Am I alone in regretting the changes in recent years to the availability of simple motor-open / motor-close (MOMO) valve actuators for domestic heating systems? The principal benefit of these actuators over spring-return valves was that they do not close unless instructed to do so by the control system logic. A valve only needs to close if another valve is open. However, there has been a move to standardise actuators so that the electrical connections are now similar to those for spring-return valves. As a result, the main benefits of the MOMO valves appear to have been lost and the valves always close when there is no open command.
I am a keen hobbyist, rather than an industry professional, so I do not install heating systems very often. The previous installation in my house used Sunvic SM5205 MOMO actuators and those lasted many years with no problems. I have just had a new system installed in my home, to incorporate a modern condensing boiler, and I specified that MOMO valves should be used in the new system. The controls use the Evohome package, with which I am very pleased. (I use zone valves but do not have any HR92 TRVs.)
However, the only MOMO valve actuators that I can find on the market now are designed to mimic the behaviour of spring-return valves. My system now has the Sunvic SZM 1801 actuators. Their wiring is such that it has a demand wire, which causes the valve to open when that demand wire is live and to close when it is not live. There is no do nothing option.
What has changed?
MOMO valves comparison table v3.jpg
So whats the problem?
The new design of MOMO valve actuator is inferior because:
It incorporates control logic that closes the valve in the absence of an open demand, whether the valve needs to close or not, thereby limiting the options for system control logic (i.e. only two options: open, close with no do nothing option);
It creates inefficiencies by requiring a boiler bypass to be used for the boiler and pump overrun period, rather than rejecting boiler heat to the last zone that was demanding heat before the system shuts down;
It cycles the valve bodies hundreds of times a year more than is necessary, creating unnecessary valve wear; and
It incorporates electronics inside the actuator which is in a hot and possibly humid atmosphere, reducing reliability and longevity.
With the new actuators, every zone valve closes once its heating demand is satisfied, whether it needs to close or not. The installer of my new boiler incorporated a standard boiler bypass arrangement, which he dutifully lagged very thoroughly (and therefore does not reject much heat at all). When the final zones heating demand is satisfied the zone valve now closes so that the system is totally closed and the boiler bypass is forced open by the pump. The pump overrun continues for a few minutes circulating water round the short circuit with very little heat loss, all rather pointlessly and unproductively.
On my old system, when the system stopped calling for heat, the boiler would simply shut down, all the valves would stay where they were, and the pump overrun would operate for a few minutes to cool the boiler. In the summer, when the only zone that was required was the hot water (DHW), the DHW valve would stay open for three months, with all other valves closed and the boiler and pump switching on and off when DHW heating is required. The old control system logic only gave a close demand to a zone valve if another valve was open, ensuring that there was always at least one zone open when the pump was running.
Now in the summer, with the new actuators, my DHW valve will cycle about three times a day (i.e. about 250 times in the season) totally unnecessarily. In the winter, the final heating zone to close as the house warms is a large one (my ground floor). I have used the Evohome controller in my new system. This cleverly learns the heating characteristics of each zone and punctuates the heating demand accordingly. However, that means that my final zone to be satisfied is receiving little sips of heat as it nears its set point. With the new valves and controls, the zone valve closes every time the heat demand ceases. The boiler bypass opens instead. Indeed, as the sips of heat get less frequent as the zone nears its set point, I notice that the return water temperature is low and could be used much more effectively to cool the boiler than using the bypass. It is wholly counter-productive to close this zone valve when no other zone requires heat. What I really want is for the final zone valve (which might be the DHW) to stay where it is and let the boiler shut down and use the final zone for its pump overrun.
Why has the design changed?
I suspect that Sunvic has changed its design to make it more attractive to installers. The wiring is now identical to the more common spring-return valves. Indeed, these MOMO valves now behave like spring-return valves, so there seems very little benefit in using them. The key benefit that the valve stays where it is unless it gets a positive signal to move has been lost. Of course, professional installers are the main market for manufacturers. Hobbyists on this forum, such as me, are a tiny market by comparison. I guess installers value simplicity so that they can diagnose problems quickly and recognise the wiring arrangements immediately. I assume therefore that Sunvic is trying to compete with the more common spring-return valves by making the installers lives easier and thus make it more likely that installers will recommend these actuators to their clients. A cynical view would be that installers care much less than their clients whether a valve and its actuator will last two or twenty years. They are also unlikely to be persuaded by engineering elegance over pragmatic simplicity.
Whats wrong with that?
I am disappointed if that reasoning is correct, as it means that the manufacturers are focusing on the requirements of the installers, rather than the end user. I fear that the actuators are now much less reliable (as they have electronics inside) and do not allow for good logical system control (i.e. valves close whether they need to or not). There is a keen critic of Sunvic who has a very comprehensive site showing the failures that now occur in these new Sunvic actuators with electronics inside them:
http://www.seered.co.uk/sunvic.htm
His commentary includes:
..older designs of motorised actuators used in housing often worked reliably for 20 or more years, yet the new 'improved' designs failed within a few months or years.
Sunvic is now trumpeting the environmental benefits of the MOMO valves. Given that it has eliminated the other benefits (operational flexibility; low valve wear; reliability) it has only this rather weak benefit to claim. A spring-return valve uses only about 6W to hold it open. Even if the actuator were energised permanently, the cost of the electricity would be only about £6 per year, which is hardly a key factor when designing a heating system for which the operating costs will be many hundreds or thousands of £ per year.
Can you not incorporate logic circuitry to overcome the problem?
I have incorporated logic into my heating control system that tries to defeat the automatic unnecessary closure of the valves. I have noticed suggestions from other hobbyists on this forum for bespoke circuitry to achieve similar results. But, in essence, I have used a standalone boiler relay (i.e. not a Honeywell BDR91) to interrupt the permanent live supply to the valves, thus stopping them from closing. I have used relays with multiple poles (one pole (switch) for each zone valve) such that the power supply to the zone valve (grey wire) is live only if (a) the boiler has heat demand (i.e. another zone is demanding heat) or (b) this zone has a demand for heat (the brown wire is live). The relay switches the grey wire between permanent live when the boiler relay is active and the brown wire when the relay is dead. Purists will point out that this is poor circuit design, as the boiler relay momentarily interrupts its own power supply when it operates. Relays with insufficient latency may therefore flutter. My relays seem to work satisfactorily however in this configuration. (I can post a circuit diagram if that is of interest.)
This arrangement is satisfactory in leaving open the final heating zone when demand for heat ceases. The boiler and pump can therefore overrun through that final zone. The DHW valve will stay open throughout the summer in this arrangement too.
So do you still have a problem?
I still regret that the valve actuators are being unnecessarily complicated through the incorporation of internal electronics which make them less reliable and less flexible for the end users (rather than for installers). I do prefer the simple mechanical MOMO valves which can be used with simple control logic. I can still get some SM5205 actuators from old stock (ebay and others) so I will swap out the newer ones that my installer provided. I regret that the modern MOMO valves are likely to fall further out of favour if they become unreliable. Their principal advantages of long reliability and low valve wear are being lost and so it will be increasingly hard to justify using MOMO actuators.
Perhaps I am just an old fogey mourning the passing of the old ways. Or does anyone else here share my regrets?