Where do I go from here?

aarmitch

Good morning!
I need some help figuring out how to heat my radiant floor. Here's the details:
- rooms to heat are a master bedroom and bath - 233 square ft total - ceilings are 12' at highest 8' a lowest
-190' single loop 1/2" pex
- floor is concrete (pex was put in before pouring) about 5 inches thick insulated with 2" xps below and fox blocks around the sides
- winters get down to -10 F, but usually only a few nights, most nights are between 0 F and 20 F
- Prefer a closed loop system at this point - no good way to run another line of domestic water and the return
- I had a friend give me a Stiebel Eltron DHC 3-1 that I wonder if it could be used for this application
- We have excellent southern exposure for solar, but don't have a good place for a large tank besides an unheated, leaky attic
I'd really appreciate some guidance so we can avoid a second winter of space heaters (none too comfortable and very expensive!) Thank you all!

hot_rod

Was a heatload calculation done? Give this a try: Do each room separately, bath and bedroom.

http://www.slantfin.com/products/virtual-heat-loss-calculator/


How much of the bath square footage is covered with cabinets, shower stalls or tubs? That will lower the output some. I like to tube bathrooms 6" on center, around 400' of tube for that size room would be ideal.

Is this the sole heating for the room? Are you looking for some floor warming? Any floor coverings in the bedroom section?

I assume the rest of the home is heated.

aarmitch

Thanks for the quick answer. The calculator was looked promising, but I got a 404 error when I tried to get the Google Play version and a timed out error when I tried the web app.
To answer your questions:
A little more than half of the bathroom is covered, leave about 20 sq ft to heat. The bedroom has a king bed and various dressers - also a rug that's 24 sq feet - totaling about 104 sq feet, leaving 87 sq feet to heat.
I'd like the radiant system to handle as much of the heating as possible, but from your post it makes me worried that we either undersized the pex or did not lay enough.
These rooms are a new addition - the rest of the house is a combination wood/electric heated. Someday I would like to install hot water radiators in the other rooms of the house, but the priority now is to get heat in the addition.
Thank you so very much for your help.

Zman

A hydronic towel bar heater or in-wall radiant is an excellent solution for the lack of available radiation you have in the bathroom

aarmitch

Zman, so are you saying that the existing radiant floor loop in the slab in the bathroom will not work, or that it won't be sufficient (I'm guessing that's what you mean by "lack of available radiant") and that I should use a hydronic towel bar or in-wall radiant to fix this? Would it make sense to have a wall mounted radiator that I also pass the radiant hot water through? Thanks for your input.

Zman

You will need to do the room by room heat loss in order to figure this out.
Bathrooms are a critical area for thermal comfort. It sounds to me like you lack the floor area and control needed to make that space comfortable.

hot_rod

The system will "work". I suspect on cold days the bedroom may be under heated with carpet, bed and furnishing limiting the radiant surface available.

May as well try it for a winter and see if you need to add additional output.

Panel radiators are a nice supplemental heat emitted for bedrooms, they would kick on when the floor is unable to keep up.

Same with a heated towel radiator or small panel.

Without an actual load number we are all speculating.

Same with the Stiebel sizing, that 3K unit gives you @ 10,230 BTU / hr of output. That will probably be adequate if you can get the energy to the space with just one 190' loop.

Since you are using electricity, you could just add electric panel radiators or electric towel bars if you need additional output.

DZoro

233 sqft with 190' of piping, a little shy but, you should be good providing you have r-21 walls and r-50 ceiling not too much glass. A towel warmer is a great idea, would help keep the bath a bit warmer than the bedroom, which is usually a good thing. Slab sensor stat, outdoor sensor on the boiler, not a big fan of electric boilers, cost of operation can be large depending on your rates.
If future radiant is in the plans, may want to go with gas now. Although almost any gas boiler you will find will be waaay to big with just the one 190' loop.

aarmitch

Wow - this is a lot of great information and great ideas. Thank you all! I estimated the heat loss using https://www.dimplex.com/en/customer_support/heat_loss_calculator - converted to BTU/h it gave me 1583 for the bath and 4756 for the bedroom. If it's accurate, it sounds like I'm within the ballpark of what the Stiebel DHC 3-1 might be able to handle so I'm willing to give it a go this winter and evaluate in the spring if we need to supplement. So, I have the pex loop and the heater - what size of pump should I be looking for? I had a friend suggest the Grunfos Alpha with the idea that the pressure could be adjusted. Thoughts?

Zman

It looks like the alpha would handle the radiant side just fine. Do you have a head loss chart for the tankless?
I would suggest using an electric boiler instead and setting it up with outdoor reset controls.
Bear in mind that concrete slabs warm up very slowly. Don't even think about setting back the t-stat with a small electric boiler.

hot_rod

Here is one choice for a nicely built small electric boiler. It comes with a 3 KW element. It is a standard 4 bolt HW element and you could downsize to a 2KW and it would match your load perfectly.

They have versions with outdoor reset function also.

Much better option compared to an electric tankless heater, but a bit more $$.

aarmitch

I'm really learning a lot - can you help me understand the advantages of the boiler over the Steibel tankless I have? I've looked at a few boilers, one that's intriguing is a NEXTGEN-4 - no need to buy the pump, expansion tank, etc. separately. But for the price I might be better off limping along with space heaters until I can do more of the house with hydronic heat and get a larger boiler then. Thoughts?

Zman

Boilers tend to be designed with less hydronic resistance. This makes it easier to pump the water through.

Being able to control the water temp based on outdoor temp is a big deal. If you give the slab the same temp water all the time, your indoor air temp will spike and fall. Using outdoor reset will make the system more efficient and comfortable.

hot_rod

Definitely wait till you can purchase a boiler if you intend on doing more hydronic heat in your home.

The tankless and boiler are designed to do opposite jobs.

Tankless heat a relatively small flow over a wide temperature. Their graph shows a 60° rise. Note the output spec on that DHC-3. usually intended to be placed under a single lav sink, for example.

A radiant boiler is designed to heat higher flow over a narrower temperature range. The boiler will have heating specific controls, and often be certified and listed as a heating appliance.

First do some research in fuel cost comparison, if a larger boiler is in the plans, electric may not be a cost effective energy source?