Retrofit thinslab radiant - floor re-engineering costs?
Last Post 20 May 2022 01:13 PM by sailawayrb. 6 Replies.
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GrantoUser is Offline
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19 May 2022 07:36 AM
We have a 2 story townhouse built 1997 in the Pacific NW. Our complex just got natural gas, so we are considering replacing the existing electric baseboard heating with a gas fired hydronic system. Upstairs we recently installed hardwood floors, so we are thinking hydronic baseboards. Downstairs we are considering a 1.5” concrete slab with PEX (suspended over crawlspace). We would install floating engineered hardwood on top. From my research, gypcrete or concrete with pea aggregate seem to be favourites for thinslabs. The downstairs floor is 820 sqft with 9.5” tall I-beam joists 19 inches apart. Because of the weight, I know I’ll have to hire an engineer for the floor. Before I hire an engineer, is there anyway to ballpark the re-engineering and construction costs for the floor? I’m trying to figure out if this hydronic heating system will fit our budget.
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19 May 2022 11:06 AM
My first house I did heated floors in I used sand topping mix by hand over a yard and 1/2 of it(that was hell to mix and dumb).....The house was 1950"s and we was able to level all the rooms and landing doing what you planning. We tiled everything.

The up stairs was new flooring but also 1-1/2 inch of concrete with pea gravel. Note it is a lot harder to finish inside floors then out door pads because you can't walk around them. We did floating floor upstairs and man did it float and never really felt like it wasn't floating. I would glue it down, our newest house we glued with some Bona glue- and has been great. This area was 1200 sqft but open to the lower floor and after the first year we turned off the heated floor because it was to hot to sleep and never turned it back on.

Current ranch house we did heated floors every where and had to turn off the basement floor for being too hot. Got to remember with heated floors you can't direct the heat....it goes up and down from the floor itself.

As far as loading goes. The original house the floors were standard and not reinforced. The newest house has floor trusses and I told the truss company I was putting in 1-1/2 inches of concrete on top. It had no effect on the size of truss we used. Floors are over engineered and mainly sized for sag and bounce. A layer of concrete helps sag and bounce, much like bridges are concrete and not asphalt.

Your method of doing the pex is the cheapest and easiest. I use chicken wire staples to hold down rebar ties and hold the tubing that way. Zap ties are slower and weaker and always tend to stick out of the concrete while pouring even if you try cutting them all off.

Good luck....

I don't have experience with baseboard heat.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
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19 May 2022 01:10 PM
Yes, have a licensed professional structural engineer sort out your floor loading for this project.

Yes, Gypcrete or similar is a great thin slab hydronic radiant emitter. WarmBoard performs nearly as well too. Avoid plate systems, especially below-floor approaches. Any R-value you put above the PEX reduces performance. Tile is a far better hydronic radiant floor finish than wood.

Hydronic radiant systems need to be properly designed in order to avoid the issues Newbostonconst mentioned. You first start with a room-by-room heat loss analysis (ACCA Manual J or similar) and then you determine your required heating zones, PEX spacing, circuit lengths, circuit flow rates, circuit resulting head losses, and circuit supply supply/return temps. Only then can you determine your pumping and heat source requirements. Be sure to use floor temp sensors and appropriate thermostats to avoid any room temp setpoint over- or under-shooting.

Many companies design and install hydronic radiant floor heating systems. Very few companies have the competency to actually do this properly and successfully. The same can be said for HVAC companies in general. Get a copy of John Siegenthaler's Modern Hydronic Heating to gain a sense of what is required to do this properly if you have any doubts about this.

Frankly, retrofitting hydronic radiant floor heating is very expensive. This heating approach makes way more economical sense in new construction when all your floors are already going to be standard concrete slabs anyhow. So yes, you might be better off with a mini split system.
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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19 May 2022 06:29 PM
newbostonconst - Thanks for the feedback about radiant slab heating. It sounds like your slab was very efficient if you had to turn it off!
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19 May 2022 06:42 PM
sailawayrb - Thanks for your frank reply about the economics of retrofitting. Perhaps I need to rethink our heating plan.

I was actually planning to contact the company in your signature link about obtaining design services. Perhaps they can suggest some other options to take advantage of our new natural gas service.
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20 May 2022 11:27 AM
As Sailawayrb stated Mini Splits are the logical choice these days, but aren't for everyone in a aesthetics view point. The big plus is they are as efficient as NG like 90% of the time and do both air conditioning and heating very cheaply.

Most places are going away from NG but we also don't know what the costs of energy will be in the future. NG has historically been the cheapest but that is changing quickly.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
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20 May 2022 01:13 PM
An efficient gas mod con boiler might have an efficiency of 95% or equivalently a Coefficient of Performance (COP) of 0.95. A mini split might have a COP of 3.0+. So you could say that a mini split performs at least 3 times better than an efficient gas mod con boiler.

Given the relatively low cost of largely renewable electricity in the PNW, I would expect that a mini split would cost far less to operate than a gas boiler. You can use calculators like these to ascertain which heat source makes the most economical sense to operate:

https://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html

https://www.borstengineeringconstruction.com/Integrated_Heating_System_Performance_Calculator.html

However, you should also consider acquisition and maintenance costs and accomplish a true Return on Investment (ROI) analysis when selecting the best heat source for your locality.
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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