Boiler AND water heater vs Combi?

notsoepic

Hi all — I’m planning on doing a radiant staple-up retrofit on my home this summer and am having some trouble deciding on the best approach. We want to go with a closed/heat exchanger system for the radiant and I have calculated my heat loss at 52,000 btu/hour using the slant fin heat loss calculator. We have 2 bathrooms and would like to get around 5gpm on the DHW. We are also looking to save as much space as possible as we finish our basement.

After looking into available Combi boilers I’ve come to the conclusion that I would have to vastly oversize the boiler to get enough DHW, leading to short cycling etc. The costs of a high end Combi also is a turn off.

So i am now thinking of pairing an ~70,000 btu condensing boiler with the closed loop radiant system and adding an on-demand water heater dedicated for DHW. It seems to me the equipment cost would be roughly the same as a high-end Combi boiler if not a bit cheaper.

I have seen a lot of recommendations of just going with a boiler and indirect to address DHW needs for a system like mine, but this would take up more space and seemingly make the system more complex for a DIY (I’m not planning on going DIY with the boiler install, but will do the radiant system myself). I don’t think I’ve ever seen a recommendation for going with two completely separate systems. Is there any reason for that? Is there something wrong with my current thinking? Is the installation costs of two separate systems going to blow the budget through the roof?

the_donut

A modulating boiler with a high enough turn down ratio would work fine without short cycling. Depends on your cold water supply temp though. Here in Indiana water temp is mid 40’s and requires 160,000 btu/hr to get hot enough water for 2 gpm. 5gpm is a lot. Are you taking 2 showers and doing the dishes at the same time?

Zman

A condensing boiler with indirect is more efficient and less expensive than the two units with venting and all.
I don't see how an indirect will take up more space than a water heater. Even a wall hung tankless needs space in front for service clearance.

You assessment of combi units is correct.

notsoepic

the_donut said:

A modulating boiler with a high enough turn down ratio would work fine without short cycling. Depends on your cold water supply temp though. Here in Indiana water temp is mid 40’s and requires 160,000 btu/hr to get hot enough water for 2 gpm. 5gpm is a lot. Are you taking 2 showers and doing the dishes at the same time?

The two modulating boilers that looked like the closest fit were the Lochinvar Noble 150 and Navien NCB -- both give around 3.6 GPM on DHW and with a 10:1 turndown minimum BTU around 15,000.

Our current showerheads are 2.5 GPM --- I know not efficient and not hard to fix. But this got me to wondering if a- equipment costs are roughly the same (or quite a bit cheaper if I were to go with the Lochinvar unit) and b - I could get ~9gpm with a tankless heater for DHW ----- is there any significant advantage to a combi over the dual heater/boiler set-up?

notsoepic

Zman said:

A condensing boiler with indirect is more efficient and less expensive than the two units with venting and all.
I don't see how an indirect will take up more space than a water heater. Even a wall hung tankless needs space in front for service clearance.

The idea I had was that the tankless and boiler would be in two different adjacent closets -- with a plan that when not being serviced I could keep things like bikes/skis/etc. (I live in Colorado) in the same room, allowing for some pretty significant efficiency in the use of space. Though I could also see some obvious space savings of consolidating into a single utility closet. My primary hesitation to doing this is that 1- I didn't know if there were any benefits (based on your response it sounds like there are some benefits) and 2- based on my (very brief) review of indirect set ups the up-front installation seemed quite a bit more complicated for a DIY project (based on the idea that in my original plan my installer would install and hook up tankless DHW and install the boiler while I would install and hook up the manifolds, zones and run the pex) . Is this a fair assessment or would the complexity be the same for either set up?

Zman

Which tankless are you considering? What altitude do you live at?
The ratings on all gas fired appliances need to be adjusted for the altitude and cold ground water temps.
Even in Denver, for an instantaneous to make 9 GPM, it would need an input rating over 400,000 btu.

lchmb

When you say 70k condensing boiler, is it capable of modulating? remember radiant system's have long run cycles and if you have an off/off system then you will maintain temp reducing your savings. although a condensing boiler may be to big for the load, with out door reset and proper programming it will only run to that btu load on extreme cold days or hot water calls.. As far as using the space in front as storage, I recommend against this. If there is an issue with the boiler and you need to access it quick (water leak), it could cause issues.

