Joules ASHP, set 2 different output temps, rads & UFH?

[Andehh]

Morning all!

 

With a joules preplumbed GEN6 system, with zone 1 and 2 on UFH and zone 3 as towel rails.... I presume you should be able to set two different heating temps ie 35degrees for UFH and 50degrees for radiators? 

 

Our towel rails are on a separate zone to the zone 1 and zone 2 for our UFH loops... I would like them to be boosted to 50degrees vs what looks to be a 37degree temp they are set to currently. 

 

I tried running the towel rails today, and the outlet temp hit 37degree for about 10mins then shut off (short cycling?).... But I'd like it to do this but at 50degrees... I don't think joules have commisioned my system it up correctly like this though per 37deg outlet temp. 

 

Short cycling i can live with, as towel rails would be boosted to 50 degrees for 15mins via a timer programmer then shut off, to give the towels some warmth /finish drying them as the rails cool down to ambiant. 

 

So just asking for advice on menus to set the zone 3 temp different to the zone 1 and 2!

 

Thanks

[JohnMo]

1 hour ago, Andehh said:

presume you should be able to set two different heating temps ie 35degrees for UFH and 50degrees for radiators?

Why do you think you can? Not that normal, unless you are controlling a mixing valve on the cooler loops.

 

Most heat pump have a single flow temp for central heating, this may change depending on outside air temp 

[Andehh]

Just a hope that it is possible, for those that have UFH downstairs (Zone 1?) then Radiators upstairs (Zone 2?) I imagine this is not unheard of?  If DHW can be ran at 50degrees, then I would like to think it is possible to get that sort of flow to one of the output zones?

Edited August 16, 2023 by Andehh

[JohnMo]

For that to occur you need one of the zones to be a mixed flow. You also need to run the heat pump at the highest temperature - that will need deep pockets.

 

Think you really need to go back to basics on how a heat pump works best. You are just making stuff ineffective 

[Andehh]

Thanks JohnMo - and I don't disagree!  I'm just trying to ascertain what i could do with my installation as is to get some use out of it, whilst not ideal running towel rails/garage radiators, I am only trying to run them for a very limited time frame so cost isn't an issue (ie a few hours one weekend to boost the garage rads/take chill off the air when im working in there is inefficient....but not ineffective).

 

Sadly I was not guided very well by our installers when I asked for these things ie towel rails...and he plumbed them in vs electric ones or heating in the garage for a future grannie annex...should have looked at A2A I think -vs- him just sticking in a couple of big rads. To then kick me again, I was charged extra for things like the zone valves as recommended by them....only to now find none of it seems to work full stop!  

 

I am now better informed (thanks to this forum) on the basics/good working practise for ASHPs, which is how 99% of the use will be managed (one big zone, low flow temps etc) I just want to see if there is a way I can get some use out of what I have, as opposed to having nothing from it. 

[Beelbeebub]

I guess it's better to run the UFH at as low a flow temp as poss, and then the rads at a higher temp than to run every thing at the higher temp and blend down. 

 

It isn't ideal, the ideal would be to get the rads large enough to run at the same low flow as the UFH, but that is fairly unrealistic. 

 

As to whether the heatpump can do it, I think it depends on the make. I think some brands allow 2 different zones, and temps. Ideally you would want weather comp on both with a different curve. 

[JamesPa]

2 hours ago, Beelbeebub said:

I guess it's better to run the UFH at as low a flow temp as poss, and then the rads at a higher temp than to run every thing at the higher temp and blend down. 

 

It isn't ideal, the ideal would be to get the rads large enough to run at the same low flow as the UFH, but that is fairly unrealistic. 

 

As to whether the heatpump can do it, I think it depends on the make. I think some brands allow 2 different zones, and temps. Ideally you would want weather comp on both with a different curve. 

There is a fundamental here, the unit can only output one temperature at any one time, given it has only one pair of output pipes.  Gas and oil boilers are exactly the same.

 

So you operate at the efficiency of the worst component unless you can time-slice it as you do with DHW vs space heating.

[Beelbeebub]

Just now, JamesPa said:

There is a fundamental here, the unit can only output one temperature at any one time, given it has only one pair of output pipes.  Gas and oil boilers are exactly the same.

 

So you operate at the efficiency of the worst component unless you can time-slice it as you do with DHW vs space heating.

Yes, I was thinking time slicing.

 

You have the efficency of running at higher rad temp all the time and blending down the UFH

 

Vs

 

The higher efficency of running half the time at lower UFH temps and half the time running at Rad temps. 

 

Because you are time slicing, you may need to run the UFH a little higher temp than if it was full time, but that would still be lower than the rad temp. Likewise the rad temp would be a tad higher than if it was full time rad temp. So you would lose a little efficency there. 

