New Boiler but radiators are not heating up

cheme7676

Hi,

I've just had a new combi boiler installed, the Viessmann Vitodens 050 35kW, along with this, I've had all the radiators in the house replaced and balanced. I have kept my old thermostat, the Hive V2.

Despite DHW working perfectly, however the Central Heating takes forever to warm up. It'll warm the house by ~3 degrees in 5 hours which is unacceptable.

Just to confirm, weather compensation is NOT on. The boiler is set to operate at a constant flow temperature of 60 Degrees C, which should be plentiful to heat the house, but clearly this isn't happening.

I was wondering if anybody had any ideas for what I can try? Thanks

FreeBear

Hive doesn't support OpenTherm (or at least I don't think it does), so that rules out the thermostat commanding a lower flow temperature. At a 60°C flow temp, the pipes going to each radiator should be nice and toasty - Can you feel the heat ?

If so, the lockshields have probably been turned down too much. I'm assuming that the radiators did heat the house up just fine before the boiler was fitted and the system was balanced afterwards and the TRVs are fully open. It may be that the engineer was sloppy in adjusting things.

If the radiators are getting hot, then there is nothing wrong with the heating system, just the radiators are too small for the rooms. Crunch the numbers with this calculator - https://www.stelrad.com/basic-heat-loss-calculator/ - That should tell you if the radiators are large enough. Don't forget to set the Δt to 40°C if you are using a 60°C flow temperature.

If all else fails, turn the flow temperature up to 70°C (Δt50°C) - That is the usual number that many plumbers use when doing the calcs for a new heating system. But in the drive for better efficiency, they should be using a Δt50°C or even Δt40°C.

Veteransaver

Is it possible to change the pump speed? It could be that the pump speed is too low. Does the boiler cycle a lot, ie heat for bit then shut off for 10 minutes while the rads are still not warm?
You could also try opening the lockshield valve fully on one or 2 radiators (note the original position though, ie count the turns), it might have been badly balanced

cheme7676

Unfortunately not much heat getting to the radiators as the pipework isn’t that hot. I also checked flue on a cold day whilst CH was on, flue was barely giving anything out.

in terms of rad sizing, I had them all replaced for identical sizes but double panel instead of single. If anything, I should be getting much more powerful heating - given that the old rads were plenty powerful enough despite being single panels, using my old Ideal Logic Combi 35 boiler.

The rads eventually get kinda warm (enough to heat up ~3 degrees in a 5 hour period), but not as hot as they should be. With my last boiler, from fully cold, all my rads would be piping hot within 30 mins. That was using the “economy” flow temperature too - which is probably around 60 Deg C?

I’m totally stumped. I haven’t checked pump speed but can get my installer to take a look next week. Not sure what else is worth checking?

I read a few people online facing similar issues with new Vitodens installs, but they don’t come back to forums and say how it was done. Unfortunately, the ones that did had weather compensation settings to change (which doesn’t apply to me)

FreeBear

Get yourself a permanent marker pen. Put a dab of ink on the lockshield spindle & body, then crack the valve open by (say) half a turn on just one radiator. If that radiator gets hot within ~30 mins, you know the system hasn't been balanced properly.

I had a Viessmann 050W combi installed last summer, and spent a fair bit of time tweaking the lockshields to get the system balanced. My radiators all heat up as expected, and the house heats up quite nicely at ~0.5°C per hour - I could get it up to temperature faster by increasing the flow temperature if desired at the expense of lower boiler efficiency.

cheme7676

Right so there’s an update, I think I realised the issue. I noticed that the boiler cycles (burner comes on then cuts off for a few mins) despite thermostat demanding heat. 

I opened all of my rads and noticed that the cycling stopped and all radiators became super hot after a while.

it seems that with too many rads closed, the boiler is overheating (i think). Although I found a solution for now, it’s not ideal because some rooms will end up super hot before the rest of the house, since all the rads need to be on flat out.

This is despite me buying an identical power boiler to my last one and upsizing all my rads to double panel…

The good news is that it’s not the boiler i guess

FreeBear

cheme7676 said: I opened all of my rads and noticed that the cycling stopped and all radiators became super hot after a while.

OK. You need to get yourself a couple of these thermometer modules plus a couple of pipe clips to hold the sensor tight against the pipe - https://www.ebay.co.uk/itm/274842104250 for example (don't buy the hygrometer version. The sensor is wrong).

Shut off all lockshields, and starting at the closest to the boiler, crack open the lockshield a little at a time. Monitor the temperature difference between flow & return, aiming for (nominally) a 10°C difference. That is assuming your flow temperature is around 50-55°C. Go slow, take your time. It will take a few minutes for the temperature to change as the lockshields are opened up.

When you are done balancing the system, you'll find the radiators will need bleeding a few times over the next few weeks. The water inside the system will contain quite a bit of dissolved air when freshly filled..

Oh, and do give some thought to fitting an OpenTherm compatible (programmable) thermostat - It will save on gas consumption and reduce the short cycling considerably. You'll need to get a Gas Safe engineer to make the connection inside the boiler as it involves removing the front of the casing.

cheme7676

FreeBear said:

cheme7676 said: I opened all of my rads and noticed that the cycling stopped and all radiators became super hot after a while.

