BDR91 stops asking for heat every 20 minutes

[adriankpope]

I have 8 zones linked to my evohome controller. Even when the system says that it has a high demand of 70%, heating the house up in the morning, it drops the demand (the green light on the BDR91 goes off) for about 3 minutes every 20 minutes. This does not lead to a happy heat pump !
The honeywell helpline said to change the honeywell system from 6 to 3 cycles per hour, but that made no difference. I also changed it over to partial load scaling, with no change.
I know that the controller talks to the BDR91 every 20 minutes but cannot see why it should drop demand for a couple of minutes as it does so.
I am running a heat pump at very low supply temperature, so the rooms are unlikely to overshoot even with our new large rads, and it typically takes about an hour to raise the temp by 1 degree if the system has been set to holiday mode whilst we are away. Overall, the system should be running slow and steady.

[Richard]

Any system using TPI and a on/off switching relay is going to do an element of proportional cycling. Hence I'm not personally a fan of heat pumps using on/off switching controls unless they are tied into a buffer cylinder (like mine is at home).

[adriankpope]

Thank you for the response, worrying but honest.
I am somewhat concerned because Honeywell say that TPI systems can be used on any system (https://youlearn.honeywellhome.com/uplo ... nality.pdf). My system does not have any of the units listed on the accompanying page (https://heatingcontrols.honeywellhome.c ... i-control/) with HR92 valves and a single round room thermostat controlling two valves.
I cannot see how TPI could possibly work by monitoring a heat pump over a week since the temperature that the water is supplied at can vary between 30 and 38 degrees over a 24 hour period due to temperature compensation. This gives a delta T of 9 to 17 degrees in a single day so the characteristics will vary massively.
In any case, when our system was working reasonably hard, and the advanced load scaling was not used, the demand went off regularly for about two minutes every 20 and our heat pump followed suit, which could break the pump. I have tried turning on the advanced load scaling, and it behaves the same way when there is a high load required, When the system was not working so hard, it was asking for heat for a variable amount of time.
What does the reported percentage listed next to "Wireless relay box" signify ? Now that I have the advanced load scaling turned on, it was turning off and on for a few minutes when the demand was 60%, and then off for 20 minutes at a time when it later showed 50%. I am very confused. Changing from 6 changes per hour to 3 (as instructed by Honeywell on the phone) had no effect at all.
We have had a 35 litre diffuser installed in the local loop, which will slightly even out the temperature fluctuations, but the honeywell system will not know anything about that since the communication is one way. Even if we had a 300 litre tank, I feel that the system would still be turning off regularly since it only responds to the rads not the heat pump.
Is there any way to fake the controller into turning off for a minimum of even ten minutes ? The chap on the phone mentioned that Honeywell now have a BDR91T for heat pumps that can be set to switch once per hour - is there a software patch to fudge this because the box is only doing what it is told and the evohome control box should be able to do whatever it chooses ?

[Richard]

As stated by Resideo (Honeywell Home) the BDR91T was launched so cycling times could be minimised. 3 cycles per hour isn't really suitable for heat pumps imho.

In my system, the heat pump always maintains the temperature of the buffer cylinder and evohome only controls the secondary pump (which pulls the energy away from the buffer cylinder). My BDR91A has no direct access to the heat pump which is weather compensated, this just runs completely independently. I've had this setup for 7 years and it works just fine.

I would suggest the design of your system isn't optimised for your heat pump. Lots of heat pumps to 'MCS standards' are oversized and cycle most of the year anyway. Cycling isn't ideal, but not the end of the world either unless it is frequent and often (hence cycle rate of 1 is optimal).

[adriankpope]

Thank you very much for this response. Bizarrely, I was trying to explain to my wife this morning how I would want a system set up, ideally, and it exactly matches what you propose. My only change would be to have the heat pump and the rads completely separated by using the equivalent of a hot water cylinder using an internal heat exchanger.
Our system is a Mitsubishi, and they specifically tell their installers that one of their major design advantages is that they do not require a buffer tank, even though they then criticised the installed system because it did not meet their local water requirements, which required an extra tank: confused ? our installers certainly were.
I will check back with them but I do not believe that Mitsi are set up to control a buffer tank in the way that you say.
Out of interest, if you do not mind me asking, how powerful is your pump and how big is your buffer tank. Ours is a 14kW, I talked the installers down to 11.5kW of radiators by taking one air change per hour in dry rooms, not 1.5 (the BS requires 0.5/1.5 dry/wet but recommends 1.5/1.5)did not fight hard enough to get a much quieter 11 kW machine. Our peak average usage over 3 days has been 8.0kW for comparison. We now have a 35l diffuser tank added to give higher local water storage but that is not set as a central store as you have.
How much are the BDR91T controllers ? I could not find one when I searched your site (user incompetence ?). I presume that changing them would be a qualified electrician job.

[Richard]

I have an 11kW GSHP and 300 litre buffer cylinder (which also has a solar thermal coil and Immersion heater). The Mitsubishi ASHP needs a 3 pipe buffer in my opinion and are giving out bad advice.

I have a brand new BDR91T here. If you purchase a BDR91A (https://theevohomeshop.co.uk/honeywell- ... a1000.html) and let me know, I will happily send this instead. Changing it is as easy as taking the front cover off the one you have and replacing it with the front cover of the one we send.

[adriankpope]

I have ordered the standard model with a note attached. It will be interesting to see how it works out
Cheers
A
ps I have passed on you views about the Mitsi setup to my installer who have installed it as Mitsi instructed, I await their thoughts.

[adriankpope]

Hi, the box arrived very quickly, but, unfortunately, despite putting a comment in with the order, and informing you in my last post, the standard type A control has been sent through.
What shall we do now ?
Cheers
Adrian

[Richard]

Hi, the box arrived very quickly, but, unfortunately, despite putting a comment in with the order, and informing you in my last post, the standard type A control has been sent through.
What shall we do now ?
Cheers
Adrian

Really sorry this occurred... Warehouse completely missed your note!

We will get the T sent to you and a prepaid returns envelope to put the other one in and drop into your nearest post box.

[adriankpope]

I have already received a delivery notification - thank you.

I always find it best to judge a company by how they react when things go wrong and you did not even break stride fixing the problem