Radiant slab heat pump and manifold sizing

[asmith1923]

I am building a Barndominium with 2 zones of 1/2” pex radiant in slab heat.

First zone 2490 SF
8 loops of 300’ and 1 loop of 200’
10 loop manifold
Estimated to need 68475 BTUs/HR

Second zone 400 SF
2 loops of 250’
2 loop manifold
Estimated to need 9100 BTUs/HR

I need help figuring out the circulating pump sizing. I know nothing of friction loss and head pressure. Can anyone help me here.

 

[Firebrick43]

You might want to break the 8 zone loop up into two itself. Keep all the loops exactly the same length and you can save a lot as you won't have to have a balancing valves on the manifolds.

Then do you want to run one pump and use a thermostatic valve and zone valves?

Or have a primary loop pump and a pump for each zone on your secondary loop?

Is this square footage shop or living space?

Insulation underneath?

Where do you live?

Do you really mean heat pump? or do you mean boiler?

 

[asmith1923]

The 400SF is living space and will be heated to 68.
The 2490SF is workshop and will be heated to 62.

The manifold have loops flow adjustments, so the one shorter loop isn’t the issue.

2” insulation under living space and border around workshop. 1” insulated border inside the 2”. No insulation in the middle of the shop floor.

I live in northern Virginia and will be using 199kBTU LP fired combo boiler for DHW and Heated Floor.

I need to know what circulator pump id need for each of the two zones.

 

[walrus]

That boiler sounds big to me. Who sized it?

 

[86turbodsl]

The workshop is almost exactly my shop size. 8 loops, 300ft each. I was able to do it with a single taco 007. The boiler is way oversize if you have code insulation in Virginia. I am in Michigan and needed only about 60,000 btu with code legal insulation and 2" under slab.

 

[asmith1923]

[SIZE=4]86turbodsl[/SIZE] I just read a bit of your barn thread - seems I'm building what you've already got. Hows your 60K BTU boiler holding up? Is it a combi boiler heating DHW as well? ​

Mine will be LP heating the floor water for 3100' of 1/2" PEX floor heating in 300' loops and on the DHW side, a full kitchen, full bath and a slop sink. The engineers at Radiantec did all the load calcs and he's the one that told me I needed a 199K BTU boiler to handle everything. They calculated I'd need:
77575 BTUs for the floor heating
Approx 120k BTUs for DHW (About 6.5 GPM at 70 degree rise to cover a shower, dishwasher, clothes washer and faucet at the same time)

This should put my combi boiler at 199K BTUs. Seems right to me.

I heard back from the Radiantec engineer and he said the Grundfos 26-99 circulator pump could handle 2600' of 1/2" PEX just fine.

 

[yeldogt]

The boilers are typically sized for making hot water .... but you don't just add them together to come up with the total. The key there is the turn down of the boiler and it being able to go low enough to run the floor on mild days.

Do you have any other heat? How is it being cooled?

Slabs are slow to heat -- so in the shoulder season that can be a problem especially with going out to design max 300 feet. Especially with only 400sf of living space ...

With propane you have to design in efficiencies ---propane is expensive. I would rethink no insulation in the middle of the shop.

Anyplace for a HP water heater and just use a small boiler for heat.

You don't need a big pump ---- get an alpha II and it will have no problem You will need primary/secondary ....

My place in PA is over 4k and has over 30 loops and the alpha has no issues

 

[Jackfre]

Turbodiesel, your math is off for the dhw. The formula is gpm x delta T x 500=BTU. Accordingly your 120K unit will provide 3.5 gpm at 70* rise.

 

[asmith1923]

The boilers are typically sized for making hot water .... but you don't just add them together to come up with the total. The key there is the turn down of the boiler and it being able to go low enough to run the floor on mild days.

Do you have any other heat? How is it being cooled?

Slabs are slow to heat -- so in the shoulder season that can be a problem especially with going out to design max 300 feet. Especially with only 400sf of living space ...

With propane you have to design in efficiencies ---propane is expensive. I would rethink no insulation in the middle of the shop.

Anyplace for a HP water heater and just use a small boiler for heat.