On the other side.. condensing boiler's need to be serviced yearly. The cost is greater on this type of unit than your old cast on/off system. So their are good and bad to both..

notsoepic

Zman said:

Which tankless are you considering? What altitude do you live at?
The ratings on all gas fired appliances need to be adjusted for the altitude and cold ground water temps.
Even in Denver, for an instantaneous to make 9 GPM, it would need an input rating over 400,000 btu.

I'm i n Denver and was looking at various Takagi and Navien tankless heaters around 199,000 BTUs-- I was looking at the advertised ratings but realize I need to see what it can do at altitude with our winter temp rise. Thanks for the head up!

notsoepic

lchmb said:

When you say 70k condensing boiler, is it capable of modulating? remember radiant system's have long run cycles and if you have an off/off system then you will maintain temp reducing your savings. although a condensing boiler may be to big for the load, with out door reset and proper programming it will only run to that btu load on extreme cold days or hot water calls.. As far as using the space in front as storage, I recommend against this. If there is an issue with the boiler and you need to access it quick (water leak), it could cause issues.

On the other side.. condensing boiler's need to be serviced yearly. The cost is greater on this type of unit than your old cast on/off system. So their are good and bad to both..

Yes I was looking at 10:1 modulating models, which I guess should be enough to eliminate short cycling.

Zman

Adding an indirect to a condensing boiler is far easier than installing a stand alone tankless. A tankless needs a large gas supply as well as it own vent and CA. Not to mention, you now have 2 appliances to maintain.

There is no reason the indirect needs to be in the same room as the boiler. An adjacent closet should not be an issue.

A 90% efficient 199,000 unit at 5,000' with an 80 degree temp rise looks like this 199,999 x .9 (alt derate) = 179,100 x .9 (efficiency derate) = 161,190 / 80 (delta t) / 500 (constant) = 4.03 GPM.

You can use the same math to evaluate your condensing boiler's ability to recover an indirect tank. With the indirect you have stored hot water so you don't need as much continuous capacity.

DZoro

Lochinvar noble, stagger showers
D

notsoepic

Zman said:

Adding an indirect to a condensing boiler is far easier than installing a stand alone tankless. A tankless needs a large gas supply as well as it own vent and CA. Not to mention, you now have 2 appliances to maintain.

There is no reason the indirect needs to be in the same room as the boiler. An adjacent closet should not be an issue.

A 90% efficient 199,000 unit at 5,000' with an 80 degree temp rise looks like this 199,999 x .9 (alt derate) = 179,100 x .9 (efficiency derate) = 161,190 / 80 (delta t) / 500 (constant) = 4.03 GPM.

You can use the same math to evaluate your condensing boiler's ability to recover an indirect tank. With the indirect you have stored hot water so you don't need as much continuous capacity.

Thanks, this is helpful! Is there a rule of thumb for calculating minimum input to prevent short cycling? I plan on two, roughly evenly divided zones with similar heat loss (~26k each).

delta T

I would recommend as others have said installing a ~85000 input 10:1 condensing boiler, with a 50 gallon IDWH and a mixing valve on the outlet of that, tank temp set at 140*. A good quality IDWH (Lochinvar squire for example, look up SIT050) has a lifetime warranty in a residential system and requires practically no maintenance. Having storage, especially storage that is mixed down from a higher temp will mean you have a very large first hour draw. I put this exact setup in a house that had three teenagers and had absoulutely no complaints. ODWHs require lots of maintenance, and usually retrofitting to install one can be problematic as you will probably need to up size your gas line, run a seperate flue, add a dedicated electrical circuit etc...

DanInNaperville

It may not be necessary to design to 5gpm of hot water. Keep in mind that the hot water is mixed with cold before it gets to the shower head. If it's 60/40, your 2.5 gpm showerhead would use about 1.5 gpm of hot water (with 55 degree "cold" and 120 degree "hot" from a conventional WH mixed to 94 degrees at the showerhead). If, upstream of the shower valve, you're mixing down 140 degree IDWH output, then you'll only use about 1 1/8 gpm of hot water to get 2.5 gpm at the shower head. 5 gpm would run 4 showers with a 1/2 gpm to spare.

udhoop

We have a Lochinvar Noble combi and it works for us. It is primarily just my husband and myself (no kids). We live in northern MI and have a pump for water. I have no idea what our ground water temp is but it doesn't seem to affect our boiler heat.