall. I have extensive knowledge of hydronics, but my knowledge specific to boilers is lacking a little.


The problem:
I am trying to help a homeless shelter for women and children that my church operates in the poor part of town. The existing system has in the wall cast iron radiators, a 50 year old Kewanee firetube boiler with a retrofitted gas burner, and one circulator for the whole house. The tag on the burner states 300,000 btuh. The system is piped with all the radiators in paralell, and all one zone. I will do a heat loss calc on the building before any work is started. The boiler system needs to be replaced.

What we would like from the system:
I would like to see some redundancy in the design. Possibly two boilers and two system pumps. I was thinking two boilers, maybe one modulating condensing and one non condesing boiler. Each boiler would be sized for 75 percent load and piped manifolded Primary Secondary. That way we could make the most of outdoor reset by using the condensing boiler when outside temps require a water temp below 140 degrees, and the other boiler when water temps are above the condensing point. The system pumps would continuously circulate. The system needs to remain one zone. I thought about using TRV's, but the people who use this place would probably crank the temp up.

What do you think? What specific controls could I use? I know the Tekmar tN4 is out, but I don't see a control that will do what I want. I went to their school in Cleveland earlier this year.

Any help will be well accepted, as I will be over seeing this project from concept to selecting the installing contractors.

Constantin

I'm a mere homeowner myself...

... but if being cost effective is one of your design goals, you might want to look into a NTI Trinity 400. With its 16:1 modulation range, it can produce anything from 25-400kBTU/hr. That should cover the heating load and leave plenty for many warm showers and other DHW loads via an indirect water heater. Ideally, choose one with a large surface area like the Triangle-Tube Phase III or the WM Gold to minimize the supply water temp requirements.

I'll let others chime in, but how often is there an issue with NTI or any other brand boilers, i.e. is the redundancy really worth the extra expense if the boilers are well-installed, well-maintained, and not abused? I would think that spending the extra $$$ elsewhere may be more rewarding, like optimizing the delivery/emitter system, installing GFX heat exchangers to stretch the hot water supply and deliver energy savings, etc..

The heat load calculation and surface area of the radiators will let you know just how hot the supply temperature has to be to keep the occupants happy on a design day. If any weatherization, insulation, etc. has been done, I'd be surprised if the place couldn't benefit from a very low reset curve, very low water temperatures, and thus very good efficiency.

Constantin

Yup...

... perhaps the question is how well the rads in their current state meet the needs of the space?

If the rads are sized such that they meet the needs in every room pretty much equally, then TRVs may be overkill - you could let the OR curve ramp the supply temp up and down. If someone is too warm, let them close the gate valve on the radiator.

Where TRVs (and thus individual room control) become really interesting is when you have a structure where the heat loss/gain is highly variable across rooms. Insolation, large occupancy swings, etc. can produce such conditions, and that pretty much describes most structures out there. Plus, TRVs are KISS, at least until one is used to using them.

TRVs with large rads really shine in responding fairly quickly, increasing comfort, etc. without the need for fancy zone control or other electronic throttling. Hence, from the little I know of the system, I would follow Mike's lead and install a low-mass, condensing, modulating boiler and spend the rest of the budget on TRVs in this application.

The one thing you might have to look into is the piping/distribution system to ensure that it can handle the added static pressure of the TRVs being inline and to put in a way for it to deal with all TRVs going shut... like PAB or a pressure sensor that adjusts the pump speed. A PAB is probably the least expensive way to do it, KISS, etc.