You don't need a big pump ---- get an alpha II and it will have no problem You will need primary/secondary ....

My place in PA is over 4k and has over 30 loops and the alpha has no issues

Thanks. Too late for the floor insulation change, tubes are done. But I will look into a heat pump option, I haven’t been too excited about propane anyway.

Concrete in 2 weeks!!!

 

[fitter30]

1/2" pex is 32btu per ft. 86200 btu and 16000 according to your loop lenghts. Using your numbers 68475 = 6.9 gpm at 20° temp drop 9100 = .91 at 20° td
.03x 300 = 9.0' head + 2' for piping and extras Taco E series 008 pump
.03 x 250 = 7.5' head + 1.5' for piping and extras.Taco E series 005 pump
Glycol and the correct inhibitors has to be minimum 30%. Glycol has to made for boilers or chillers. Hard freeze protection is not necessary can be slush protection. Boiler glycol has temp chart for slush. For air eleminator recommend Spirovent Jr.
System piped primary/secondary with pex piped reverse return.

 

[asmith1923]

1/2" pex is 32btu per ft. 86200 btu and 16000 according to your loop lenghts. Using your numbers 68475 = 6.9 gpm at 20° temp drop 9100 = .91 at 20° td
.03x 300 = 9.0' head + 2' for piping and extras Taco E series 008 pump
.03 x 250 = 7.5' head + 1.5' for piping and extras.Taco E series 005 pump
Glycol and the correct inhibitors has to be minimum 30%. Glycol has to made for boilers or chillers. Hard freeze protection is not necessary can be slush protection. Boiler glycol has temp chart for slush. For air eleminator recommend Spirovent Jr.
System piped primary/secondary with pex piped reverse return.

Fitter30 - wow, I should’ve had you design my system! Dumb question, your calculated 9’ of head for a 300’ line, does that include the entire zone of 9 loops or is that 9’ per loop?

I have a 10 loop manifold, 8 - 300’ loops and 1 - 210’ loop

 

[Jackfre]

You are at a point where you have time to decide on the piping lay-out. I encourage you to order Dan Holahan’s, “Primary-Secondary Piping”. It is short sweet and easy to understand for the layman.

 

[yeldogt]

will add:

load will dictate flow rates .... ideally under 4gpm w/ 1/2 pex

Flow through the ID of the pipe using the longest loop will dictate the head. flow is additive -- head is not.

 

[fitter30]

Fitter30 - wow, I should’ve had you design my system! Dumb question, your calculated 9’ of head for a 300’ line, does that include the entire zone of 9 loops or is that 9’ per loop?

I have a 10 loop manifold, 8 - 300’ loops and 1 - 210’ loop

9' is total +2' pipe and extras = 11' head

 

[yeldogt]

The Alpha II is around $200 .... is fully automatic .... solves every hydronic problem and can do a head of 19.

There is zero thought required. As I said -- I have 2200 ft of 3/8 retrofit and about 3000 ft of 1/2 PALP in my new build. 4 manifolds w/ two of them remote from the mechanical room.

Ideally -- not having loops out to the max of 300 is IMO better as it makes for quicker response and does lower the head. But -- you have what you have. I always add a loop if near max -- that cuts them all down ... buying a one larger manifold is pennies.

Head is a strange thing. A proper length manifold of 3/8 has the same head as a proper length of 1/2 ---- you have to make sure you have the proper supply sizes on the secondary .... Pex is restrictive vs other types of hydronic so the secondary loop has to be designed properly. They make less expensive pumps for the primary --- you still want a newer type as they use so much less power.

A typical brute or 007 can use near 100 watts and that is a combined 200 watts running .... newest advanced pumps use a fraction. When my system is running it's using 14w and 24w .... so 38watts 24/7 in the heating season vs the almost 200

 

[86turbodsl]

Turbodiesel, your math is off for the dhw. The formula is gpm x delta T x 500=BTU. Accordingly your 120K unit will provide 3.5 gpm at 70* rise.

I didn't say anything about DHW. I said i moved 8 300ft 1/2" pex loops with a Taco 007.

 

[Jackfre]

My mistake. Sorry for that.

