Hydronic floor water flow

Gta · November 23

Hi I have a house with radiant floor heat , it has 11 runs of 300 ft . It’s not zoned so it’s all or nothing on or off . The pump that runs this zone is a bell and gossett NRF-25 the zone valves are currently wide open and show about .75 gal per min each with the pump in high speed . Is this correct? I almost feel the pump is too small as the far end of the house seems too cool thx

Jamie Hall · November 23

What's the delta T on the runs?

Gta · November 23

it’s about 12-15 f

Jamie Hall · November 23

That's about what one would expect — and want. You're getting around 6,000 BTUh per run, or call it 65,000 BTUh for the entire set of them.

The problem isn't a flow problem, but most likely is something about the way the runs are laid out. In the nature of things, the heat delivery and temperature in any one run will be greater at the beginning of the run than at the end.

Do you have any plan for how the piping runs are laid out?

Another factor with radiant floors is the floor material and any floor coverings. Do those differ in various locations?

Gta · November 23

it’s actually 15-19 f

I just rechecked.. here are the plans ..

I have 3 pumps one for the main house / one for the garage and one for the bonus room

Gta · November 23

oh house is 2000 sq ft

Garage is about 1600 / bonus room is 800 ish snd change … house is stone tile garage concrete of course and bonus room is a wood cover 😡 I know it’s a poor choice for radiant heat . lol spent 6 years in court after I sued the original contractor 🫣.

DCContrarian · November 23

Has the system ever worked well?

hot_rod · November 23

what is the supply temperature?

.50-.65 is typical for 1/2” loops, so .75 is a good flow rate with that circulator

HomerJSmith · November 23

Not the way I would have done the tube layout, but as my Dad said, "What is, is. Life is a big compromise."

I would like to know what kind of manifolds are used. What the kind of zone valves are used and what the .75 GPM refers to and how was it determined? What pumps manifold #2?

Where are you located? It makes a difference in what you can get away with. (compromise)

HomerJSmith · November 23

" lol spent 6 years in court after I sued the original contractor 🫣." hmmmm

I was called out on a job and the customer during the conversation mentioned that he sued the previous contractor. I didn't go into the details, but it was a big red flag for me the size of the Queen Mary. I passed on it and gave the lead to another contractor, informing him as to why. (professional courtesy)

"May your life be filled with Doctors and Lawyers." An old Chinese curse. (not)

Gta · November 23

the supply temp for the main zone of 11 is about 90f when it runs the smaller zones like the garage at 5 runs or the upper level at 3 runs it will go over the 100 mark

no I have never been happy with it , here is the manifold pic you asked for , a pic of pump set up

Gta · November 23

Gta · November 23

Also I should mention it’s a air to water heat pump

Gta · November 23

it used to be a 23 kw electric boiler ! I changed it bout few years ago to this cold climate air to water unit it’s much much better 👍😁

Gta · November 23

up in Canada … this is my heat source all the way down to -35c … when it goes beyond that there is a 5 kw back up to help if needed

DCContrarian · November 23

Flow is much less important than water temperature.

Back of the envelope: 300 feet of tubing means 200 square feet of heated floor at 8" centers. 11 coils means 2200 square feet of heating. The heat pump is nominally 58,000 BTU/hr, that's 26 BTU/hr per square foot of heated floor. To achieve that you need a floor temperature 13F above room temperature, so a floor temperature of 83F for 70F indoor. I don't know what your floor construction is but the first question I would have is whether your water temperature is hot enough to get the surface of the floor to 83F. Because if it isn't it doesn't matter what the flow is. You can try to calculate what it should be based on the construction, that's what should have been done pre-construction, but once it's built the thing to do is to take a thermometer and measure it.

The entire heated area is 4400 square feet, so the total capacity works out to 13 BTU/hr per square foot. Considering that almost half of the area is garage and you're in a place that gets pretty cold that sounds somewhat low. That would manifest itself in the system working fine in milder weather but being unable to keep up when it gets colder. I'm not familiar with the heat pump you have but you also want to check its performance curve to see what the actual output is in cold weather.

Was a Manual J heating load calculation done as part of the design? Often installers just fill the room with tubing and assume that's got to be enough, that isn't always the case. The reason I asked a few posts ago whether the system had ever worked well is if it wasn't properly designed to begin with it's going to be a long road to get it working.

DCContrarian · November 23

If the leaving water temperature is 100F and you're seeing 12-15F drop, that puts your return water temperature at 85-87F and your average water temperature 93-94F range. Just my opinion, but I think you're going to have trouble achieving a floor temperature of 83F with water at 93F.

