wet UFH - ashp fast cycling

[BEJB]

A couple of years ago, I bought a house heated by a 14kw Ecodan ASHP with a Mitsubishi pre-plumbed cylinder system, that was installed in 2015. It has zone 1 to heat 10 radiators and zone 2 to heat 3 loop wet UFH for the kitchen and utility - also heats DHW.

 

During the colder winter months and late frosts this year, it was clear that the UFH was failing to heat the kitchen/utility floor. The input to zone 1 is physically higher on the low loss header, so if both zone 1 and zone 2 are on at the same time, zone 1 takes the heat, and no heat travels down the low loss header to zone 2.

 

If just zone 2 is on, some heat goes into the zone 2, but much of it travels down the low loss header to the return. This causes the return to be almost as hot as the flow, so the Melcloud reports show the return temperature tracking the flow temperature. When it reaches the peak temperature, it cuts out and flushes with cold water from zone 1. This shows as a sawtooth effect of fast cycling - the cycling is very regular, every 10 minutes. If zone 1 is on by itself, this does not happen - the return remains lower than the flow and there is no cycling (I've attached a melcloud report showing this). 

 

The system has been serviced, I've flushed the UFH loops out (via hose - a small amount of muck came out but not much and the flow was OK) and inserted more biocide. I've tried altering the pump speeds. To reduce any flow obstructions, I've removed the flow meters and used blanking plates. I've replaced the Heatmiser UH2 with a standard Ecodan wireless thermostat, and Salus THB23030 actuators. I still have the 10 minute cycling.

 

I have a feeling that the ASHP is too big for the UFH by itself - but not big enough to heat both zone 1 and zone 2 together.

 

I've tried asking UFH and ASHP installers and maintainers, but they look even more baffled than I do and offer no help (with all the green deal stuff, they are too busy installing new systems to bother with maintaining existing ones). I just wondered if anyone had any thoughts or ideas as to anything I could try?

 

Many thanks

 

Eric

[MikeSharp01]

Sounds like a flow control problem - well it would. I would have thought you should be able to balance the zones out in any pattern you like through flow control, some of which can be passive EG flow control valves / needles and some of which could be active IE just switching off one zone, or proportionally switching off one zone (such as your actuators - which are Self balancing or should be) to allow the other to get the flow until some set point parameter is reached. The later 13:00-14:00 portions look like the 7deg delta that the Salus valves are trying to achieve. I wonder if a control valve for zone 1 has been fitted wrongly, or forgotten, somewhere allowing a return path that should not be there when only zone 2 is active? Can you post the install schematic?

 

[BEJB]

Many thanks for replying so quickly.

 

The later portions of the charts are where zone 1 cuts in and takes all the heat. If I could get zone 2 to behave like that and heat the kitchen, I would be happy!

 

I'm not sure if I have anything as grand as an install schematic. However, it is a standard Ecodan pre-plumbed FTC5 cylinder, so I have attached a couple of diagrams that show that.

In the first one, number 22 shows the low loss header, with the two spare ports for zone 2.

 

The second diagram shows zone 1 and zone 2, with the pumps associated with those zones.

 

Does that help understand the configuration?

 

Thanks again

 

Eric

 

[joth]

Can you confirm it has 3 pumps (primary, and one per zone)?

Or 2, sharing a single secondary pump between the zones using a 3 position valve? My installer tried to do it this way initially, but I insisted on a pump per zone as this just seemed a better idea. I can see various failure modes where a single pump would fail to balance between the 2 zones it was expected to drive. 

Edited July 26, 2021 by joth

[BEJB]

Yes, the heating system has three pumps, primary and one per zone. There are no valves in the heating circuits.

 

The system also provides DHW in a tank. So there is another pump for that and the three position valve selects whether to send the heat to the DHW heat exchanger or heating. It can't go to both at the same time - hot water takes priority.

[MikeSharp01]

OK, thanks for the schematic, can you show us how the Salus valves are connected is it just as this: https://www.vpsunderfloorheating.co.uk/wp-content/uploads/2019/10/THB230-Quick-Guide-V008.pdf ? 

