View Full Version : Please educate me on combi boiler/dhw heaters
07-21-2012, 05:48 PM
Hello all. I had an energy audit since my last post. Turns out I don't need whole house ventilation since I get about double the air changes I should (haha...old houses). One of the recommendations was to replace our Weil Mclain boiler (which I believe is about 20-25 years old) with a wall hung Navien Combi. I don't know anything about these systems but did read that it was important that the unit be rated as a real boiler and not just a heat exchanger. Could anyone tell me whether the Navien is a good unit or if there is a better one I should be considering? We have hot water radiators. Thanks.
07-21-2012, 06:38 PM
Look into triangle tube boilers, I have seen one navien but don't know much about them. The excellence is a combi
07-21-2012, 10:21 PM
I just read Jtemple's long thread on her troubles with her combi installation. Because I know nothing about these units, I'm not sure it's the way to go but I do trust my HVAC company. Can anyone tell me the advantage to a combi unit as opposed to a high efficiency boiler and separate tankless hot water heater? For instance, the installation of the combi is very expensive. Would the install of high efficiency boiler be as involved? (ie. would the old storage tank be removed or would the new boiler utilize it and the existing pipes). Would the combi be able to handle several long showers running simultaneously (when the college age girls are home)? The washer and showers and DW all at the same time? Our house is a 100 yr old stone colonial, 3 stories, cast iron radiators for heat, near Philadelphia (cold but not severe in winter). I would love to know more about these so any information would be really appreciated. Thanks again.
07-21-2012, 10:29 PM
Like i said triangle tube, but with a boiler mate, with a mixing valve you would have plenty of hot water, and cast rads the condensing boiler would be really efficient, low temp. The down side is when your boiler has a problem you have no hot water. The cost is high compared to an 80% boiler and a draft vent water heater. The company should be able to estimate your payback. There are pros and cons but I would have your hvac company give you some info.
07-22-2012, 11:32 AM
Thanks for the replies. Think I'll stay away from the Navien. I've been doing more reading and am pretty confused at this point. What I would like to have is a boiler/hot water system that is as energy efficient as possible. I'm not overly concerned about initial costs for new equipment/installation but at the same time, I don't want to invest a large amount of money and get a system that isn't anymore efficient than a newer version of what I already have. Here are a few questions I have.
1) is it possible in a retrofit to get the return water to be a low enough temperature to operate a condensing boiler at high efficiency?
2) how difficult is it to "un-noticeably" vent the condensate? I read about problems with billowing steam and icing.
3) can I make my existing system more efficient with just a new boiler? I already have some kind of outdoor temperature sensor.
4) how good are indirect hot water systems?
5) is there a book I can read or a website that would explain all of this in layman's terms?
Thanks for your help.
07-22-2012, 01:49 PM
To improve effciency you can use a outdoor rest ot adjust water temperatures based on outdoor temperatures...ie. reduce system capacity during lower periods of demand.
Otherwise, it will depend on your home. Odds are good that you have more radiator capacity than your really need and/or you've added insulation, improved windows or added storm windows, etc to reduce your heat loss.
I'm pretty happy so far with my Navien tankless water heater with the built-in buffer tank and circulation pump, a unique feature they offer... but I don't have any experience with the Combi boiler. I think Navien's expertise is more in the tankless market, and the boiler operation probably needs some work. It's quite a bit cheaper than the competition and I tend to be a believer that you get what you pay for. Unless it's a very small condo unit with low heat loss, I'd stick with a more established boiler company. You might still look at a wall mount condensing boiler and use a seperate indirect water heater as an alternate option.
07-22-2012, 02:21 PM
The triangle tube is a condensing, modulating boiler with outdoor reset.
07-22-2012, 02:22 PM
95% wall hung, very popular her in upper Michigan
07-22-2012, 03:13 PM
As you probably know, a high efficiency gas boiler only will provide heat for the home. What you may not know is that the rating of the boiler is based on return water temperature. So if you're home is heated by radiant heat, including floors, free standing cast iron radiators, radiant steel panels or cast iron baseboard, you can take advantage of cooler water temperatures at moderate outdoor ambient through the use of an outdoor temperature reset control. On the other hand, if your home heats with hydr-air or aluminum finned copper baseboard, you'll not recognize either the efficiency or as much of a lower operating cost as you may have otherwise been led to believe.
With a combi-boiler, no matter who's it is, there is always a heat exchanger involved whereby the house heating water is on one side of the exchanger and the domestic water is on the other side. This means there must be a way for the internal circulator to know whether to run the water through the heat exchanger or directly to the heating system. Triangle Tube has been mentioned a few times in the thread. FYI, they make an excellent boiler from my experience but my experience with them is limited to the Prestige Series, Solo (no dhw) and Excellence (combi-boiler). They also have the Challenger combi series at a lower price point but I've not experience with those. Other combi boilers we've used include the Trinity (not happy with our experience) and Navien. We've only installed a couple of Navien Combi boilers but our manufacturer's rep for them advises against using the smaller of the 3 options, the 3 being the CH-150, CH-210 & CH-240. Since they're a modulating boiler, the 210 fits the bill as well as the 150 without the seemingly inherent problems the 150 has had. In using the 210, we've had no bad experiences.
If you choose to NOT use a combi boiler, I'd then recommend that you instead consider a high efficiency heating only boiler with an indirect water heater for your DHW source. One note about that, however. Each indirect water heater requires a certain minimum boiler capacity to satisfy the recovery needs of the water heater. This point is missed by a lot of installers, whereby they'll install an 80-gallon indirect with an 80,000 Btu boiler. The manufacturer of the indirect would specify a much larger boiler if you want the indirect to operate as designed.
07-22-2012, 05:55 PM
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07-22-2012, 07:02 PM
Lots of good information. Thanks all. I'm pretty sure that a combi unit won't work for us. Our house is about 3400 sq ft with 3 baths. The girls are at school now most of the year but you never know. In this economy they may still be living here for quite a while so I have to assume high demand for dhw. We have replaced all of the windows and are going to have the attic sealed and better insulated this fall and other air leaks fixed. The HVAC company did the audit (for a fee) but didn't include detailed analysis in the report so I don't know what size unit they have spec'd. The report says that the blower door results are 4675 cfm at 50 Pa, 1.94 times higher than ASHRAE standard. After what I've read here, I think we should wait until the house is better sealed and then have a new load calculation done. I really like these guys and they have a great reputation but I'm a little concerned that they recommended a unit that would not have worked for our needs. I read that the Triange Tube combis are best for small houses/condos and I assume the Navien fits in this catagory as well.
I'd still like to know what you all think about whether it is worth it to retrofit a condensing boiler v installing a new standard one. I'm very interested in reducing our carbon footprint wherever possible. Also, which is more efficient, indirect dhw or separate tankless? Thanks again for your help.
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