New boiler questions

mpage

First off, I'm not a pro, I'm a hobbyist, and this system is not to heat my entire house. Thought I should get that out of the way for those that don't want to answer tech questions to non pro's.

I have a pex radiant system which i laid in my garage and basement. Heat loss calculations were 19k Btuh garage, and 25k Btuh basement. These numbers have proven to be way too conservative as I keep the garage around 50* and only want it at 60-65 on the weekends. The basement is getting a lot of heat from the force air ductwork above.

I haven't had the funds to finish off the system until recently, but 5 years ago i came upon a rheem 74pvn on demand water heater which I've wrestled with each winter and have actually gotten pretty decent results. On to the question.

I want to buy a condensing boiler. The caveat is that my wife wants me to install a pool heater also. So I'm wanting to buy a condensing boiler and heat exchanger for the pool. The pool is a 27' above ground in the Chicago area, which I've found the recommended size is 100k to 200k heater. I'm willing to under-size the heat source and run the pump longer if i need a bigger jump in temps.

1) Will i be sorry if i do this? If so why?
2) I'm leaning towards HTP Munchkin contender MC80 or Navien NHB-110. Opinions on either company or product line?
3) Is it a bad idea to oversize the boiler even though it modulates down the range i need for heating? I know the ideal for heating would be the smaller 50 for each brand but thought i should up it for the pool's btu need.

I'm leaning towards the Navien, even though i would prefer to support an American company. The Navien has wider range which seems to be what i need for what i'm trying to do.

Thanks for the great site with some great information. If you guys want a laugh i'll post pictures my on demand water heater which i've decided to sacrifice as a pool heater until i put the final system in.

njtommy

The worst thing you can do is over size a Mod con boiler.
The Navien is a nice unit I have there Combi boiler in my own house. HTP makes some great stuff to. The biggest problem with Tankless like the Navien is theres no water storage for the system. Your btus of your heat loss change as the outside air temp warms up. Its Possible that the boiler will short cycle on a 40 or 50 degree days. Theres ways around this by adding buffer tanks, but its additional cost and you may be better off with a mod con boiler that has some storage capabilities. Are you plaining on keeping your forced hot air system to?

Paul48

http://www.comfort-calc.net/Dont_Oversize_Mod_Con_Boiler.html

This saves me some typing....

hot_rod

some options: look into a boiler with adjustable outputs. Lochinvar has a ramp delay function that allows you to "derate" the boiler output during heating season. So size to the pool load, maybe 180- 199K? Add reverse buffer that would give you DHW and buffer for heating. Depends on temperature requirements for the heating loads.

Or a heater dedicated to pool load. What about a heat pump pool heater? Maybe run the numbers on that. There really is no inexpensive way to keep a pool warn, insulated cover, keep the wind off it with fences or hedge rows, will help.

mpage

Thanks for the answers, I read through the oversizing mod con article and I think I understand but need to re-read it again. I'm still a little hung up on how/why something like NHB-110 that modulates from 10k to 100k would be oversized.

The pool heat question:
1: 27' round 4' deep
2: Specific temp, the kids use the pool almost every day when it's 80* or above
3: I think 82*-85* water temp.
4: I think the pool wouldn't be used on a day less than 80*

I came up with 100k/200k on some general guidelines from pool heater websites. I know where to find the design temps for hvac systems in my area, but i'm not really sure where to look for what to use for a pool.

RobG

I'm of the opinion that with an above ground pool and kids, you will probably be ready to remove it in a few years. Why bother heating it? The reason we go into pools is to cool off. The worst part is the initial shock of the cold water, and then all is good. They're kids they'll survive. I did. JMHO

mpage

Question on oversizing mod con: I read that recomended confort calc article. If I look at models within the Navien NHB and compare the NHB-110(10kbtu-110kbtu) to NHB-80(8kbtu-80kbtu). Not real life but lets just say there was a fixed load of 15k, would there still be issues with the 110 even though it's turn down ratio is higher and can therefore modulate almost as low as the 80 or 50? The statement in the article when talking about the oversized boiler "This boiler requires somewhere around 15 gpm of flow", is that an issue with Navien 110 vs 80 or 50? I don't see a higher minimum flow requirement in the manual anywhere. I guess they recomend the same pump for the 50,80,110 but the 110 shows lower headloss so therefore it will be flowing higher gpm.

http://www.comfort-calc.net/Dont_Oversize_Mod_Con_Boiler.html

Rob G. You are spot on, and we did get by last year with solar cover only. But the wife is pushing for the heater and want a real boiler for my garage and basement heat.

Paul48

OK....Here's the deal. You size the mod/con, based on the heat loss. If the area to be heated requires 100k/btus at design day temp ( maybe 7 days a yr), it makes no sense to put a 200k/btu unit in, even though the larger boilers turn-down ratio may allow it to fire down to your heat loss. You will need to take advantage of all of the turn-down ratio of a properly sized unit, in order to get the most savings.

Paul48

I understand.....Here's the thing....Large boiler, or small, he'll use the same amount of fuel to heat the pool. The only difference would be time. Why sacrifice the efficiency, all winter long?

mpage

Ha, you guys are the best, even looking out for my personal life. I'm leaning towards a Navien NHB line.
Last question, if i want to set myself up to be able to go with an indirect water heater in the future would that make the NHB-100 definite over trying to go in the middle with the NHB-80. Below are the current parameters of my water heating that i would want to match or better. Also, do mod-con boilers condense while running an indirect water heater? Or does the water temp have to be raised so high that there is no condensing? Thanks again for all the help.

50* to 55* from my well
Flow 6gpm at tub hot water tap
Rheem 6G75PV-75FW
75 Btu/h
First hour GPH 130
Recovery capacity 72.8 gph
Energy factor 0.59

4Johnpipe

Another often overlooked item...The flow rates are for the fixture capacity. Unless you run straight hot water we always mix with cold. Being conservative it would be 50% cold and 50% hot water. In reality it is closer to 60/40 cold / hot. That 6 GPM tub would be fine with a piece of equipment that provides 3.5 GPM of just hot water. Worst case scenario it may take a a minute longer to fill the tub.

njtommy

I would definitely run an inderect tank. Like every one stated above make sure you have enough storage for the big tub. Once the sizing is done you will never run out of hot water. No tankless will give you that flow rate and temp raise in the winter when wells temps drop into the 40s.

4Johnpipe

Sorry pre senior moment...Ha! I did flip that ratio. Too much going on at once. One thing about instantaneous water heaters though is they will slow the flow through them to maintain the set point water tempurature. Yes colder inlet water will reduce the output. I cannot attach the document showing a storage tank with a SS circulator and aquastat pippied from the domestic in and out of the EFTC 140. It is really simple and would add that ready demand for the high volume fixture. Or you could wait a minute longer to fill the tub. You wont run out of hot water it will just take little longer to fill. By the way 6GPM thats a big tub filler...

hot_rod

Also consider wintertime cold water temperatures. Public water in many places, like Milwaukee can drop below 40F, actually into the mid 30's.
That's when you get the no, or low flow DHW call

njtommy

Well you could always preheat the incoming hot water if need be to with either a plate heat exchanger or add another tank to the system. Ever notice that with Hydronics there's too many options? lol.