How it all works

DaveGateway

Here I go again... I'm a homeowner that wants to really understand how these radiant systems work. I read Dan's book, good stuff, but of course now I just want to know more.

Here's where I'm having problems...

Let's take a middle of the road condensing boiler like the MZ 40. Burns at 71K or 142K.

Let's also assume I have a simple system with radiant floors (and DHW). I'm looking at a retrofit with Stadler climate panels under tile or hardwood, so I'm assuming this is considered a relatively low thermal mass system (responds quickly to heat input/loss).

In any heating system that's not running full out (most of the time), there will be some oscillation around the desired setpoint temp. Given the low thermal mass, that would imply more frequent on/off from the thermostat.

Question/Assumption 1) The circulator for the radiant floor zone only runs periodically (when the thermostat calls for heat). Or does the circulator run constantly?

Question/Assumption 2) During the call for heat, the boiler would only need to fire for short periods since the heat loss will never equal the instantaneous boiler output.

Question 3) So how does this all end up working? If your primary loop is short (not much fluid), that means there isn't much thermal mass, and I'd assume the boiler will be cycling all the time (which I know is undesirable).

I'm guessing Question 3 is addressed by one of the following:
1) Get a modulating boiler
2) Add thermal mass to the primary loop
or 3) Allow the temp of the primary loop to vary widely (store more heat in the same amount of water).

Thanks ahead of time.

Chuckles_2

cycle rate

> In any

> heating system that's not running full out (most

> of the time), there will be some oscillation

> around the desired setpoint temp. Given the low

> thermal mass, that would imply more frequent

> on/off from the thermostat.


If you mean the aquastat that controls the water temperature, it has a fairly large deltaT (deadband) between where the boiler starts heating and where it stops. For example, it might stop heating at 120, then let the water cool to 100 before starting again. This reduces short cycling.

If you mean the room thermostat, the relevant thermal mass is that of the whole house and so the oscillations will be small.

Hydronic heat is a chain with two links. Heat goes fom the boiler to the water; heat goes fom the water to the house.
On the average the heat flow is the same in each of these links. If the second link (water to house) requires few BTUs, either because of mild weather or because there is inadequate radiator capacity, then yes, the boiler will go on and off a lot (unless you have a modulating boiler) and efficiency will be poor. Often a buffer tank is added to increase the thermal mass and reduce the number of cycles. But if sufficient heat is being transferred from the water to the house, the thermal mass of the heating system does not matter as much.

> Question/Assumption 1) The

> circulator for the radiant floor zone only runs

> periodically (when the thermostat calls for

> heat). Or does the circulator run

> constantly?


Either way. Constant circulation is better in some ways.

> Question/Assumption 2) During the

> call for heat, the boiler would only need to fire

> for short periods since the heat loss will never

> equal the instantaneous boiler output.


If "heat loss will never equal the instantanous boiler output", you have oversized your boiler, that's your problem. Of course, it has to be sized for the coldest weather, so in any other weather the boiler will go on and off, but only very short cycles are a problem. Whether it runs for short or long cycles depends on the heat load and the size of your boiler.

> Question 3) So how does this all end up working?

> If your primary loop is short (not much fluid),

> that means there isn't much thermal mass, and I'd

> assume the boiler will be cycling all the time

> (which I know is undesirable).


Again, the main issue is, does the water cool significantly when going through the radiant floor? If yes, if there is sufficient heat load, the boiler will not short cycle (it may still cycle, but only short cycles are undesirable). If there is no heat load, then why do you need this heating system?

DaveGateway

Thanks Chuckles

that helps.

The aquastat delta-t really answers the gist of my question (storing more heat in the boiler loop). Is the aquastat delta-t typically programmable/controllable in most boilers?

As far as sizing, I know the "design" temp of the system isn't encountered often, so I was worried about the rest of the time when the heat load is much less than the boiler capacity.

What are the downsides to adding a buffer tank? Higher standby losses?

Thanks again,

Steve

Plumbob

yes

> The aquastat delta-t really

> answers the gist of my question (storing more

> heat in the boiler loop). Is the aquastat

> delta-t typically programmable/controllable in

> most boilers?


Some boilers. And if you have an outdoor reset controller (you should), the Tekmars will control and optimize both the T and the deltaT.

> As far as sizing, I know the

> "design" temp of the system isn't encountered

> often, so I was worried about the rest of the

> time when the heat load is much less than the

> boiler capacity.


To reemphasize, it is not cycling but excessive cycling (short-cycling) that is the problem. Let's say your design temp is 0F. When it is 60F outside, sure, you'll short cycle but no big deal since not much gas is being consumed. When it is 20F, a typical winter day, you'll cycle but you won't short-cycle...UNLESS you have an oversized boiler.

> What are the downsides to

> adding a buffer tank? Higher standby losses?


Yes, but I can't tell you if this is a big deal or not. (I am merely a relatively knowledgeable homeowner). I believe they are used only when you have a small zone connected to a large boiler serving several zones...when only the small zone calls for heat, you can get 'microloading' and thus short-cycling. The buffer tank reduces that. If that is NOT your situation, i.e. if your radiant zone will cover the whole house, you don't need a buffer tank. Just make sure you don't oversize the boiler...in the US, maybe 75% of boilers are grossly oversized.

DaveGateway

Mix valve or Injection Pump

Good stuff, I was looking at the Tekmar 363 info, now I see the delta-t you're talking about.

So another question, what are the advantages/disadvantages to using a Mix valve versus Injection pump to control the radiant floor zone temp?

Plumbob

> another question, what are the

> advantages/disadvantages to using a Mix valve

> versus Injection pump to control the radiant

> floor zone temp?


Don't know; I am sure someone else here will. But the MZ is a condensing boiler, you don't need any mixing unless you need different temperatures for different zones. You just need to have the Tekmar run the MZ at the necessary temperature. Condensing boilers are ideal for radiant applications because they can run at low water temperatures, while cast iron boilers need to have >140F return water.