The 'art' of balancing a heating system

MrThrifty

Right, I've upgraded the controls for the central heating system and all is well now... apart from the fact that the rads aren't properly balanced. There seem to be lots of schools of thought out there on how this should be done - e.g. get a difference of 12 deg across all rads, or get all rads to heat up to the same temperature, or get the outlet temperature of all rads the same. I'm wondering which method folk think is best. Also, after balancing the rads, what's the best way to balance the cylinder (presumbly done with all rad zones open)?

deandean

I saw a good write up somewhere....I think one of those yellow books "Plumbing for Dummies". Here's what I did:

Ensure circ pump is matched to the application, especially that it'll pump to the required head.

Get two pipe thermostats.

Aim for 11DegC temperature drop across the boiler

For a condensing boiler your return water should be ?40DegC? max

Aim for 4DegC across a radiator, although I was only getting about 2DegC.

Throttle down the valves closer to the boiler. I had one b1tch of a rad a long run away and to heat it I ended up throttling down practically all other valves in the system.

For the cylinder, I just barely crack open the isolation valve and it's enough.
It takes a good few evenings to get it reasonably balanced.

Best of luck!

MrThrifty

The house has never been so hot! Had a go earlier at balancing the system but there seemed to be some strange things going on. For one thing, I could not get the temperature across the boiler to less than 20degC, even with the pump at max speed (which I know is too high considering the size of the house and also coz you start hearing noise in the pipes).

Also noticed funny things happening when measuring the temperature difference across the rads, with it seeming dependent on the overall rad temperature. Gave up in the end and resorted to adjusting all rads so that the rad temperatures were equal when the boiler is running.

Anyone know what the Grundfos pump speed should typically be set for, in the average sized house? Also, how bad is it just to adjust rads so that they all heat up at the same rate rather than trying to get the differential right across them?

heinbloed

Gave up in the end and resorted to adjusting all rads so that the rad temperatures were equal when the boiler is running.

Well, that is how it should not be.

The outlet of all radiators ( prof. term is " return ") should have the same temperature when the thermostatic valves are open.

Therefore the outlet valves have to be adjusted, the flow limited.

Here 2 little commercials aimed at the US market which describes the situation:

http://heating.danfoss.com/PCMPDF/Flash%20animation%20about%20hydraulic%20balancing.swf

and here

http://www.radiatorcontrol.com/benefits.aspx

(press onto "see how they work")

An intelligent A-rated circulation pump will automatically adjust it's speed, the volume it pumps, according to actual demand.

With fewer radiators asking for thermal energy the pump can reduce it's throughput, saving on electric energy as well as on wear and tear. Increasing it's life time.
In combination with a modulating boiler that is about how efficient a standard central heating system based on combustion can be. The maximum fuel efficiency.

MrThrifty

heinbloed wrote: »

The outlet of all radiators ( prof. term is " return ") should have the same temperature when the thermostatic valves are open.

Interesting... Had another go at balancing the system after and things started making sense this time - think the issue was that if the boiler is cycling near its setpoint temperature then you get lots of funny stuff happening if trying to measure temperatures at the rad valves. Anyway, I adjusted the heaters so that the temperature difference between flow and return pipes were similar (~12 degC) - turned out that the rad temperatures were actually very similar when this was done, maybe because most rads are of the same size in the house.

What you say about only needing to measure the outlet temperature makes a lot of sense since the inlet temperature is going to be pretty much constant along the flow circuit. Must have a look at this out of curiosity, as it would save having to measure 2 temperatures at each rad and some maths!

One spanner in the works is that I found that certain rooms in the house with poor insulation tended to have a greater heat loss and therefore took much longer to heat up when the system was balanced. So what I did, and I know this is probably going to receive some cringes, was deliberately unbalance the system so that the rads in these rooms had a greater flow through them and so got hotter at the same rate as the other 'better-insulated' rooms. Of course, these same rooms tend to loose their heat faster than the better-insulated rooms so that inevitably probably means I'll probably have to place the main control stats in them.

heinbloed

Well done Mr. Thrifty!

To combat the uneven heating-up time of the various rooms (caused by bad thermal insulation) two things can be done after balancing the system:

First: all radiators should be equipped with thermostatic radiator valves. (Or alternativly with a room temperature sensor switching the radiator on or off.)

The room temperature should be meassured with a thermometer and - once the desired temperature can be shown - the flow temperature of the boiler should be reduced.
Reduced so far that these most demanding rooms are still kept warm at the desired room temperature.

The second thing which can be done is to install a second radiator or exchanging the existing one for a larger one.

But harvest the low hanging fruits first: balancing, reducing the flow temperature.

Make sure that the return-temperature is not to low, that it does not get below the allowed min.temperature. Which is boiler- and fuel dependant.
No problem with a condensing boiler, go as low as possible. But with non-condensing boilers a condensation should not happen. In this case the flue gas should never condense, neither within the boiler nor within the flue/chimney. Check the boiler's technical manual.

What type of boiler do you have, what type of fuel is used?