Posted by: @peterr

Hi,

This is probably a silly question, but do radiators fitted to ASHP's need to be balanced in the same way as for conventional boiler systems?

The reason I ask is that when our system was installed 7 out of the 8 radiators in the house were changed for higher output ones. The only one not changed was the "designer" one in one of the bedrooms. I am pretty sure that the plumbers did not balance the radiators before they left, because they left before the electrician had finished work on the controller!  At the moment, 2 of the radiators are turned off, 3 have the TRV's fully open and 3 are somewhere in between.  2 of the 3 radiators that are fully open are uniformly warm to the touch, the third (the one that wasn't changed) is warm at the top, but cold at the bottom.

Thanks

Hi Peter,

Many things in life are a balancing act, so why should radiators be any different. The simple answer to your question is yes, your radiators may need balancing.

The reason that most of your radiators were changed for ones with a higher output capacity, is because heat pumps are more efficient when operating at lower water flow temperatures. So, if one of your rooms has a heat loss of say 30W per degree C, then with an ambient air temperature of 0C and an indoor air temperature of 21C, the total heat loss would be 30 x 21 = 630W.

If the original radiator had a nominal output rating of 1000W at a Delta T of 50C, then for it to provide a heat output of 630W would require a water flow temperature of approximately 56C, but by installing a radiator with a nominal output of 2000W, then 630W of heat energy could be supplied with a water flow temperature of only 41.6C.

For a completely balanced system, radiators would need to be sized to match the heat loss of each individual room, but of course radiators come in standard sizes, but rooms do not. Good practice is therefore to select the next higher output radiator that meets the anticipated maximum heat demand for that particular room.

TRV's help to overcome the radiator balancing problem, by regulating the water flow to the radiator and hence the heat output, and so helping to match the heat output to the demand. Using TRV's in an ASHP based heating system may cause lower efficiency if a buffer tank or low loss header is installed. I would suggest that you open the closed valves, even slightly, and see how this affects the water flow temperature.

In the rooms where the TRV's are fully open, is this because the rooms cannot get up to temperature or because thermostats are controlling the temperature?

Shutting off radiators by closing valves, may cause the remaining radiators to work harder and in turn require a higher water flow temperature to meet the total heat demand. This in turn may reduce the overall efficiency of the whole system, and so could be counter productive.

The radiator that is warm at the top and cooler lower down has probably just had time to cool to some extent. I would suggest checking it again when the ASHP and central heating pump are both running. If the radiator were cold at the top and warm at the bottom then it would indicate that the radiator needed bleeding.

@peterr I would balance them. My house was cold and I thought the ASHP was crap and not delivering at lower temperatures. Turns out my radiators were not balanced and the room I really needed the heat had the coldest ones. Once I fixed it, the house was suddenly lovely and warm and my wife stopped mentioning how cold it was...

It's quite easy, but will take a few hours, or 30mins a day over a few days. 

Step one: look on the other side of the radiator (not the trv). You'll see a cap probably. Take this off and there will be a little bit of metal sticking up. Get some pliers and you can twist this. Right for more open, left for more closed. Or you can just twist it to see up (open) and down (closed).

What you want to do is find the coldest radiators and open these fully (if they aren't already). Then go and find the closest radiators to the heat valve. If you don't know, just find the hottest radiators. These you want to close down. You can be as aggressive as you like. As you turn down the hottest rads (those with the highest flow) you'll find the other radiators start to heat up as more water pressure is reaching them.

Then it's just a game of tweak and wait. I would suggest you do this over a few hours and just get on with your life in between. In the end you are aiming for them all to be the same temperature. Measure however you want, ideally with a thermal gun, but the good old mark 1 hand also works fine.

@derek-m 

Hi Derek,

TRV's are closed in my wife and I's bedroom because we like a cooler room to sleep in.  TRV's are fully open on the first floor landing and in the two second floor rooms.  It is one of the second floor ones that is cooler at the bottom.

Thanks,

Peter.

@batalto 

Thanks for the suggestion - I'll give it a go.

@peterr, @batalto is spot on.

We had our rads rebalanced in April and it's made a world of difference. The installers did it for us, and followed the process @batalto described. They started with the coldest rads (ones that struggled to come to temperature), opened the release valve (not TRV side) to the maximum. You can do this exercise by touch, but it's obviously not as accurate. They (since they're professionals) used heat sensors and probes they connected to the incoming and outgoing valves and tried to get as close to a 5C differential between incoming and outgoing.

In our case, the exit valves on the furtherest rads were fully opened and the closer we got our heat distribution the valves were almost fully closed. 

It's worked brilliantly, but it takes a lot of time and trial and error.

@peterr i did a blog on this once https://www.linkedin.com/pulse/balancing-guide-how-make-your-heating-system-amazing-graham-hendra/

Hi guys,

So when I had my Gas Boiler I religiously went round each radiator and balanced the system throughout the home and achieved a Delta across each Rad of around 12C.   Now I'm in the process of having a Daikin Altherma M installed I am looking at whether or not that Delta should be changed (I will be asking this question to the installers next week but want a second opinion).

many thanks

My understanding is this Balance for correct distribution of heat to radiators.heat pumps systems benefit from high flow rates so no need to look for a specific delta t, you do want a small delta t across radiator as the temperature is lower..

@alec-morrow

So I'm going to take a different approach and balance the rooms and not worry about the Radiator Delta.  My installer has said the rads don't need upgrading (mostly T2s) but I'm thinking they do.   As it happens this weeks forecast is 7 degrees, which is where my ASHP is rated at for 4.3 COP, so its a good week to look at room temperatures and the HP flow temp.   I will start high and check all the rooms can get up to 21C, those which overshoot i will throttle back on the rads and those which undershoot i will open up the rads.  I will continue to do this while dropping the HP flow temperature until I cannot hit the desired room temps.   At the end of the week I should be able to see what my lowest flow temp is and if that is nowhere near 35C at 7Deg outside then i will know my rads are undersized.

There is a buffer in the system which reading through the forum will degrade performance, I've opened up all the EVOhome TRVs to high temperatures which wont be reached.  The Danfoss system pump is set to Constant Press II, I have it set there as it was the recommended setting for TRVs with a buffer but I may look at a different setting as I have the TRVs open.

@saf1973 

with on -off controls it may be beneficial to change radiators, but with weather compensation controls with a compensation room unit you almost certainly don’t need to do so..

@saf1973, after installing our ASHP, we had our rads rebalanced because there were rooms that were struggling to come to temperature. Air source heat pumps usually have a delta T of 5, and when our installer’s heat engineer rebalanced our rads he was striving to get a heat differential of 5C from entry to exit in our rads. We got close in most instances and it made a big difference.