Radiator sizing: to oversize or not?

[ectoplasmosis]

We’re having a Vaillant 7kW ASHP installed in our Victorian semi as part of a full refurb. Wet UFH in new 35m2 ground floor extension and 1st floor bathroom, rads everywhere else. 
 

Had MCS survey and calcs done (properly; the installer has done several friends’ houses and is very good). We have a couple of smaller bedrooms, ~9m2 on first and second floors, in which we can afford the space to double the size of the rads specified by the calcs. 
 

Is oversizing rads a good idea? I’ve read conflicting reports. We are looking to use smart/dumb TRVs on all rads. 
 

How would an oversize rad, throttled by a TRV, affect the function of the rest of the system?

[ReedRichards]

The proper MCS calcs should match the size of your radiators to the needs of your house and thus determine the required power output from your heat pump (with allowance for time off heating hot water instead of the house).  The calculation will have been made at a particular Leaving Water Temperature and a particular outside temperature.  If you oversize all your radiators by roughly the same percentage you would be able to use a lower LWT than the one assumed in the calculation, which would enable you to operate the heat pump more economically.  If you would like to use all bigger rads, get the MCS survey to redo the calculation assuming a lower LWT.  

 

 

Edited December 12, 2023 by ReedRichards

[ReedRichards]

If you only oversize some of the radiators then those rooms will heat up most quickly and the room with the smallest radiator, as a fraction of the calculated size, will be the slowest to heat up.  This may or may not work for you.  You could also end up with a situation where most of your radiators have been throttled by TRVs and your heat pump is only working to supply heat to a few remaining ones.  This might well cause your heat pump to cycle and if these cycles are short that is generally thought to be a bad thing.  The same thing would happen with a conventional boiler.

 

Really hard core heat pump users don't believe in any sort of thermostatic controls and just use Weather Compensation and radiator balancing to give you the inside temperature you require.  If you go down that road then there is absolutely no point in oversizing radiators. 

Edited December 12, 2023 by ReedRichards

[ectoplasmosis]

Thank you, this makes sense. 
 

We will have 9x radiators, and I am only considering oversizing 2x of them (in the two small ~9m2 kids bedrooms), my thinking being that being able to temporarily boost the temp in these small rooms would be handy. 

[ReedRichards]

If the boost is to be temporary, you might need Smart TRVs.  Or you remember to turn up the TRVs when you want to boost the heat and turn them down afterwards.  Even then, if there is a room thermostat somewhere controlling when the heat pump is on or off it might not be on at the right time.  In my experience, a Smart TRV will override a room thermostat so you can be sure the heat pump will run when you need it to.  But if it ends up only heating the two boosted rooms then it might cycle.   

[SimonD]

1 hour ago, ectoplasmosis said:

my thinking being that being able to temporarily boost the temp in these small rooms would be handy. 

 

Why? If the heating system is designed, installed and commissioned correctly with appropriate controls there shouldn't be a need to boost individual rooms.

[HughF]

Fit the largest radiators you can get away with, in all rooms. You can always turn the flow temperature down…

 

Be aware that flow temps of below 37.5 will result in rads that don’t feel warm to the touch - that might be something you want to consider.

 

I sized ours for 45 flow at -4 outdoors and 21 in the house. It turns out that it rarely gets that cold so the house sits at 22.5-23 most of the time, just running on weather comp 24/7 without any thermostats.

 

Edited December 12, 2023 by HughF

[ReedRichards]

1 hour ago, HughF said:

the house sits at 22.5-23 most of the time, just running on weather comp

Too hot for me, I would adjust the Weather Compensation settings.

[ectoplasmosis]

2 hours ago, HughF said:

Fit the largest radiators you can get away with, in all rooms. You can always turn the flow temperature down…

 

Be aware that flow temps of below 37.5 will result in rads that don’t feel warm to the touch - that might be something you want to consider.

 

I sized ours for 45 flow at -4 outdoors and 21 in the house. It turns out that it rarely gets that cold so the house sits at 22.5-23 most of the time, just running on weather comp 24/7 without any thermostats.

 

 

Thank you, very helpful.

 

Do you use TRVs on the rads, or just a balanced system with no individual controls/zoning?

 

[Dillsue]

8 hours ago, ectoplasmosis said:

 

Thank you, very helpful.

 

Do you use TRVs on the rads, or just a balanced system with no individual controls/zoning?

 

With a multistorey house I'd definitely zone it based on occupancy. If your first and second floors are bedrooms only,  there's likely no point in heating them during the day. We have the upstairs split from the downstairs and only heat the upstairs late evening and occasionally early morning. Rest of the time upstairs only gets what trickles up from the ground floor. You may need to slightly oversize ground floor emitters to account for bigger losses to the cooler upstairs??

[JohnMo]

8 hours ago, ectoplasmosis said:

 

Do you use TRVs on the rads, or just a balanced system with no individual controls/zoning?

 

That will get two answers, no zoning or zoning.

 

But really depends on how you will use the system. WC requires long run times you could just balance the system on a single zone, keep it simple. Out all day house empty you may be cheaper flowing a slightly higher temperature for a shorter time period and rely on trv's to modulate the room. temperature.

 

I would, based on my experience of starting complex and moving to more simple, start as simple as you can, zero controls. We are currently running an electric mixer valve on our UFH set to 28 degs, house temp has varied around 0.5 degs in the last couple of weeks with outside temp swings from +10 to -2.  Heat pump just does it's own thing with very little load on it and no external inputs.  Have a thermostat but it just increases the mixer valve set point to 32 degrees, if the house temp drops below 20. Currently sitting at 20.5.

[SimonD]

8 hours ago, ectoplasmosis said:

Do you use TRVs on the rads, or just a balanced system with no individual controls/zoning?

 

Context is everything. You need to consider your personal setup and lifestyle within this.

 

Also, regarding radiator sizing, your system designer can tell you whether they considered intermittent use as part of the heat calcs. When considering intermittent use, this can significantly increase the emitter sizes as well as energy consumption depending on calculated re-heat times of the property. There needs to be a careful balance of this, that isn't just about your rad sizes and a single set flow temperature.

 

When running with trvs if they turn off flow to your rads there will be less volume in the system for the heat pump to warm. You'll gradually get to a point where the system cycles due to low volume of system fluid. If you're running a thermosat in the house it requires some good setting of trvs and room stats to ensure the system isn't still calling for heat when all your trvs are closed.

 

People often incorrectly think setting up a whole house heating system to run efficiently is a simple thing, even with weather comp it takes a long time to get it set up correctly. On here you'll find a lot of people who tinker with their systems until they get it right, because they have the interest and make the time to do it. Also on here you'll get a lot of people who have installed just weather comp, but depending on how you live, you may find load compensation or a controller that combines load/weather comp to be better for your needs.

[HughF]

12 hours ago, ReedRichards said:

Too hot for me, I would adjust the Weather Compensation settings.

Spot on for my wife, she’s the one who lives there most of the time. Apparently I should have designed for 23 degrees...

Edited December 13, 2023 by HughF

[HughF]

10 hours ago, ectoplasmosis said:

 

Thank you, very helpful.

 

Do you use TRVs on the rads, or just a balanced system with no individual controls/zoning?

 

TRVs on the bedroom rads only, mostly set to 3.5/4 out of 6. No zoning, no buffer/LLH/extra pump.