[MikeSharp01]

I am still worried about return flow from zone 1 when it is not active, this must be coming from somewhere and the only place it can come from is through zone 1 and that must be getting heat at the header but as this is not getting back to the header it must be quite a low flow which means that the pump, although not running is allowing some flow through, albeit very low. Is there a manual valve you can use to isolate zone 1 and observe then?

[BEJB]

36 minutes ago, MikeSharp01 said:

OK, thanks for the schematic, can you show us how the Salus valves are connected is it just as this: https://www.vpsunderfloorheating.co.uk/wp-content/uploads/2019/10/THB230-Quick-Guide-V008.pdf ? 

Yes, I followed the instructions as carefully as I could. I've attached a photo below.

 

The Heatmiser is still in place, but is entirely disconnected and turned off. I'll get rid of it when I am absolutely 100% sure I'm not going to go back to it! When I do that, I'll arrange the power feed to the Salus units more neatly.

 

The only bit where I improvised regarding the Salus was to cable tie some foam lagging around each of the thermistors. This was just to make 100% that they were at the temperatire of the pipe and were not influenced by other hot things around them. It seemed fairly sensible.

 

 

[BEJB]

55 minutes ago, MikeSharp01 said:

I am still worried about return flow from zone 1 when it is not active, this must be coming from somewhere and the only place it can come from is through zone 1 and that must be getting heat at the header but as this is not getting back to the header it must be quite a low flow which means that the pump, although not running is allowing some flow through, albeit very low. Is there a manual valve you can use to isolate zone 1 and observe then?

 

I don't think there is a valve of any sort that can help here. Obviously there is the three way valve to select if the heat goes to DHW or to heating. There are on-off valves either side of the magnetic trap. There are on-off valves either side of the UFH. But I think that is it  

 

I'm just wondering what your thoughts are about return flow from zone 1. From my perception, zone 2 operates, but a substantial amount of the flow goes down the low loss header rather than the UFH. As a result the return thermistor returns a reading that is only marginally below the flow. The temperature of both rises, until the the heat pump cuts out. At that point, the pump for zone 1 cuts in (and zone 2 pump continues). This seems to result in some heat passing into zone 1 and heating the radiators a bit. colder water emerges from zone 1, cooling the water through the heat pump, low loss header and the start of zone 1 & 2. After a pause, the cycle starts again.

 

 

Edited July 27, 2021 by BEJB

[BEJB]

I am becoming increasingly convinced that the UFH just doesn't have the capacity to handle the heat coming from the heat pump, and hence the fast cycling. So there are a couple of 'solutions' that I'm thinking about at the moment (but always open to instructions not to be a dolt, or other options to consider!).

 

First, I wonder about adding additional items to the UFH heat manifold, to 'absorb' the heat and hopefully heat the kitchen. I could extend the manifold and connect a radiator into the room (as it is an ASHP, it would have to be a large triple radiator). Alternatively, there is a lobby areas just outside the kitchen - I could disconnect the radiator in that (currently in zone 1) and connect it to the UFH manifold. Neither are great solutions, but they might help.

 

The other option I'm considering is a 'feature' of the Ecodan FTC5. I've attached a copy of the outputs for the FTC5,

For 2 zone heating with pumps for each zone, Mitsubishi say that zone 2 pump should be connected to OUT3 and zone 1 pump should be connected to OUT13 (apparently, the 0.1A is a typo!). One reason for this is that, if zone 2 is on (OUT3), then there is also power at OUT2. If the zone 1 pump were connected to OUT2 (as intuitively seems sensible), then it would also come on when the zone 2 pump (OUT3) came on.

 

So I'm wondering about connecting the zone 1 pump to OUT2, rather than OUT13. So when the zone 2 UFH came on, so would the zone 1 radiators. So there would be enough radiator capacity to handle the heat provided by the heat pump, and there would be no fast cycling. It is true that the heat would go to the highest point in the low loss header (zone 1) . So I'm thinking about balancing the zone 1 radiators down, so there is remaining capacity to pass to zone 2. 

