I’m putting together the heating system for my pole barn and need some help in two areas. The building is a 24’ by 40’ by 10’ (940 sq feet) slab on grade with an open ceiling. The slab is 4” deep with 2” ridged insulation across the entire bottom and 6” up the band board. The walls and roof deck are covered with 2” of 2 lb closed cell foam. It has one 2’ by 3’ insulated glass window, two insulated steel doors and an insulated 8’ by 9’ roll-up door. The building was air tested with a result of .66 ACH50. The slab has ½” pex tubing, 8” spacing, 4 loops approximately 240’ per loop. My calculations show that I should be able to heat the place with about 18k to 20K btus. I only have electricity available, so I’m looking at about 5500 watts of heat input. I figure I could get by with 4500 watts because I only need to heat it to 55 degF. The building is located in Mount Airy MD 21771. We have about 4200 HDD per year. My question is, would I be better off using a tank water heater or an instantaneous water heater? I had planned on using a 20 gallon water heater, but I can’t find one that size with a 4500 watt element (all small water heaters seem to have 1200 or 1500 watt elements these days), plus they have double in cost since the last time I priced them. Cost is a factor. Also, I had a 1/25 hp Grunfoss Type UP 15-42 SF pump that I would like to use. Will this pump work with my application? Thanks, Ron C

heating only? if so, use the cheapest unit that gives you the output you need, assuming the unit is located in the heated space.

Yes, the unit is in the heated space. I want to mount the heating unit up on the wall to save floor space, but I'm not too happy about mounting a 30 or 40 gallon tank in the air. All of the small WHs are 120v/1200 watts, so they won't work. All of the instananous heaters I've looked at in the 7500 watt range have 3/8" fittings, so I don't think I can get enough flow thru one. The budget dosen't allow for a hi-buck heater/boiler. We just need a low cost heat source for determining our base line requirements for heat. The building will be used as a test bed for solar and other technologies, so the electric heater would be used for backup heat. It is a working shop, so space is concideration. I'm open for ideas! Thanks,

electric on demands/boilers are typically pretty cheap.. under $1k.

or you can use the 7500 watt one piped primary/secondary.

Electric boilers are easy to use, wall mounted, efficient, will modulate to out door conditions, and are completely off when not in use(no stand by loss). I am not a fan of tank water heaters, small on demand units as they will not have the life of a boiler or the efficiency. Have a look at the Thermolec, here is a page to give you an idea of cost, BTU, Here is a link to ours, there are other units out there as well,
http://www.blueridgecompany.com/radiant/hydronic/325/thermolec-boilers
Good luck,
Dan

there isn't really any standby loss in this case (tank is in the heated space) but I do agree electric boilers are good choices.

Rob, What about stand by loss in the late spring - early fall????

if the space is conditioned, I wouldn't be particularly worried about it. it also wouldn't be very much energy. it's a slab heating system... that would be of vastly more concern if you are turning the system off and on irregularly...

I would rather use a boiler, but it's just not in the budget right now. What is the max temperature that I can/should run in a slab instalation?

the max you can is 140. max you should depends on your heat load and thermostats. low as you can.

the difference in price here in only a couple hundred bucks, I think, though those thermolecs appear to be a bit pricier. There are at least a few brands significantly under a grand.

I am not so sure about that, here are a couple Delivered Thermolec prices
But certainly if you have a 200.00 water heater budget, these are more. Up shot is they are a real heating appliance.
B-6U M Thermolec 20,400 BTU, 6KW@240V/1 "mono" electric boiler $892.71
B-8U M Thermolec 27,300 BTU, 8KW@240V/1 "mono" electric boiler $976.43
B-10U M Thermolec 34,000 BTU, 10KW@240V/1 "mono" Electric Boiler $988.79
Dan

Domestic hot water heaters are designed to deal with incoming water at ground temperatures. Heat transfer fluids coming from a pole barn slab to the boiler could be quite cold on start up. Can a boiler handle those temps OK?

I hold the idea that a boiler is designed for space heating. It will have a greater capacity to emit heat when sized to load. From cold start to design temp it will give it all shes got until at design temp then modulate down. Short answer, Yes.
Dan