 

[Firebrick43]

​

I heard back from the Radiantec engineer and he said the Grundfos 26-99 circulator pump could handle 2600' of 1/2" PEX just fine.

Radiantec is the last place you should be dealing with, bunch of crooks.

I agree with yeldogt on the alpha pump. My I use a vavle zone approach which historically had issues as zones closed and opened with flow rates. I put the pump in constant pressure mode and it self adjust depending on demand. I two only see about 18-20 watts useage, 22 watts if everything opens up.

 

[asmith1923]

The boilers are typically sized for making hot water .... but you don't just add them together to come up with the total. The key there is the turn down of the boiler and it being able to go low enough to run the floor on mild days.

Do you have any other heat? How is it being cooled?

Slabs are slow to heat -- so in the shoulder season that can be a problem especially with going out to design max 300 feet. Especially with only 400sf of living space ...

With propane you have to design in efficiencies ---propane is expensive. I would rethink no insulation in the middle of the shop.

Anyplace for a HP water heater and just use a small boiler for heat.

You don't need a big pump ---- get an alpha II and it will have no problem You will need primary/secondary ....

My place in PA is over 4k and has over 30 loops and the alpha has no

 

[asmith1923]

I’ll be doing a mini split ducted ac system zoned. I am thinking to zone it for upstairs and down, because I may need a little extra heat upstairs as I’ve only got radiant in the slab. I assume I can get heat out of those mini split heat pumps? Do they have gas backup like the regular units?

 

[Firebrick43]

If your doing ducted, don't do mini splits. Its not as efficient(in ducts) nor cost effective nor is there gas back up. Stick with a central system if you are going to have ducts.

But for 400 sqaure feet there is nothing wrong with a standard minisplit head. In fact if you actually insulate the apartment area well and have an open stairwell one unit mounted upstairs will work, Probably even a small 18K unit will be overkill. Cold air falls so it will cool the downstairs as well. I know of one house that is 3000 two story well insulated with a single mini mounted in the top of the stairwell that cools the whole house to an acceptable degree. It is an open floor plan house.

Better minisplits such as the mitsubishi hyper heats have heat down to -15 degrees.

Also if you are worried about heat, why not add heating loops to the bottom of the upstairs floor when you build it. Just have and cap a port on one of the heating manifolds so when you construct it you can place it. Frame and wire/plumb the upstairs first and staple up the tubing to the underside before putting in the ceiling down stairs.

 

[asmith1923]

Thanks. I did design the stairwell to be open to help the heat get upstairs. I could totally put a unit right against the ceiling in the stairwell. Plenty of room there. You think a single ductless unit will work for AC and backup heat for the whole apartment? It’s 400 SF downstairs (kitchen, bath and mechanical) and another 500 SF upstairs (LR & BR).

I could easily change out my 2 zone manifold for a 4 and add tubes under the 2nd floor. Worth considering.

 

[Firebrick43]

Thanks. I did design the stairwell to be open to help the heat get upstairs. I could totally put a unit right against the ceiling in the stairwell. Plenty of room there. You think a single ductless unit will work for AC and backup heat for the whole apartment? It’s 400 SF downstairs (kitchen, bath and mechanical) and another 500 SF upstairs (LR & BR).

I could easily change out my 2 zone manifold for a 4 and add tubes under the 2nd floor. Worth considering.

Might want a 24k unit for 900 feet but yes, one can. I have an 18k unit doing 1450' in a super insulated icf build with radiant suspended slab. Some rooms maybe warmer/cooler than others but if you keep the doors open it wont be bad. I have been in poorly ducted Central systems that were worse. Insulation is really the key.

The mini split heating is ideal for shoulder systems. When nights dip down into the 40's but days are in the 60's and you really don't want to fire up the slab yet as you will overheat in the daytime. I don't fire the slab up until late October. I rarely use the mini in the winter time even if it will provide heat.

 

[Ak Jim]

So you are running a radiant slab with no insulation under it or did I misread what you said?

 

[Firebrick43]

So you are running a radiant slab with no insulation under it or did I misread what you said?

Me or the OP?

 

[Ak Jim]

Me or the OP?

The OP. Sorry if I wasn’t clear in my post.