Hydronic System in Small Space — Need a Buffer Tank?

evelyntoronto

We’re looking to install a hydronic in-floor heating system in our basement in conjunction with an upcoming renovation (we’re in Toronto, Canada). Our space is small (~500sf), and so our HVAC specialist recommended a buffer tank, as he’s worried the minimum BTU of the combi boiler will be higher than what’s needed for the space, creating a cycling problem with the boiler, burning it out prematurely. I’m inclined to defer to his judgment, but we’re trying to minimize space consumption, so I want to make sure it’s absolutely necessary. I've tried researching the topic, and what I've read suggests that we may need 30BTU per SF, or 15,000 BTU for our full basement. If this is the case, I would think we’d be fine without a buffer tank, as I believe there are combi boilers with a minimum BTU in the 15k range. Is the 30BTU per SF estimate sound? How should I be thinking about this project? As background, we’ll only be heating the basement with hydronic in-floor heating — the rest of our house is heated with forced air (gas furnace). Any and all perspectives appreciated!

EdTheHeaterMan

actually you will need to have someone do a load calculation for that space. Being a basement, there may be a percentage that is below grade. That looses heat differently. Also are you going to insulate below the floor tubing? that may change your 30/SqFt to 20/SqFt. And that number is only needed when the outdoor temperature is at its coldest. You need much less heat when the outdoor temperature is only 45°F. so maybe 5 BTUh.SqFt. The buffer tank need not be that large, and it can fit under or over the heater location, or even up on a shelf out of the way somewhere. Just make it easy to service.

hot_rod

If you decide it is needed, a 20 gallon tank could  fit below a wall hung boiler

GroundUp

The system can be manipulated to put more BTU into the slab than it needs in shorter bursts rather than less BTU over a longer period, but that's a pretty poor practice and the efficiency as well as comfort level will suffer. Most new construction basements I do come in at less than 10 BTU/sq ft, so long story short, yes you should have a buffer tank.

Jamie Hall

30 BTUh per square foot is rather on the high side for a floor -- you may find the floor is unacceptably warm under foot.

Rich_49

You need a buffer tank .

EBEBRATT-Ed

Whatever your load is that is on a design day. If you had a boiler that would modulate down to 15,000 btu and you have a 15000 btu load on the coldest day when it is warmer out (but you still need heat) your load will no longer be 15000 btu and the boiler will short cycle.

evelyntoronto

Well, the answer seems pretty unanimous - thanks all for the perspective!

hot_rod

What size combi are you considering? In an area that sees cold incoming water in winter months, be sure to look closely at the output charts. Some show a 70 degree rise, some a 77.
Next how much DHW are you expecting. A 120 would give you a couple gallons a minute. If you need more DHW capacity, you might look at a 150 or larger

50 square feet with maybe a 10 btu/ft load is 5000 btu/ hr on a design day. Gosh a small 2 kw electric boiler might be more appropiate. Then the correct size tankless for DHW

A load calc for the basement could pin down the actual load better. Pretty rare to see a 30 btu/ ft load in a basement. Unless there is some tube outside😳

Teemok

I'm assuming this is a high mass floor. Insulated and 2.5" minimum thickness pour. The amount of cycling the boiler would see has a lot to do with spaces heat loss and what your thermostat is asking for. Floor coverings can change a lot. A single 500 sq ft zone will ask for more than 15k/hr on the cold start up. Short cycling lock out works wonders for a situation like this. Pulsing calls for heat with the thermostat programing is another option for control. A buffer tank is the best design call for optimum comfort. DHW need and how the space is going to be used would be a factor in decision making for me, along with mechanical space and budget. There are a few creative ways to meet that load well.

bio_guy

What are you ditching that you are using for DHW now?

bio_guy

Just use a high efficiency tank (that's your buffer) water heater and install a flat plate heat exchanger to get heat and DHW out of the same unit.

JMWHVAC

If short cycling "burns out the boiler prematurely" that means it will burn out anyway as the true definition of short cycling is combi and tankless units starting and stopping every time someone uses a squirt of hot water. I did buffer tanks on several systems but I would much rather use a unit with sufficient control settings to mitigate short cycling.
Short cycling lockout was mentioned, add to that the burner on/off differentials, ramp up time, etc., and you can tailor its operation to an amazing degree. Not all manufacturers have the settings available that I like to use.

Teemok

A friend with a very limited budget asked me to help him get heat to a 500-600 sqft single high mass zone cabin. I know it's blasphemes and that haters will hate but a Rinnai RUCS65IN 10,300 min btu (my friends was a TKjr.) with a stainless 008, DHW expansion tank, Honeywell air sep. and a TT "low water cut off" AKA a pressure switch and 5 short loops. It's a 3gpm value. I know it's not the best but the poor make compromises and it works. Way out lasted a few munchkins I've known. My delay on break anti-short cycle control idea was not needed. I've used a Ranco on the return with it's anti cycle feature (supply water a bit hotter than normal becomes a pulse system near room set point) Granted people see that kind of limited simple set up and think they can ad zones or heat a big house with it and they are wrong.