 

One advantage would be:

 

If you run the "traditional" HP outputs huger rad temps and UFH blends down approach, you can it have weather compensation for the UFH flow temp. It is stuck at whatever the blending valve is set to (unless you use an electronically adjustable blending valve). 

 

If you use the time slice approach there is nothing stopping you using a WC curve for UFH and one for the radiators. That may yield a bit more efficiency. 

 

[JamesPa]

At first this seemed logical but Im not sure the maths works out. 

 

If you run UFH half the time you need to run it at twice the delta T (room-UFH), so about 45-50 rather than 35. 

 

With the rads its not so bad because the output goes as deltaT (room-rad)^1.3, so a 70 % uplift from say from 45->55.  Thats a 10%+ hit on both and more to the point the UFH has to run at the same temp as the rads would be running if everything were run full time

 

WC means its not quite as bad as this (because most of the time the actual deltaT is less than the design deltaT), but overall I suspect you might still be just as well off, possibly better, running the UFH at the same temp as the rads.  

Edited August 16, 2023 by JamesPa

[Beelbeebub]

Yeah, I'm pondering the maths too. 

 

The only thing that your analysis might be missing is the flow rate issue. 

 

Energy delivered is proportional to dT across emiiters and flow rate. 

 

If we double the flow (q) through the system we double the energy, assuming same dT. 

 

For rads, we are limited by the emitter area. As we up the flow rate the dT tends to drop. So the flow temp of the rads will need to be a bit higher to compensate for the intermittent heating.

But for UFH, at least the heavy slab type, the slab tends to absorb much more energy than you can throw at it. 

 

My slab returns the flow at pretty much the slab temp until I reach a high enough flow that the water can't transfer enough heat before it leaves.

 

(as an aside I am tinkering with this as a way of measuring the slab temp -then controlling the actual slab temp to a weather compensation curve by "blipping" the flow from my Thermal store) 

 

So if we assume the heat transfer to the slab is well below the "saturation" rate, we could inject double the energy into the slab by doubling the flow rate at the same flow temp during the "on" time slice. We can do that because out HP is directing all it's output to the UFH and not dividing it between UFH and rads. 

 

Does that make sense? 

 

As I say, it's just a gut feeling rather than hard maths at the mo

 

Could be wrong. 

[JohnMo]

He is trying to heat up a couple towel rads for 15mins at a time.  No idea why or what advantage it serves over running for a longer period at a lower temp. Add an electric element to the rad on a timer - or just let the towel rads be warm when the balance of the system is on - or both.  Anything else is stupid complicated for no real reason and most (time splicing for example) almost impossible to program with a normal heat pump.

1 hour ago, Beelbeebub said:

You have the efficency of running at higher rad temp all the time and blending down the UFH

should this read inefficiency?

 

 

[JamesPa]

5 hours ago, Beelbeebub said:

For rads, we are limited by the emitter area. As we up the flow rate the dT tends to drop. So the flow temp of the rads will need to be a bit higher to compensate for the intermittent heating.

Yes and no.  The flow of energy to the rads is only one part of the equation, it's the flow from the rads that matters and is the limiting factor.  So if I heat the rads half time I do need a 70pc higher delta T between rads and room to get the same energy from rads to room.  I am assuming that the rads respond quickly to turning off the flow, which typically they do.

 

5 hours ago, Beelbeebub said:

My slab returns the flow at pretty much the slab temp until I reach a high enough flow that the water can't transfer enough heat before it leaves.

 

(as an aside I am tinkering with this as a way of measuring the slab temp -then controlling the actual slab temp to a weather compensation curve by "blipping" the flow from my Thermal store) 

 

So if we assume the heat transfer to the slab is well below the "saturation" rate, we could inject double the energy into the slab by doubling the flow rate at the same flow temp during the "on" time slice. We can do that because out HP is directing all it's output to the UFH and not dividing it between UFH and rads. 

 

Does that make sense? 

I think it might.  This is almost the opposite, as you say, to the rad situation.  You can get all the energy out of the slab that you can put in.  So if you can put more in by a faster flow at the same deltaT across the flow/return, then you will get more out.  Because the slab stores energy the on/off cycles have less effect.

 

Not sure if it helps op and @JohnMo makes a very good point, but as another variable to play with its interesting.

[Beelbeebub]

7 hours ago, JohnMo said:

He is trying to heat up a couple towel rads for 15mins at a time.  No idea why or what advantage it serves over running for a longer period at a lower temp. Add an electric element to the rad on a timer - or just let the towel rads be warm when the balance of the system is on - or both.  Anything else is stupid complicated for no real reason and most (time splicing for example) almost impossible to program with a normal heat pump.

I think you are right, just an interesting thought experiment 

 

7 hours ago, JohnMo said:

should this read inefficiency?

Yes