Increasing the flow is going to have limited effectiveness. Let's say right now you have a 12F drop, average water temperature of 94F. At 5200 BTU/hr from each coil you'd expect a flow of 0.9 GPM. To get your average water temperature to 97F you'd need a drop of 6F. That would increase your output by 12% but to get that you'd have to increase the flow by 125% to 2.25 GPM. You quickly hit diminishing returns on increased flow.

Gta · November 23

the entire house is ICF construction.. I agree that it’s just barely able to keep up . I wish I had the individual rooms zoned out … the full 2000 square ft at once I’d just too much to heat at once.

it does it but the run time is pretty high to achieve the desired temp 68-69 is about max or she’s running constantly

Kaos · November 24

With a heat pump, it is actually better not to zone. You want the full flow from the heat pump to go through your emitters and adjust house temperature by changing the reset curve on your heat pump.

If your heat pump would be keeping up with your load, you could fix the cold zone by reduce the flow through the rest but this won't work in your case. Maybe you can look at areas of the house that are less used and turn down the flow to those zones a bit.

What is the design heat load of your place (minus exterior buildings)? Most air to water heat pumps loose about 40% to 50% their nameplate capacity in very cold, so maybe the unit is simply just a bit too small.

P.S. is there a thermal break around your slab edge or are they connected to your footing? Also how was the rigid under the slab done and where are the pipes?

hot_rod · November 24

I think all the a2whp now have variable speed drives on the compressors The circulator pump also modulates. I have watched my 2 ton VitoCal modulate from 3.2 to 8 gpm based on the load. With that function and a small buffer, zoning should not be an issue

That ICF construction should present some very low heat load numbers. Ive seen some in the single digits

DCContrarian · November 24

https://forum.heatinghelp.com/discussion/comment/1822043#Comment_1822043

A heat pump should run continuously with all but the lowest loads. They modulate, they can go down to about 25% of rated capacity and run continuously. That's how they're happiest.

If it's running continuously and meeting the set point, it's operating properly.

What people often find in well-insulated homes is that the under-floor heating doesn't give the "toasty toes" feeling they were expecting. In mild weather your system might be able to keep the house warm with floors that are not perceptibly above room temperature, maybe a couple of degrees.

Gta · November 25

100 percent true …,the mild weather outside makes it feel coolish in here for sure

hot_rod · November 25

75° floor temperature in a room at 70° will provide 10 btu per square foot. That is a very possible ICF home heat load.

At some point radiant floors heat is not the best match for low load homes. If you are expecting warm floors.

One option for a radiant slab in a low load home is to not install tube for 2-3' around the perimeter or outside walls. Just tube the areas where you walk or have furniture.

Or just use panel radiators.

DCContrarian · November 25

"At some point radiant floors heat is not the best match for low load homes. If you are expecting warm floors."

Too late for this house, but my current preference is not to size heated floors for design heat loss, but to limit them to maybe one quarter of that. That way where you have heated floors you can have them on full blast for almost the entire heating season without overheating the space. Use a 2-stage thermostat so that the heated floors always come on first, and whatever other heat only comes on when the heated floors at full blast don't heat the space.

The other heat can be panel radiators, but if you live in a climate where is cooling is needed in the summer you can use the air handler you'll need for cooling.

GGross · November 25

To the OP if all of this is a single zone, is it possible that your thermostat is just located in an area with less heat loss than the area that is too cold? If everything else seems to be in order maybe the simplest answer is the correct one

Gta · November 25

the t stat is located in the middle of the interior wall I’ve changed it to be sure .
The floors don’t ever feel warm but the house is up to temp . My concern is the long run times but it’s a low temp system so maybe that’s normal?

Gta · November 25

here is what I’m talking about.. see the almost constant runtime? I guess got what it’s heating it’s doing decent

DCContrarian · November 25

That looks to me like it's cycling on and off as I would expect.

"The floors don’t ever feel warm but the house is up to temp . My concern is the long run times but it’s a low temp system so maybe that’s normal?'

With your design that's what I would expect if the system were working as designed.

The one thing that is unexpected is that when it's on it seems to always be drawing 5.4 kW, which according to the nameplate in the picture you posted above is the rated capacity. Most air to water heat pumps modulate quite a bit, the run continuously but they adjust the compressor speed to match the heating load. So rather than cycling on and off at 5.4kW, I'd expect it to be not cycling but running at a lower load.

Now, the unit you have is not familiar to me, it may just be that it's not a modulating unit. In any case it's not going to be a big difference in electricity used.

Hydronic floor water flow

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