Regulating Radiant Heat in Warm Condo

[C Hawk]

We live in a top-floor Seattle-area condo (3 stories) that is south and west facing. Because of the moderate climate, exposure to what little winter sun we might get, and top-floor location, the unit stays relatively warm, even with the heat off. On a "cold" winter morning, the temperature might get down to 62 degrees without heat. While great for utility cost, the problem is the in-floor radiant heat, which seems to be overkill for our heating requirements. In a nutshell, we can’t heat the condo without the temperature swinging 6 or more degrees, heating to 73, dropping to 67, etc.

We have a 60-gallon, natural gas hot water tank for the 2,300 square-foot condo that serves all hot-water needs in addition to supplying hot water for the radiant heat. I have the water temperature around 130 degrees for dishwasher, showers, etc. Turning on the radiant heat at the wall thermostat starts the pump, which drains the hot water tank (no more hot showers!) while the gas kicks in to try and maintain temperature. After about 3 or 4 hours, the hot-water tank catches up, water in the radiant heat pipes is hot, and the condo is a degree or two warmer. At this point, I need to manually turn off the radiant heat (the wall thermostat won’t have clicked off yet), since the temperature will continue to increase from the warm floor and pipes. Even without the pump on, the temperature will go up another 3 or 4 degrees over the next couple of hours before leveling off and starting to cool.

The real problems are 1) having to regulate the system manually, meaning I have to shut the system down at night, when we leave for a few hours, or before the condo gets unbearably hot; and 2) my wife complaining loudly when there’s no hot water in the morning for a shower.

Ideally, I would like to have the system keep the temperature at a constant 70 during the day and 66 at night. What do I need to do? Different thermostat? Some sort of valve to lower the water temperature in the radiant heat pipes?

I’m a scientist, not an HVAC professional. Any advice would be appreciated.

 

[WorthFlorida]

Is it a standard residential water heater or a furnace designed for a heating system? Is this water heater just for your unit? It is on your floor or in a basement.

What is the floor made of, concrete or wood? If it is concrete it is a lot of mass and the concrete must be heated before the air is warmed up to satisfy the thermostat before the circulatory pump is turned off. A concrete floor is loaded with thermo energy and it will continue to warm the room until it cools down. If you ever boil water on an electric stove and just before it does comes to a boil you turn off the heat, but the water temp keeps rising and the water boils.

If your heating load and usage is low it may be worth installing a water heater just for domestic hot water if you have the space. The existing heater temperature can be lowered to your comfort. For domestic hot water should not be more than 120 degrees. If you're tight for space and venting for a gas water heater, an electric tankless heater would be a good choice as long as your electric panel can handle the additional load.

 

[Sponsor]

 

[C Hawk]

Is it a standard residential water heater or a furnace designed for a heating system? Is this water heater just for your unit? It is on your floor or in a basement.

What is the floor made of, concrete or wood? If it is concrete it is a lot of mass and the concrete must be heated before the air is warmed up to satisfy the thermostat before the circulatory pump is turned off. A concrete floor is loaded with thermo energy and it will continue to warm the room until it cools down. If you ever boil water on an electric stove and just before it does comes to a boil you turn off the heat, but the water temp keeps rising and the water boils.

If your heating load and usage is low it may be worth installing a water heater just for domestic hot water if you have the space. The existing heater temperature can be lowered to your comfort. For domestic hot water should not be more than 120 degrees. If you're tight for space and venting for a gas water heater, an electric instant hot would be a good choice as long as your electric panel can handle the additional load.

Standard residential water heater, just for our unit. No basement; condo is a one floor unit with water heater adjacent to the manifold for radiant heat pipes. Pipes are set in Gyp-Crete, which is why it takes a while to heat, then retains heat for so long.

We don't have the space for an additional water heater. I like the idea of an electric instant hot. Electricity is cheap here. So if I had an electric instant hot installed, could I set it at a low temperature (let's say 90-100 degrees) so the pump runs longer and evens out the temperature swings? The idea is that the floor would stay just warm enough to heat the unit to around 70 degrees.

 

[WorthFlorida]

#1 problem is a water heater is not designed for heating space. The heat recovery rate is just not there as you stated that it takes forever to get hot water again
Time to call a heating contractor to look into an electric tankless unit. My brother heats his entire Vermont log home with a propane tankless heater. With your light heating load a heating specialist should be able to size it up with the right size electric unit.