OK. You need to get yourself a couple of these thermometer modules plus a couple of pipe clips to hold the sensor tight against the pipe - https://www.ebay.co.uk/itm/274842104250 for example (don't buy the hygrometer version. The sensor is wrong).

Shut off all lockshields, and starting at the closest to the boiler, crack open the lockshield a little at a time. Monitor the temperature difference between flow & return, aiming for (nominally) a 10°C difference. That is assuming your flow temperature is around 50-55°C. Go slow, take your time. It will take a few minutes for the temperature to change as the lockshields are opened up.

When you are done balancing the system, you'll find the radiators will need bleeding a few times over the next few weeks. The water inside the system will contain quite a bit of dissolved air when freshly filled..

Oh, and do give some thought to fitting an OpenTherm compatible (programmable) thermostat - It will save on gas consumption and reduce the short cycling considerably. You'll need to get a Gas Safe engineer to make the connection inside the boiler as it involves removing the front of the casing.

I’ll either try this or otherwise my gas engineer is calling back at some point next week to fix. I’m in half a mind to try myself but might let him do his attempt first.

Just to confirm, this doesn’t necessary mean my boiler is oversized right? Given that my old boiler was a 35kW running on smaller rads.

In terms of opentherm thermostat, I don’t have any issues changing to one but it’s just not convenient for me right now - I’ve spent so heavily on the boiler and new rads so I need a couple months to breath. 

FreeBear

A combi boiler should be sized according to the cold water flow rate. The 050 35KW is suitable for flow rates up to ~14l/min. For heating, it will deliver between 3.2-32KW @ 50°C flow temp (2.9-29.3KW @ 60°C). You can limit the maximum CH heat output if desired - I have mine limited to 35% which suits my installation. Much of the time though, it is ticking over at 3-6KW.

Thanks to the OpenTherm interface, I can log data such as modulation level and boiler temperatures to keep track of how well it is performing.

One thing that is worth doing if you haven't already done so - Put the DHW in to Eco mode. This will stop the boiler firing up every 30-60mins just to keep the system ready to deliver hot water. Downside is it takes a little longer for hot water to come out of the taps, but it does save a significant amount of gas.

cheme7676

FreeBear said:

A combi boiler should be sized according to the cold water flow rate. The 050 35KW is suitable for flow rates up to ~14l/min. For heating, it will deliver between 3.2-32KW @ 50°C flow temp (2.9-29.3KW @ 60°C). You can limit the maximum CH heat output if desired - I have mine limited to 35% which suits my installation. Much of the time though, it is ticking over at 3-6KW.

Thanks to the OpenTherm interface, I can log data such as modulation level and boiler temperatures to keep track of how well it is performing.

One thing that is worth doing if you haven't already done so - Put the DHW in to Eco mode. This will stop the boiler firing up every 30-60mins just to keep the system ready to deliver hot water. Downside is it takes a little longer for hot water to come out of the taps, but it does save a significant amount of gas.

Perfect thank you.

By the way I reached out to Viessmann who didn’t mention the radiators but said the exact same thing as you. They sent me instructions to limit the CH output %, is it probably worth trying this before trying to rebalance the radiators? My plumber did say he’s confident he balanced them correctly but said he will call back to rebalance if necessary.

Installing opentherm is definitely on my to-do-list over the next few months, then I’ll be looking to change the boiler over to weather compensation at the same time because I’ve read online that it’s criminal to run these boilers without using WC. I think il need to install an outdoor sensor for this too if I recall correctly?

FreeBear

cheme7676 said:

FreeBear said:

A combi boiler should be sized according to the cold water flow rate. The 050 35KW is suitable for flow rates up to ~14l/min. For heating, it will deliver between 3.2-32KW @ 50°C flow temp (2.9-29.3KW @ 60°C). You can limit the maximum CH heat output if desired - I have mine limited to 35% which suits my installation. Much of the time though, it is ticking over at 3-6KW.

Thanks to the OpenTherm interface, I can log data such as modulation level and boiler temperatures to keep track of how well it is performing.

One thing that is worth doing if you haven't already done so - Put the DHW in to Eco mode. This will stop the boiler firing up every 30-60mins just to keep the system ready to deliver hot water. Downside is it takes a little longer for hot water to come out of the taps, but it does save a significant amount of gas.

Perfect thank you.

By the way I reached out to Viessmann who didn’t mention the radiators but said the exact same thing as you. They sent me instructions to limit the CH output %, is it probably worth trying this before trying to rebalance the radiators? My plumber did say he’s confident he balanced them correctly but said he will call back to rebalance if necessary.

Installing opentherm is definitely on my to-do-list over the next few months, then I’ll be looking to change the boiler over to weather compensation at the same time because I’ve read online that it’s criminal to run these boilers without using WC. I think il need to install an outdoor sensor for this too if I recall correctly?

You don't have to limit the CH output - Once the water circulating in the system gets up to the set temperature, the boiler automatically modulates down (or at least it does with mine). Although I'm not sure if this applies if you are using basic on/off control. Wouldn't hurt to knock the output back to say 25% while you are balancing the radiators.

I've not bothered with weather compensation yet - It is something I have plans for at some time, but it will be part of my custom control system https://github.com/FreeBear-nc/esphome-opentherm & https://www.home-assistant.io/ - I begrudge spending £30 or more for a plastic box containing a component that costs pennies. Already have a temperature sensor outside, so no need for another one.