What size boiler based on heat load

[79rallysport]

So I'm installing a closed hydronic radiant floor heat in my new shop (28'x30'x12'). I did a Manual J to calculate the heat load. The heat load is 21,095BTU/hr.

Base on the heat load, how much boiler should I get? Do I get one that is 100% of the heat load? 120%? 150%? 200%?

If it matters, I'm planning on using a Seisco tankless hot water heater, 1 zone, 3 or 4 loops (haven't decided). Seisco makes a number of different size boilers, 7kW, 9kW, 11kW, etc.

 

[Jackfre]

A Man J reflects "design condition". What that means is that on the coldest day of the year the unit, itf properly sized will run continuously but maintain 70* in the space. 3,414 btu/kw. I'd get the 7kw unit

 

[JCByrd24]

A couple of notes based on your low heat load number:

Given that this is a shop, is there an overhead door? The R-value ratings of overhead doors are often grossly overrated, as they are the value of the center of the panel itself, not counting the thermal bridging at the joints/edges, and more importantly, their leakiness. I'm not sure what number you used for air infiltration, but make sure it's conservative to account for an OVHD door.

Given that you are using radiant I would assume you're going to heat pretty consistently vice just when you're out there. Which addresses my next point that a "right sized" system will not give that quick warm up that one oversized will. As an example I have a 30K unit heater and it's not exactly a huge burst of heat. I run a 5kW electric as well for the first half hour or so when it's cold out (I don't heat the shop all the time).

A slightly oversized electric boiler will not have the downsides that for example an oversized gas or oil boiler would, so don't worry too much about going a little conservative. On the flip side however, your floor will only transfer so many BTU/hr, so you don't want to overshoot what your floor is capable of. Obviously the cost of the unit and install go up with kW.

Weigh your confidence in your calc against your tolerance for the shop being a little cooler when it's really cold out.

 

[Highbeam]

That does sound low on btu required. Usually we see much larger boilers for your space size.

Electric boilers can short cycle with no efficiency loss. Really, you can't hardly compare radiant slab heat to forced air heat when considering btu because the thermal mass takes away anything called "rapid".

 

[79rallysport]

Heat load sounds low? OK I'm new at this. Maybe I did something wrong in the calculation. What type of number would you expect?

The shop does have two 8'x8' overhead doors and one man door.

 

[Jackfre]

I don't think that number is low. Check the numbers, but once you have done so, go with the number.

 

[Highbeam]

Your calculation certainly assumed a certain number of air exchanges per hour. You had to tell it how leaky your building is. With two overhead doors your building is very leaky. When it is windy, those doors let lots of air in. Standard installation of those doors leaves plenty of gaps and when the wind blows they get bigger.

Also, what degree difference did you specify. If you want to keep it 70 inside when it is 0 degrees outside that will take way more heat than keeping it 50 inside when it is 32 outside. Perhaps that is why your load is so low.

I expect double your btu load.

Check the "post pics of your radiant floor heat system" thread for a good selection of example boiler installations. To compare size of building, climate, and boiler size.

 

[Shop Specialties]

I do not think his number is low for in-floor heat in that small of a shop.

 

[79rallysport]

I included

.35 air changes/hr.
70*F room temp
4*F outside temp
82*F surface temp of concrete (not an input to the heat load calculation, the is a result of the calculation.
20*F delta from supply to return temp of water/glycol mix


So I still cannot find an answer to my original question anywhere on the net. When you know your heat load (BTU/hr.), how to size the heat source (BTU/hr.)? Maybe I'm making this more complicated than it needs to be. I understand that a bigger heat source will heat the room up faster than a smaller heat source. But...if I have a heat load, does the heat source need to be 120% bigger, or 150% bigger...or is it just 100%?

 

[Highbeam]

An electric boiler can be oversized and it won't matter, it will cycle on and off if needed with no damage to the boiler or to efficiency.

Since you specified the smaller single stage electric boilers you can know that they will cycle if oversized and that is fine. You determine the delivery of heat to the floor by flow rates. The boiler will try to maintain the temperature in the boiler. A right sized boiler will have less cycles. An undersized boiler will stay on 100% and never get the job done.

Size for over 100% but consider cost of the boiler and wire. If it is cheap, I would rather go up a size.

 

[mygarageone]

I do not think his number is low for in-floor heat in that small of a shop.

It isn't . My shop is identical to this one and my loss was 23,000, I have 2 ovh doors and 2 man doors .
Not a bit of problem and we have had -30 for several days. For 2 winters now .
My system was designed at a 70 TD .

Heat loss calculators are designed with much fudge .