 

This doesn't seem ideal. I wonder if that might mean that when zone 1 is on by itself, the radiators might then have insufficient capacity to handle the heat - so once again heat would go down the low loss header and I'd get cycling.  The balancing seems quite sensitive. It feels like I need a variable valve that allows full flow to zone 1 when it is operating alone, but restricts the flow to zone 1 when zone 2 is operating.

 

As you can tell, I am very much an amateur at this, and easily get myself into a tizz. Any help is much appreciated

 

Eric

[MikeSharp01]

1 hour ago, BEJB said:

I'm just wondering what your thoughts are about return flow from zone 1. From my perception, zone 2 operates, but a substantial amount of the flow goes down the low loss header rather than the UFH.

Yep - that looks like the nub of it and it means that the two pumps (the primary loop & zone 2) are not balanced if they were everything coming in the top of the header would flow through the UFH to ensure this you need to get the UFH pump creating a slight negative pressure at the top of the header. From all this it looks like the flow rate capacity of the UFH is well out of whack with the primary circuit. Also your control table seems to suggest that the OUT3 controls both pump 3 and a 2 way valve and the *2 note indicates a ON/OFF valve somewhere. There is some good theory on the LLH - https://www.flexej.co.uk/what-is-a-low-loss-header/ which explains the various modes of operation and gives you a feel for the way the flow rates mingle and what happens in the header. Possible investigations / experimentation might be, in no particular order or with any analysis of consequences: Restrict the primary flow pump so the UFH can take all the flow or increase the zone two pump capacity somewhat.  Not sure linking the two zones together will make any difference it will just hit the HP capacity too hard.

[joth]

FWIW I have a similar issue with zone 1 (fancoil) in cooling mode not transferring enough cooling energy and hence the ecodan (8.5kW R32 model) is short cycling.

I have to say, I hadn't realized the Melcloud UI allowed useful hourly charts (I'd been scrapping it via Home assistant into Grafana) but similar to you I see 8-10min temperature cycling. This is what people would call short cycling, right?

 

Edited July 27, 2021 by joth

[MikeSharp01]

4 hours ago, joth said:

This is what people would call short cycling, right?

 

 

Yes and short cycling is symptom of low demand where the HP cannot modulate down far enough to make only the heat that is required.

[BEJB]

8 hours ago, MikeSharp01 said:

Yep - that looks like the nub of it and it means that the two pumps (the primary loop & zone 2) are not balanced if they were everything coming in the top of the header would flow through the UFH to ensure this you need to get the UFH pump creating a slight negative pressure at the top of the header. From all this it looks like the flow rate capacity of the UFH is well out of whack with the primary circuit. Also your control table seems to suggest that the OUT3 controls both pump 3 and a 2 way valve and the *2 note indicates a ON/OFF valve somewhere. There is some good theory on the LLH - https://www.flexej.co.uk/what-is-a-low-loss-header/ which explains the various modes of operation and gives you a feel for the way the flow rates mingle and what happens in the header. Possible investigations / experimentation might be, in no particular order or with any analysis of consequences: Restrict the primary flow pump so the UFH can take all the flow or increase the zone two pump capacity somewhat.  Not sure linking the two zones together will make any difference it will just hit the HP capacity too hard.

The Ecodan documentation says that the flow rate for the heating primary for a 14kw Ecodan is between 17.9 and 40.1 L/min. The service says mine has a flow rate of 18L/min.

 

I have three loops on the UFH, each with a flow meter that goes up to 4l/min. So the total is 12L/min as a maximum (it is actually below that - say about 10. I did remove the flow meters and put blanking plates in to remove any obstruction. Actually that increased the rate of the cycling. I think the hot water was travelling so fast that it wasn't losing heat fast enough.

 

I think I have hit a case of very bad heating system design  

[BEJB]

8 hours ago, joth said:

FWIW I have a similar issue with zone 1 (fancoil) in cooling mode not transferring enough cooling energy and hence the ecodan (8.5kW R32 model) is short cycling.

