Trench Heating

Depends on cfm of air flow, and gpm of water flow, and incoming water temperature supply...but you seem to be approaching this in a bassackwards way....First you would determine the btu load requirement for heating the space, then you design the delivery system.....in any case here is a calculator for convection heating...

...pure convection heating calculator....

https://www.engineersedge.com/calculators/heat_sink_convection_with_fins_calculator_10048.htm

Hi solaredge,

The client is a friend of mine and he doesn’t want radiators. I’ve been told the space I can use for these underfloor radiators, so there is only so much pipework I can fit in.

The room requires a heat load of 1.5 kw or 5118 btu’s. I only have space in two areas for 12m of 15mm copper pipe so around 480w of heat, not enough, so from an educated guess of modern radiators and trench heating with grills I said I’ll need to add fins. From an engineers point of view would adding fins along the length of the pipe increase heat transfer from the water to the room?

I get your point on the flow of water through the convector, I will be able to adjust the flow rate once installed.

There are many products and strategies for hydronic radiant floor heating that include finned pipes....

Steam/Hydronic Heater Element, Length 3 ft., Tube Size 1-1/4 In., Tubing Material Steel, Fin Material Steel, Inlet/Outlet NPT 1-1/4 In., Max. Fluid Temp. 650 Degrees F, Max. Pressure 660 psi, Output @ 2 PSI Steam 1600/2250 BtuH, Max. Heat/Linear Ft. @ 150 F Water Temp. 720/1010 BtuH, Max. Heat/Linear Ft. @ 160 F Water Temp. 850/1190 BtuH, Max. Heat/Linear Ft. @ 170 F Water Temp. 975/1370 BtuH, Max. Heat/Linear Ft. @ 180 F Water Temp. 1080/1640 BtuH

<...would adding fins along the length of the pipe increase heat transfer from the water to the room?...> Naturally, though how much depends upon a range of factors, not least of which is the air flow past those fins.

There are also off-the-shelf electric underfloor heating systems available for a range of applications.

Worth mentioning, the fins that I am familiar with had 2 opposing edges folded so that the butted up to the next fin. This formed a channel to encourage convection air flow through the fins rather than relying on radiation.

Good point. Corrugated ones may also be found.

"Trench" heating? Will this system be buried. or just under the floor?

Some issues I see with this.

1. Energy loss. How will the water be heated? What is the cost per BTU per unit volume to heat the water? How will you pump (move) the water through the system?

2. System heat loss. As heat is transferred out of the water and into the room the useful radiation will diminish. Flow (velocity) will be critical, as in high.

3. System cost. Would it be less expensive over time to simply install electric convection heat, without the cost of a massive water heater, copper pipe, aluminum fins and pump?

Hydraulic systems are the normal method of heating homes in the UK. Pumps generally use under 100W, and have a flow rate of 5-20 litres a minute depending on plumbing complexity.

Two ways to increase heat transfer are increasing the surface area or increasing the flow of air.

You could use 10mm pipe with a manifold each end to get larger surface area.

You could add PC fans to assist airflow: these can be almost silent if running slowly, but be able to boost the heat transfer for warming up. Many newer emitters have some form of fan fitted so they work with heat pumps. A well insulated house can have a flow temperature of 40C and keep the space at 20C.

Thank you for all the input. I’ll be using a 43 Kw vaillant boiler https://www.vaillant.co.uk/homeowners/products/ecotec-exclusive-with-green-iq-combi-24448.html

After giving it thought and exposing the joists, it’s a house built in 18th century, I do not have enough room to get adequate heat into the area, which is a shame. Going for standard panel and old style school radiators as a feature piece.

thanks for the input guys.

I would be concerned about the electro-chemical reaction of mounting copper in close proximity to aluminum. The two metals are not compatible and the aluminum will suffer from serious corrosion with just atmospheric moisture acting as the electrolyte between the two metals. The resulting aluminum oxide formed adjacent to the copper is not a good heat conductor and will form an insulating barrier so the performance will quickly degrade to the equivalent of just having the copper pipe.

I worked in this field some time ago for about 7 years designing perimeter & trench heating for mainly commercial buildings. The core of both products was aluminium fins on copper pipe, the fins being loosely assembled onto the pipe which was then expanded by ball-sizing to lock the fins in place. I don't recall hearing of any problems due to electrolytic corrosion in that time. Our local hospital has one of the systems I designed in their entrance lobby which is still in use after about 30 years. I guess the risk is there but minimised by the fairly benign environment it operates in.

If after 30 years you can still observe the color of copper rather than the green of verdigris then there is a good chance that the copper had some protective coating sprayed onto it that formed a barrier to prevent the aluminum corroding. Bare copper in contact with aluminum over that period would certainly show signs of corrosion on the aluminum with a puffy white oxide covering the areas in direct contact.

Alu fin on copper tube is the normal construction of any outdoor unit of an air conditioner or heat pump. The alu is coated to reduce corrosion, water adhesion and to aid sliding onto the copper tubes.

Similar to my reply to jhhassociates the units I saw being made had no coating unless the raw ali sheet was pre-coated. The fins came off of the press & straight into assembly.

Certainly, the units I saw being made in the factory had no coating. The copper pipe was cut, fins punched into shape, then assembled.

If my memory serves correctly, the two will form a battery and drastically increase corrosion. But even copper on its own buried needs a plastic sheath to stop contact with possible moisture!

See here:- https://www.finishing.com/78/97.shtml

Alu on its own MIGHT be OK.....guessing only.

Most people use special flexible plastic hoses as far as I am aware laid under a special concrete (as a name to use!) floor.

Many experience leaks years later, very damaging and expensive leaks, taking months to find and dry out!! BIG PROBLEMS....

It took my daughter and her husband 6 months, with no proper bathroom for all that time! And 2 small children!! Luckily it was mostly summer....

Yes good point on the Electrolisis, like copper to galvanised steel is a big no no with water, didn’t realise copper and Alu had the same effect. Yes for underfloor heating you run plastic pipes around the floor area on top of insulation buried in a screed. I was thinking trench heating but no longer doing this.

https://www.trenchstyle.co.uk/jaga-canal-compact-36-cm-width.html

good for when building from new to place in front backdoors to eliminate the cold spot, unless triple glazing is used!