Temperature regulation in radiant floor heat system.

_big_

Our house has a DHW/radiant heat system. Consisting of a SuperHot boiler, an 80 gallon indirect water tank and 6 floor zones.

The system employs a single circulator for the boiler loop, the DHW "zone" and the 6 floor zones.

Water for the floor zones is generated by a Tekmar 4 way mixing valve similar to this: https://www.watts.com/products/hvac-hot-water-solutions/controls/control-accessories/016/016

The Tekmar 4 way mixing valve is a very simple static device without any sort of thermostatic control.

Our house is a ranch with walkout basement. The lower lever is PEX under concrete slab and the main level is PEX in gypcrete.

All the heat comes through the floor, except for one zone on the main floor with a baseboard radiator under the hot tub. This single baseboard radiator is the high point in the system.

The Tekmar 4 way valve has the undesirable characteristic that the mixed output pressure and mixed output temperature are directly related, ie if you adjust it for more pressure, you get higher mixed temperatures too.

The basic problem is that if I adjust the Tekmar 4 way mixing valve to get reliable flow through the one zone, the mixed water temperature is too high.

My solution is to reduce the boiler's aquastat set point down to about 145 and adjust the Tekmar 4 way valve to reliably have enough pressure on the mixed side to push the water through the baseboard radiator.

With this I can get hot water of about 115 and mixed floor water of about 120. Too low and too high .

It seems to me that I should be increasing the boiler set point to 155 or more to get the DHW temp up and then install some sort of thermostatically controlled mixing system, probably with another circulator to achieve proper temperature and pressure for the floor zones.

Its sort of a mystery to me why the plumber installed such simple system?? Cost effective and simple yes, but the lack of thermostatic control on the floor loops just doesn't seem like a proper system to me.

There are at least two different ways to generate temperature controlled radiant floor water. Some thermostatic valve, and variable speed injection mixing.

The question is, what are the tradeoffs of these two systems?

Big Ed_4

It is a basic radiant installation . I would like to see more control . Floor sensors and a mix station would be very nice ... I would use a circulator per zone . The hot tub warmer I would keep simple ..

Jamie Hall

Can you draw a diagram of how this thing is piped -- especially where the valve is and any pujps? For starters, the baseboard should be on an entirely different zone -- not the radiant If that bit of baseboard is piped as part of a radiant zone, you'll never get it right.

The whole thing will likely be better off piped primary/secondary, and a third pump for the radiant zone taking water from a thermostatic mixing valve, one input of which is the return from the radiant and the other is from the boiler primary loop.

hot_rod

Need to see some pics. Not a lot of value in a 4 way mix valve that doesn’t have an actuator and control. Usually on outdoor reset control

MikeL_2

  The Viega hydronic mixing block is our favorite radiant floor operating system. Easy to install & it uses constant circulation, outdoor reset, and a programable / adjustable heating curve; no thermostat required or needed. 

_big_

System diagram attached. Its a pretty simple system.

At this point, its not possible to move the baseboard radiator. A dedicated circulator might help.

hot_rod

Which SuperHot boiler is that?

Here are two piping options for a 4 way valve.

If the boiler has a high pressure drop, or a specific minimum flow, you might want two circulators as in figure 6-12.

A cast iron boiler would be fine with one pump.

In either case I like to see the circ downstream from the expansion tank. Also downstream of the mix valve. So you pull flow thru the valve from the two ports for good mixing.

Four way valves are not so common anymore. used on cast boilers or any non condensing boilers they can provide return temperature protection and radiant mix down.

A 4 way valve has two mix points, going on.

_big_

Boiler is a SuperHot Saturn Series 270K BTU.

Can you give me a part number for that 4 way mixing valve?

hot_rod

That's a substantial boiler size, how big is this job?

tekmar is a common brand, ESBE and Belimo are some other 4 ways.

_big_

5000 sq ft residence plus a three car (radiant) heated garage.

hot_rod

One way to look at boiler sizing.

About 25 BTU/ sq ft is about what you can comfortably get from a residential floor. Above that the floors tend to get pretty hot, un-comfortable to walk on.

So 5000 X 25= 125,000 btu/ hr load.
The garage? call it 800 sq ft? 800 X 25= 20,000 btu/hr load. Total load 145,000 btu/ hr.

So a boiler in the 175,000- 180,000 btu/ hr range should cover your application.

Oversizing, grossly over sizing leads to boiler short cycling, lowered boiler efficiency, excessive wear. Not good.

If you have a radiant design, it should spell out the boiler sizing?

New, well insulated, tightly constructed homes could Have loads in the teens maybe single digits as in BTU/ sq ft.

This recently built home up around Utica, NY has a heat load of just over 10 btu/ sq ft.

So a 5000 square foot home could require a 50,000 btu/ hr boiler in this case. A 270K boiler would be near 5X oversized!

So confirm some load numbers before you go way overboard on the boiler sizing.