I have to say, I hadn't realized the Melcloud UI allowed useful hourly charts (I'd been scrapping it via Home assistant into Grafana) but similar to you I see 8-10min temperature cycling. This is what people would call short cycling, right?

 

I'm glad you found a use for those reports too.

 

I'm also very envious. Despite having thermistors on the zones, I can't get the flow temperatures for the zones like you can. I asked Mitsubishi support and they said you can't get them on a simple 2 zone system. For something like the problem I am having, it would be very useful if I could!

[MikeSharp01]

6 minutes ago, BEJB said:

I think I have hit a case of very bad heating system design

Don't think so, looks like it is a standard design so its got to be a tuning issue. If you are right that the zone 2 flow is so high it cannot loose enough heat in the UFH then some other aspects don't make sense do they.

[BEJB]

9 hours ago, MikeSharp01 said:

Don't think so, looks like it is a standard design so its got to be a tuning issue. If you are right that the zone 2 flow is so high it cannot loose enough heat in the UFH then some other aspects don't make sense do they.

I suppose what I meant was the sizing is all wrong. The ASHP is around double the capacity that the UFH has to handle the flow and dissipate heat. Does that make sense?

 

I'm not sure I follow your comment about zone 2? What I was trying to describe were my attempts to raise the flow levels to match the primary. I removed the flow meters (which restrict the flow and I didn't have the actuators fitted, so the actuator valves were completely open. So there was no real restriction (apart from the pipe capacity/friction) and the water could flow was fast as it liked. When I did that, there seemed to be about 10 cycles an hour rather than the normal 6. The only thing I could think was that more of the heat was getting to the return than before - so maybe it was moving so fast, it didn't have time to dissipate the heat into the floor slab. There is probably another explanation but I couldn't think of one. Any other ideas very gratefully received!

[MikeSharp01]

Just while I remember, daft thought but worth eliminating - there is no chance that the low loss header has been put in upside down?

[PeterW]

17 minutes ago, MikeSharp01 said:

Just while I remember, daft thought but worth eliminating - there is no chance that the low loss header has been put in upside down?

LLH only has a vent at one end on most and two pairs of connections top and bottom. Can’t see how it would have an issue upside down..?

 

@Nickfromwales..?

[Nickfromwales]

59 minutes ago, PeterW said:

LLH only has a vent at one end on most and two pairs of connections top and bottom. Can’t see how it would have an issue upside down..?

 

@Nickfromwales..?

Depends whether or not it has baffles / vanes inside tbh. Then it becomes flow / return / load specific. 

[BEJB]

15 hours ago, MikeSharp01 said:

Just while I remember, daft thought but worth eliminating - there is no chance that the low loss header has been put in upside down?

I'm pretty sure the LLH is on the right way up. It is a pre-plumbed unit, looks exactly the same as all the photos, with the vent at the top and the correct inlets/outlets in the right places as you go down. 100% certainty is impossible but I think that is unlikely to be the problem.

[MikeSharp01]

3 hours ago, BEJB said:

I'm pretty sure the LLH is on the right way up.

Good to hear, it was a long shot anyway. 

 

 

[Nickfromwales]

Have you closed all of the pump isolation valves fully, and then reopened them fully so you know they're all open 100%?

[BEJB]

On 29/07/2021 at 14:06, Nickfromwales said:

Have you closed all of the pump isolation valves fully, and then reopened them fully so you know they're all open 100%?

I had done the pump to the ufh but not the others. I've done them all now and they all seem to be ok. At least the temperature has dropped a bit now, so testing the heating is better received by the rest of the family!

 

I've removed the Salus actuators now - I'm not sure they are set up correctly and also they seem to close the actuator valves down a bit when you attach them, even when they are supposed to be open. At least I should be able to see if I get some heat through to the UFH at all.

[SteamyTea]

On 28/07/2021 at 09:27, BEJB said:

The ASHP is around double the capacity that the UFH has to handle the flow and dissipate heat. Does that make sense?

Have you done some room by room heat loss calculations?

Takes a lot of the guess work out of it, and not as messy as plumbing.