Help sizing boiler for tankless replacement

2000xpsd

Need some help sizing a wall boiler to replace a tankless water heater thats used in a closed loop radiant floor system. Yes I know tankless aren’t designed for radiants …blah blah blah…but I just slapped this system in to get it up running quick and the tankless was free so in it went. Actually it’s been running for 6+ years and has been fine…other than my delta t sucks because of the tankless restriction but other than that it runs pretty good. But I think i pushed my luck far enough with it and it’s time to get it out. I was looking a a wall heat only Weil-mclain aquabalance series. They make a 80,120 and a 155k btu. I’m running a takagi th3jdvn with is 160k btu. Tubes are 4 loops of 350’ in concrete garage floor, floor and walls are all well insulated, run a taco vfd pump. I have all my heat loss calcs somewhere but i cant find them right now…pretty basic/simple system tho…im thinking 80k will be fine but get a 120k to be safe. Any input would be appreciated

Hot_water_fan

Hopefully you can find the heat loss. It’s unlikely you need more than 40kbtu, so the 80 will work just fine. if you’re invested, tell us how much gas you used last year and it’s easy to ballpark it from there. 

hot_rod

The heat load and the DHW load?
.Good chance the DHW load is bigger and that is what you size to. If you have cold I coming water below 45F, you may not want to go below a 110 or 120K. Turndown for heating is a about the same as an 80k

If the heat load is small, below 60k buy a combi that has all the nice features like boost, ramp delay, fire rate limiting, etc. Then it will handle micro loads nicely and still provide 3 gpm or more DHW

2000xpsd

Found some heat loss data…only have 1 gas meter and its tied to 2 other furnaces and stuff, so who know the gas usage. Garage is detached, no hot water…closed loop system

2000xpsd

Hot_water_fan

There’s your answer: smaller is better. 

2000xpsd

Cool, 80k it is. Any recommendations on brands, models etc??

Hot_water_fan

Installer and part availability is more important, the specs are pretty similar. 

hot_rod

Here is a 110 Lochinvar, as you can see about 2.6 gpm with a 77°. temperature rise.
typical shower flow is 2.5 gpm, 2 gpm on water sense heads.

if you can find an 80K, combi, it may not keep up with a single shower flow. It will be under 2 gpm at 77° rise.
Turndown on heating is 11K on this Lochinvar, so it covers that 29K load nicely.

How much DHW flow is enough for your lifestyle? The DHW is the load you need to size for.

Hot_water_fan

@hot_rod I think it’s central heating only

2000xpsd

Yeah heat only…closed loop glycol 

hot_rod

2000xpsd said:

Yeah heat only…closed loop glycol 

If 29 K is the total load, consider the Lochinvar Knight 55,000. it has a great control and also ships with a variable speed boiler pump to help keep it condensing.

I have one in my shop, very quiet and efficient. The bottom circ feeds a flat plate HX for instant DHW.