HomerJSmith

So, it is a Saturn gas boiler with stainless steel burners with Steel Fin-Tube burners. Industrial steel boiler tube (heat exchangers?).
The only reason to use a Tekmar 4 way mixing valve is to make sure the return water to the boiler is above 130 deg to prevent sustained flue gas condensation. That is the reason that you set the manual lever on the 4 way mixing valve to a return water of 130 deg regardless of the setting for the heating load.
Tekmar does make a motorized mixing controller and display with two sensors the would provide 130 deg return water to the boiler. One sensor is on the return piping to keep the return water above 130 deg to the boiler and one sensor is on the heating load piping. The display set the temperatures.
I recently replaced a cast iron boiler that had that a Tekmar 4 way valve and display. Unfortunately, the installers used only one sensor on the heating load and failed to put one on the boiler return. The result was a failed boiler because of sustained flue gas condensation (it took a long time). The only regulation was the heating load.
I would want a snap-on or well installed thermostat gauge on the return piping to the boiler or you could use a pipe thermo-couple clamp and a multi-meter to measure the return water temp and set the 4 way valve lever.
OR...you could change that manual valve to a motorized valve add on with display.

https://www.pmengineer.com/articles/87534-injection-versus-4-way-valves

HomerJSmith

_big_, your diagram of your setup is not how I would have done it. I think it is done incorrectly, but what da I know. I don't know what the pressure loss is in the boiler heat exchangers. It could be that you are pumping into the Ex-tank if the boiler HX are at a low head, like .4 'hd to .75 'hd., but at what GPM. The higher the GPM the higher the pressure loss in 'of hd.
I would want the Ex-tank installed on the input to the boiler pump.
I wouldn't want the DHW indirect in parallel with the 4 way mixing valve with out a check valve and a zone valve that is controlled by an aquastat on the indirect tank.
I would want a circulator, perhaps, a ECM on the load side of the 4 way mixing valve depending on how zoning is controlled. I prefer Caleffi twist-flow manifolds so I can regulate (balance) the flows thru the different zones.
I would want check valves on every zone return.
I would definitely motorize the 4 way mixing valve.
Those are some of the changes I would make.

_big_

hot_rod said:

One way to look at boiler sizing.

About 25 BTU/ sq ft is about what you can comfortably get from a residential floor. Above that the floors tend to get pretty hot, un-comfortable to walk on.

So 5000 X 25= 125,000 btu/ hr load.
The garage? call it 800 sq ft? 800 X 25= 20,000 btu/hr load. Total load 145,000 btu/ hr.

So a boiler in the 175,000- 180,000 btu/ hr range should cover your application.

Oversizing, grossly over sizing leads to boiler short cycling, lowered boiler efficiency, excessive wear. Not good.

If you have a radiant design, it should spell out the boiler sizing?

New, well insulated, tightly constructed homes could Have loads in the teens maybe single digits as in BTU/ sq ft.

This recently built home up around Utica, NY has a heat load of just over 10 btu/ sq ft.

So a 5000 square foot home could require a 50,000 btu/ hr boiler in this case. A 270K boiler would be near 5X oversized!

So confirm some load numbers before you go way overboard on the boiler sizing.

The house is 20 years old and reasonably high quality with 2x6 exterior walls and high quality windows.

But without supporting data, I've always thought the 270K boiler was a bit large. And yes, in my observations, it short cycles.

hot_rod

_big_ said:

hot_rod said:

One way to look at boiler sizing.

About 25 BTU/ sq ft is about what you can comfortably get from a residential floor. Above that the floors tend to get pretty hot, un-comfortable to walk on.

So 5000 X 25= 125,000 btu/ hr load.
The garage? call it 800 sq ft? 800 X 25= 20,000 btu/hr load. Total load 145,000 btu/ hr.

So a boiler in the 175,000- 180,000 btu/ hr range should cover your application.

Oversizing, grossly over sizing leads to boiler short cycling, lowered boiler efficiency, excessive wear. Not good.

If you have a radiant design, it should spell out the boiler sizing?

New, well insulated, tightly constructed homes could Have loads in the teens maybe single digits as in BTU/ sq ft.

This recently built home up around Utica, NY has a heat load of just over 10 btu/ sq ft.

So a 5000 square foot home could require a 50,000 btu/ hr boiler in this case. A 270K boiler would be near 5X oversized!

So confirm some load numbers before you go way overboard on the boiler sizing.

The house is 20 years old and reasonably high quality with 2x6 exterior walls and high quality windows.

But without supporting data, I've always thought the 270K boiler was a bit large. And yes, in my observations, it short cycles.

It depends a bit on the garage load. How warm, how tightly do the doors seal. That can be a big load if you are looking to maintain 68- 70F.

A WAG, that 270 could be twice the size you need.

_big_

The house has a "walkout" lower level. The garage/shop (that's why its got radiant floor heat) is part of the lower level. About two walls are exterior. The ceiling and the other two walls are insulated, but are shared with the heated house. There are two 8'x8' garage door openings. One has a insulated overhead door, the other is a dual glazed patio door.

Probably the seals for the overhead door are poor and this is the largest heat loss.

I frequently turn the heat off in the winter. Even on cold days (-10F) it doesn't get below about 50F. I don't think its a huge load on those days I turn the heat on. Probably not much worse than a standard interior room.

What about the sidearm DHW tank? How BTU's do I need for it?

hot_rod

Same answer for sizing the boiler for the indirect, how much hot water you want and how quickly.
A typical gas fired water heater is 35,000 BTU/hr which is plenty for most homes.

So a boiler with 50,000 btu/ hr would be close to what you would get from a gas fired tank. The more boiler you throw at it the faster the tank will recover and provide DHW.
Assuming it is piped properly, adequately sized piping, and a priority control.

My crystal ball tells me you need 150- 175K which would provide plenty of DHW.

The indirect tank manufacturers often show tank output with various sized boiler input if you want a hard number

SummitMechanic

Seems to me that you have a 2 temperature system with DHW priority. Ideally you want around 120 going to your floors and 180 going to your water heater and the baseboard heating loop. the DHW problem is an easy solve with the installation of a primary/secondary closely spaced Tee and having your DHW run off of priority control at 180. As far as the baseboard, unless you want to spend a bunch of money on a repipe to separate it from your floor system with a heat exchanger, i would just accept 120-130 degree water flowing through it.