 

[BadgerBoilerMN]

One of the many reasons I have been railing against used tankless water heaters for space heating for years.

You are missing a key element in radiant comfort control, i.e. supply temperature. Outdoor reset would be the answer.

I don't know what an "instant hot" is, but an electric boiler with the indispensable outdoor reset feature would be in order.

I use Thermolec and Electro boilers with an Ecobee thermostat at home in my everyday design work.

It all starts with a Manual 'J' heat load.

 

[WorthFlorida]

Sorry, I meant tankless heater. I see them as an "instant" hot water heater because on demand the water is heated almost immediately.

FAQ from this site:
What is the temperature of the water?

• The power mode control can be adjusted to increase or decrease water temperature.

http://www.tanklesswaterheater.com/products1.html
http://www.tanklesswaterheater.com/products4.html
Laser Instant Electronic Water Heater

The Laser will supply endless amounts of hot water, saving as much as 60% on water heating costs. The heating elements are shielded incoloy elements, used for reliability and long life.
The Laser Electronic Linear Temperature control system makes it the most versatile unit in its class on the market today. Only one heater is needed for an entire apartment, small house, business, etc. It is compact in size, energy efficient and very easy to install. No venting or pressure relief valves are necessary.

• Laser Units Are For Export Sale Only

 

[WorthFlorida]

C Hawk: You have I think an unusual set up. With a hot water heating system, you need a pump and tank to circulate heated water through the pipes and back again to be reheated, however you're also using the same water for domestic hot water? Right. Not sure if that is really desirable. Just adding a tankless system just for your domestic needs will at least solve one problem.

 

[Jadnashua]

For that type of system, it's not uncommon for the inlet water to be closer to maybe 100-degrees than 130 on a mild day. I'm hoping that you have a heat exchanger between the potable water and the radiant zones. It's not a great idea to be drinking the water or even showering with it that went through the probably 100's of feet of pipe, and then, might sit there for awhile. Low temperatures can let things grow, which is why separating the systems is prudent.

There are thermostats that are designed so there isn't as much overshoot. Some of them have many modes, and you need to tell it you have a radiant system. Some of them learn the response of your system, and then adjust when to turn it on, and then turn it off so you can maintain a pretty stable temperature. A thermostat setup for hot air will not work well for a radiant system. With the gypcrete, you have a moderate amount of thermal mass, so once it is hot, it will stay hot, and if you don't turn it off early enough, that is why you're seeing the temperature continue to rise along with the relatively high temperature. If the slab doesn't get that hot, and your inlet water is low enough, it could run longer and not really cause the room temperature to rise, or the floor to get hot. You normally don't want the floor surface temp to be hotter than 100-degrees, and lower is usually more than sufficient. It's really hard to do when the inlet water temp is too high. It's not like a hot air system...the pot of water example is a good analogy. A hot pot will stay hot for a long time. THe air coming out of a duct does not. It's all about the mass.

 

[Dana]

A mixing valve with outdoor reset control would fix most of those problems once it's dialed in. Your peak heat load is WELL below the burner output of a typical 60 gallon tank- the only reason you can't take showers when the heat is running is likely to be the thermal mass of the radiation (concrete slab, is it?). With an outdoor reset control mixer and an ECM drive pump running constantly you can probably skip the thermostat once it's dialed-in.

Tankless on-demand electric hot water heaters big enough to supply even a low flow shower at Seattle's incoming water temps require dedicated 100A 240VAC circuits, and are usually a lousy "solution" to heating hot water in cool-water locations. Anywhere north of US climate zone 2 (Seattle is zone 4) they are usually not very satisfactory.

[edited to add] With radiant floors it's generally more comfortable AND more efficient to take "set and forget" strategy on the thermostat rather than overnight setbacks. This is particularly the case with high mass radiation such as radiant slabs. By setting back overnight you're then having to max-out the heat rate drawn from the system to support the heating ramp-up, leaving the unhappy spouse to deal with a tepid showering experience.

To get around that while still using a set back approach, with a programmable thermostat you can try starting the recovery ramp early, but then set it back again 15-30 minutes before the anticipated showering time. The floor will be nice & warm even if the room isn't fully up to temp, but the tank will have had plenty of time to recover from the slug of heat withdrawn for the heating recovery.