Balancing radiators

[Potatoman]

Hi, does anybody know a good way to balance radiators on a heat pump system

[JohnMo]

8 minutes ago, Potatoman said:

Hi, does anybody know a good way to balance radiators on a heat pump system

No real difference from any other heat source. Heat Geek has a write up on several options.

[ReedRichards]

One thing I have never been clear about is how the temperature drop across your radiator interacts with weather compensation.  For example, if I set up a 5 C drop across the radiator when the input water temperature is 50 C, what should that be when the input water temperature is 40 C or 30 C.  And presumably that differs if my heat pump is trying to maintain a fixed flow or maintain a fixed temperature differential between the leaving and return water temperatures?   

[JohnMo]

24 minutes ago, ReedRichards said:

One thing I have never been clear about is how the temperature drop across your radiator interacts with weather compensation

Not sure matters for radiators or UFH. You adapt the WC curve to keep rooms at the temperature you want or need. So you set the balance at flow temperature point within a curve, dT will vary slightly with flow temperature, but the output is varied with difference in flow temp. Every house, every heat pump will react differently, you just fine tune the curve to suit.

[Beelbeebub]

I have been wondering, what is the effect of having an oversized radiator in your system.

 

Say you have a perfectly set up system, the radiator in every room is the exact size required.

 

The DT across each radiator is 5C.

 

Now you swap one room for a gigantic raiditor, 3x bigger than needed.

 

All else being equal, that room will be hotter if the DT across the rad is still 5C

 

As you can't reduce the flow temp to reduce output because the other rooms will be too cold, your only option is to reduce the flow.

 

But then the DT across that rad will be greater than 5C and that water will mix with the return from other rads so the DT the HP sees might be 7C.

 

How does that work? Do we have to decrease the flow temp slightly and increace the flow rate so the DT. Across the other rads is smaller and the mixed DT is back to the 5C target?

[PhilT]

25 minutes ago, Beelbeebub said:

I have been wondering, what is the effect of having an oversized radiator in your system.

 

Say you have a perfectly set up system, the radiator in every room is the exact size required.

 

The DT across each radiator is 5C.

 

Now you swap one room for a gigantic raiditor, 3x bigger than needed.

 

All else being equal, that room will be hotter if the DT across the rad is still 5C

 

As you can't reduce the flow temp to reduce output because the other rooms will be too cold, your only option is to reduce the flow.

 

But then the DT across that rad will be greater than 5C and that water will mix with the return from other rads so the DT the HP sees might be 7C.

 

How does that work? Do we have to decrease the flow temp slightly and increace the flow rate so the DT. Across the other rads is smaller and the mixed DT is back to the 5C target?

The premise is wrong, every rad will have a different ΔT depending on the surface area of the rad, with an overall average ΔT of 5°C

[Beelbeebub]

56 minutes ago, PhilT said:

The premise is wrong, every rad will have a different ΔT depending on the surface area of the rad, with an overall average ΔT of 5°C

Yes but if you have one oversized rad the DT will be larger, which means the DT of the other rads will need to be smaller. Assuming the lockshield etc are already at max your only option is to increace the flow through is to boost the pump head. Is that what happens?

[sharpener]

I think none of the above, quite.

 

Heat loss from rad is proportional to (mean rad temp - room temp) x (rad area)

 

Heat supplied by heating circuit is proportional to (flow rate) x (delta T)

 

In the steady state these must balance. Most HPs will modulate their output in an attempt to maintain the delta T, so typically the flow will emerge 5 deg hotter than the water returns to it (assuming the HP has enough power to meet the total load).

 

If you have an oversize rad in yr system then it is always possible to turn its flow rate down until the heat input to it matches the loss to the room with the specified 5 deg delta T. Then the return temp will be the same as the other (correctly sized) rads.

 

Yes if you overdo this the return water will be colder than that and dilute the return from the other rads, in theory the flow temp will fall slightly as a result and the whole system will reach a new equilibrium at fractionally lower output.

 

But I don't think this will be enough to worrry about, in the limit of course the rad will be off and will then not affect the return temp at all.

[MrPotts]

I have used a very simple way of balancing my radiators. All lockshields fully open, start heating and check all radiators get hot at roughly the same time, if you have some rads hot and others warm/cold then turn down the hot rads very slightly. Repeat until all rads get hot. I was lucky, all my rads get hot virtually at once with all the lockshields virtually fully open.

 

 

[sharpener]

This is bc the volume of water in a rad is approx proportional to its surface area, and so is its output (assuming all have same construction i.e. 1 set of convector fins per heated element, or not). So you need a flow rate proportional to volume which gives you both constant dT and a constant warmup time.

AFAIR @JamesPadidn't like this method <g> when I first proposed it so good to see it come up again!

[JamesPa]

10 hours ago, sharpener said:

AFAIR @JamesPadidn't like this method <g> when I first proposed it so good to see it come up again!

I think that maybe a bit strong.  Heat geek have a lot to say about rad balancing and they correctly point out that the real objective is to balance for room temperature not deltaT.  They aren't the same because the rads won't be perfectly sized.  Their comment therefore is not that spend large amounts of time balancing for deltaT on the grounds it's not actually what we want. It's a fair comment imho.

[sharpener]

Was tongue in cheek.

 

But as a starting point for fine tuning I think it has its merits, as quick to do and certainly a refinement on the "open all lockshields fully" approach if you have a variety of rad sizes and pipe run lengths.